Chapter 1: Reproduction in Lower and Higher Plants

In the closing years of the 1890s, a Russian botanist named Sergei Nawaschin spent long hours bent over his microscope, studying wafer-thin slices of lily and Fritillaria ovules. Like every biologist of his time, he expected fertilization to be a simple affair: one male cell fusing with one egg, just as it happens in animals. But what he saw did not fit the textbook. Inside the ovule, one male gamete fused with the egg, as expected — yet a second male gamete travelled deeper and fused with another cell entirely. Two fusions, not one, from a single pollen grain.

At first this seemed impossible. Why would a plant “waste” a second gamete on something other than the egg? Nawaschin had, in fact, stumbled upon one of the most elegant tricks in the living world — a double fertilization found nowhere except in flowering plants. That second fusion, we now know, builds the food store that feeds the growing embryo, so a plant commits its energy to a seed only after the egg is safely fertilized.

That single observation opened up a whole hidden drama that unfolds inside every flower — from the dust-like pollen grain to the ovule, the pollen tube, the seed, and finally the fruit you eat.

Diagram

Fig. 1.0 – An insect visiting an open flower, with a cutaway of the pistil showing a pollen tube growing down the style into the ovule; label anther, stigma, style, ovary, ovule.

Curiosity Corner

Q1. Why do flowering plants use two male gametes for one act of fertilization, when a single one seems enough? Q2. A banana has no seeds inside it, yet it still grows into a full fruit — how is that possible? Q3. How can a single orange seed sometimes give rise to several seedlings instead of one? Q4. Why can a grain of pollen survive as a fossil for millions of years, when most living tissue quickly rots away? Q5. Some plants produce seeds without any fertilization taking place at all — how do they manage it?

As you work through this chapter, you will be able to answer each of these questions for yourself.

1.1 Asexual Reproduction

Reproduction is the production of young ones that resemble their parents. It is one of the most essential of all life processes, because it is what carries a species forward generation after generation and keeps the thread of life unbroken. No individual lives forever, so unless organisms reproduce, a species would simply disappear.

Every kind of organism has its own particular way of reproducing, but all these ways fall into two broad categories: asexual reproduction and sexual reproduction. This chapter looks closely at how both of these work in plants, from simple algae to flowering plants.

In asexual reproduction, there is no fusion of two gametes (sex cells). A single parent, on its own, gives rise to new individuals. Because the offspring are produced from one parent and inherit exactly the same set of genes, they are genetically identical to that parent and to one another. Such morphologically and genetically identical individuals are called clones.

Definition

Asexual reproduction is the production of genetically identical offspring (clones) from a single parent, without the fusion of gametes.

The whole scheme of how organisms reproduce, and how the different asexual methods branch off, is summarised in FC 1.1.

Flowchart

FC 1.1 – Classification of reproduction: Reproduction → branches into Asexual reproduction and Sexual reproduction; Asexual reproduction → Fragmentation, Budding, Spore (zoospore) formation, Binary fission, Conidia formation, Gemma formation

Exam Corner

Q. If asexual offspring are exact copies of the parent, is that an advantage or a disadvantage?

A. It is both. Being identical means every well-adapted trait of the parent is preserved perfectly and quickly, which is useful in a stable environment. But because there is no genetic variation, if conditions change or a disease strikes, the whole clone population can be affected in the same way. Variation, which comes from sexual reproduction, is what helps a species survive change over time.

Common Modes of Asexual Reproduction

Organisms carry out asexual reproduction in several different ways.

Fragmentation. Some multicellular organisms simply break into pieces (fragments) for one reason or another, and each fragment then grows into a complete new individual. The filamentous green alga Spirogyra is a common example — a broken piece of its thread-like body can develop into a whole new filament.

Budding. This is the most common method of asexual reproduction in the unicellular alga Protosiphon and in yeast. It usually takes place under favourable conditions, when the parent cell throws out one or more small outgrowths called buds. Each bud, once it separates, develops into a new individual. In yeast, several buds may even remain attached for a while, forming a short chain, as shown in Fig. 1.1.

Diagram

Fig. 1.1 – Budding in yeast: a mother cell with a chain of buds emerging from it; label mother cell and chain of buds.

Spore formation. In Chlamydomonas, asexual reproduction takes place through special spores called zoospores. These zoospores are flagellated and motile, meaning they can swim, and each one can grow independently into a new individual (Fig. 1.2). Notice in Fig. 1.2 how the zoospores are formed inside the parent cell, which acts as a zoosporangium.

Diagram

Fig. 1.2 – Zoospores in Chlamydomonas: a rounded parent cell (zoosporangium) enclosing several flagellated zoospores; label zoospores and parent cell (zoosporangium).

Besides these, there are a few other well-known asexual methods. Binary fission, in which the body simply splits into two, occurs in Chlorella, diatoms and Chlamydomonas. Conidia formation — the pinching off of chains of spore-like conidia — is seen in the fungus Penicillium. Gemma formation, in which small green buds called gemmae detach and grow into new plants, occurs in the liverwort Marchantia.

Try Yourself

Sprinkle a small spoonful of yeast pellets or powder over warm water, then add some sugar. Cover it and wait for about 10 minutes. The yeast becomes bubbly over the water — this shows that the yeast is alive and active.

Vegetative Reproduction

In plants, asexual reproduction very often happens through ordinary vegetative parts of the plant body — that is, through roots, stems and leaves rather than through seeds. Because no gametes are involved, the new plants that form this way are genetically identical to the parent plant.

Natural Vegetative Propagation

Many plants multiply naturally through their vegetative parts, without any human help. Roots do this in sweet potato, Asparagus and Dahlia. Leaves do it in Bryophyllum, Kalanchoe and Begonia, where tiny plantlets sprout along the leaf margins. Stems do it in several forms — the underground rhizome of turmeric, the tuber of potato, and the bulb of onion each can give rise to new plants.

Try Yourself

Look around a garden or field and note how vegetative propagation happens on its own in nature. Which part of the plant — root, stem or leaf — is doing the job in each case?

Artificial Propagation Methods

There are also methods that would not happen naturally in plants, but which agriculture and horticulture deliberately use to multiply fresh stocks of a plant. These artificial methods let growers rapidly produce many plants of a desired variety, according to human requirements. The main methods are as follows.

Cutting. A small piece of any vegetative part of a plant, carrying one or more buds, is cut and used to raise a new plant. Stem cuttings are used in Rose and Bougainvillea, leaf cuttings in Sansvieria, and root cuttings in Blackberry.

Grafting. Here, parts of two plants are joined so that they grow together as a single plant. A piece of stem carrying more than one bud, called the scion, is joined onto a separate rooted plant called the stock. This joining is grafting, and it is used in Apple, Pear and Rose. When only a single bud is joined onto the stock, the process is instead called bud grafting (Fig. 1.3).

Diagram

Fig. 1.3 – Grafting in Rose: a scion (upper shoot with buds) bound onto a rooted stock, shown before and after joining; label scion and stock.

Do Not Mix Up

Budding vs Bud grafting (Budding is a natural asexual method in which a yeast or Protosiphon cell forms outgrowths that become new individuals. Bud grafting is an artificial horticultural technique in which a single bud of one plant is grafted onto the stock of another. Same word “bud”, completely different processes.)

Tissue culture. In this method, a very small amount of plant tissue is taken and carefully grown under controlled conditions to give a large number of tiny plantlets. This growing of plants from small tissue pieces is also called micropropagation, and it is widely used today.

Real-World Application

Micropropagation through tissue culture is used commercially to raise thousands of identical, disease-free plants of banana, orchids, sugarcane and many ornamental species from a single parent, all year round and in a small space. Because the plants are clones, every one carries the same desirable qualities as the selected parent — an important advantage for nurseries and farmers.

Side Note

Why does a gardener often choose to propagate plants asexually rather than by seed? Because asexual (vegetative) propagation produces clones — the new plants are exact genetic copies of the parent, so every prized quality (flower colour, fruit taste, disease resistance) is preserved. It is also faster than growing from seed, and it lets a gardener multiply varieties that produce few or no seeds.

1.2 Sexual Reproduction

Sexual reproduction is very different from the asexual methods you have just seen. Here, two compatible gametes (sex cells) — one male and one female — come together and fuse. A gamete carries only a single set of chromosomes, so it is described as haploid, written as n.

Before a plant can reproduce sexually, it must first grow up. Every organism passes through a juvenile or vegetative phase, and only after reaching maturity does it enter the reproductive phase. In higher plants this change is easy to spot, because the beginning of the reproductive phase is marked by the appearance of flowers.

Definition

Sexual reproduction is the type of reproduction that involves the fusion of two compatible haploid gametes (male and female) to form a diploid zygote, and it produces genetically variable offspring.

The Flower and Its Whorls

The flower is the specialised reproductive structure of a plant, and it is inside the flower that sexual reproduction takes place. The flower has two jobs: to produce the haploid gametes, and to make sure that fertilization (the fusion of gametes) can actually happen.

A typical flower is built from four different whorls arranged one within another. From outside to inside these are the calyx (the green sepals), the corolla (the coloured petals), the androecium (the male whorl), and the gynoecium (the female whorl). The androecium and gynoecium are the truly reproductive whorls, while the calyx and corolla protect the flower and help attract pollinators.

Try Yourself

On a labelled diagram of a flower, mark the four whorls — calyx, corolla, androecium and gynoecium — and note which two are directly involved in reproduction.

Events and Stages of Sexual Reproduction

Sexual reproduction turns on two major events. The first is meiosis, a special type of cell division that halves the chromosome number, producing haploid cells. The second is the fusion of gametes, which brings two haploid sets back together to form a diploid zygote (2n) — the first cell of the new individual.

Because the two gametes come from different sources, the offspring are genetically dissimilar, both from the parents and from one another. This built-in variation is extremely valuable: it gives a species the raw material to adapt, survive and evolve as the environment changes over time.

Overall, sexual reproduction is characterised by the fusion of male and female gametes (fertilization), the formation of the zygote, and its growth into an embryo (embryogenesis). The sequence of events is grouped into three distinct stages: pre-fertilization events, fertilization, and post-fertilization events. This chapter follows exactly this order, and the three-stage sequence is summarised in FC 1.2.

Flowchart

FC 1.2 – Stages of sexual reproduction: Pre-fertilization events → Fertilization → Post-fertilization events

Side Note

In all angiosperms (flowering plants), the diploid sporophyte is the main, dominant plant body. It is within this sporophyte, in the flower, that meiosis produces haploid spores. These spores give rise to the gametophyte — the gamete-producing generation. In flowering plants the gametophytes are greatly reduced in size and develop hidden inside the flower, yet it is they that produce the actual gametes. This regular hand-over between a spore-forming sporophyte and a gamete-forming gametophyte is called alternation of generations.

The Androecium and the Stamen

The male reproductive whorl of the flower is the androecium. Each individual member of the androecium is called a stamen. A single stamen is made of three parts: a stalk-like filament, a small connective at the top, and the anther, which is the pollen-bearing part. The anther typically has two lobes, each lobe being called a theca.

Zoom In / Out

the anther in the flower (Zoom out: the anther is part of one stamen, many stamens make up the androecium, and the androecium is one whorl of the whole flower — which is a reproductive organ of the plant. Zoom in: each anther contains pollen sacs, and inside the pollen sacs lie the microspore mother cells that will form pollen.)

Structure of the Anther

An anther is generally dithecous, meaning it has two lobes. Each lobe contains two pollen sacs, so a dithecous anther has four pollen sacs in all. Because it bears four pollen (microsporangium) sacs, the anther is described as tetrasporangiate.

At a very young stage, the anther is simply a rounded mass of parenchymatous (thin-walled packing) tissue wrapped in a single-layered epidermis. Differentiation begins when certain cells just below the epidermis — the hypodermal cells — enlarge and become distinct as archesporial cells. These archesporial cells are the starting point for everything that follows inside the anther.

Transverse Section of the Anther

Each archesporial cell then divides into two: an inner sporogenous cell and an outer primary parietal cell. The two products go on to build two quite different things, as traced in FC 1.3.

Flowchart

FC 1.3 – Anther development from the archesporial cell: Archesporial cell → divides into outer Primary parietal cell + inner Sporogenous cell; Primary parietal cell → repeated divisions → four anther wall layers (Epidermis → Endothecium → Middle layers → Tapetum); Sporogenous cell → Sporogenous tissue → each cell acts as a microspore mother cell → microspore tetrad

The sporogenous cell multiplies to form the sporogenous tissue, and each cell of this tissue is capable of giving rise to a microspore tetrad — a group of four microspores. The primary parietal cell, meanwhile, divides to build the layers of the anther wall.

The wall of a mature anther has four layers, seen clearly in a transverse section (Fig. 1.4a). From outside to inside these are: the epidermis, the outermost protective layer of flattened (tabular) cells; the endothecium, a sub-epidermal layer of radially elongated cells with fibrous thickenings; the middle layer, one or two layers of thin-walled cells that often break down as the anther matures; and the tapetum, the innermost, nutritive layer that immediately surrounds the sporogenous tissue (the microspore mother cells) and nourishes them. When the anther is ripe, it splits open, or dehisces, to release the pollen grains (Fig. 1.4b).

Diagram

Fig. 1.4 – (a) Transverse section of an anther showing the four pollen sacs and the wall layers (label connective, epidermis, endothecium, middle layers, tapetum, sporogenous tissue); (b) a dehisced anther releasing pollen grains (label pollen grains).

Structure and Function

Endothecium of the anther wall (The endothecium has cells with fibrous (band-like) wall thickenings → these thickenings create uneven drying stresses as the anther matures → the anther wall is pulled apart along a weak line → the anther dehisces and releases its pollen. Form here directly enables pollen release.)

Memory Aid

order of anther wall layers, outside to inside (Epidermis → Endothecium → Middle layers → Tapetum: remember “Every Egg Must Travel.” The tapetum, being innermost and nourishing, is the layer touching the pollen mother cells.)

1.3 Microsporogenesis

You have seen that the sporogenous tissue inside a pollen sac is made of microspore mother cells. Microsporogenesis is the process by which these cells give rise to microspores.

Each microspore mother cell is diploid (2n). It divides by meiosis, the reduction division, to form a tetrad — a group of four haploid microspores. These haploid microspores are the young pollen grains. Because meiosis halves the chromosome number, the resulting pollen grains carry only a single set of chromosomes (n).

Definition

Microsporogenesis is the formation of haploid microspores (pollen grains) from a diploid microspore mother cell through meiosis.

The full journey from the mother cell to the mature, gamete-bearing pollen grain is traced in FC 1.4.

Flowchart

FC 1.4 – Microsporogenesis and male gametophyte development: Microspore mother cell (2n) → meiosis → tetrad of four haploid microspores (pollen grains, n) → each pollen grain → first mitotic division → vegetative cell + generative cell (2-celled stage, usually shed here) → generative cell → second mitotic division → two non-motile male gametes

Structure of the Pollen Grain (Microspore)

A typical pollen grain is a non-motile (non-moving), haploid, unicellular body with a single nucleus. It is protected by a two-layered wall called the sporoderm.

The outer layer of this wall is the exine. It is thick and built from a complex substance called sporopollenin, which is one of the most resistant organic materials known — it does not break down easily and strongly resists chemicals. The exine surface may be smooth or beautifully sculptured, and the pattern is often characteristic of the species, so much so that pollen can be identified by it.

At certain spots the exine is very thin. These thin areas are the germ-pores, and it is through a germ-pore that the pollen tube grows out when the pollen germinates.

The inner wall layer is the intine, a thin, delicate layer made of cellulose and pectin. Together, exine and intine give the pollen grain both toughness on the outside and flexibility on the inside.

Compare Table

Structure and Function

germ-pore of the pollen grain (At a germ-pore the tough exine is thin or absent → this is a weak spot in the otherwise hard wall → the soft intine can bulge out through it → the pollen tube emerges here during germination. A gap in the armour, placed exactly where growth must happen.)

Curiosity Corner

Q4. Why can a grain of pollen survive as a fossil for millions of years, when most living tissue quickly rots away?

A. Because its outer wall, the exine, is made of sporopollenin — an extremely tough, non-biodegradable substance that resists heat, acids, alkalis and the enzymes of decay-causing microbes. Ordinary cells rot because microbes break them down, but sporopollenin defeats them, so the exine (and its sculptured pattern) can survive for millions of years.

Real-World Application

Because sporopollenin is so durable, pollen grains are preserved in rock, peat and ancient mud. Scientists (palynologists) recover and identify this fossil pollen to reconstruct what plants grew in a region long ago, and thereby to work out past climates and vegetation. The same species-specific patterns are even used in forensic investigations to link soil or objects to a place.

Side Note

Pollen viability is the functional ability of a pollen grain to germinate and develop into the male gametophyte. It depends on environmental conditions such as temperature and humidity. In rice and wheat the pollen stays viable for only about 30 minutes, whereas in some members of the families Solanaceae, Rosaceae and Leguminosae it can remain viable for months.

Development of the Male Gametophyte

The pollen grain is not just a spore — it marks the beginning of the male gametophyte, the tiny gamete-producing generation of the male side.

Inside the pollen grain, the single nucleus undergoes the first mitotic division. This produces two unequal cells: a large, naked vegetative cell and a small, thin-walled generative cell (Fig. 1.5). The vegetative cell is rich in stored food and has an irregularly shaped nucleus, while the smaller generative cell floats freely within the cytoplasm of the vegetative cell.

Diagram

Fig. 1.5 – Development of the male gametophyte in stages: (A) pollen grain with exine, intine, cytoplasm; (B) vegetative cell and generative cell; (C) generative nucleus and vegetative nucleus; (D–F) germinating pollen with pollen tube, tube nucleus and two male gametes; label pollen grain, exine, intine, germ pore, vegetative cell, generative cell, vegetative nucleus, generative nucleus, pollen tube, tube nucleus, male gametes.

The second mitotic division involves only the generative cell, and it gives rise to two non-motile male gametes. This division may take place either while the grain is still inside the pollen grain, or later, in the growing pollen tube. In most angiosperms the pollen grain is shed from the anther at the two-celled stage — that is, while it still contains only the vegetative cell and the generative cell — and the generative cell divides afterwards.

Do Not Mix Up

Pollen grain vs Male gamete (A pollen grain is the whole male gametophyte — a small structure that goes on to produce gametes. The male gametes are the two tiny cells formed inside it by division of the generative cell. So one pollen grain is not a gamete; it carries two gametes.)

1.4 Structure of Anatropous Ovule

The Gynoecium: The Female Whorl

Just as the androecium is the male whorl of the flower, the gynoecium (also called the pistil) is the female whorl. Each individual member of the gynoecium is a carpel, and because the carpel is the structure that bears and produces megaspores, it is also called a megasporophyll.

A single flower may have its carpels either separate or joined. When a flower has many free (unfused) carpels, it is described as apocarpous, as in Michelia. When it has many carpels fused together into one unit, it is syncarpous, as in brinjal.

A typical carpel has three parts: a swollen basal ovary, a slender style rising from it, and a stigma at the tip that receives pollen. The ovary encloses one or more ovules, which are the structures that later become seeds. The number of ovules varies from plant to plant: paddy, wheat and mango are uniovulate (a single ovule per ovary), while tomato and lady’s finger are multiovulate (many ovules per ovary).

Compare Table

Zoom In / Out

where the ovule sits (Zoom out: the ovule lies inside the ovary, the ovary is the base of a carpel, many carpels form the gynoecium, and the gynoecium is one whorl of the flower. Zoom in: inside each ovule is the nucellus, and embedded in the nucellus is the embryo sac, which contains the egg.)

The Ovule and Its Parts

Each ovule develops inside the ovary and hangs from a special cushion of tissue called the placenta. It is attached to the placenta by a small stalk, the funiculus. The exact point where the funiculus joins the main body of the ovule is called the hilum.

In angiosperms the most common type of ovule is the anatropous ovule. In this type the body of the ovule is turned over (inverted) so that the micropyle points downwards and lies right next to the funiculus, as shown in Fig. 1.6.

Definition

An anatropous ovule is an inverted ovule in which the micropyle is bent down to lie close beside the funiculus. It is the most common type of ovule in angiosperms.

The bulk of the ovule is a central mass of parenchymatous tissue called the nucellus. This is usually wrapped in two protective coverings, the integuments — an outer integument and an inner integument. At the apex of the ovule these integuments leave a narrow opening called the micropyle. Directly opposite the micropyle, at the base of the ovule, is the chalaza. Deep inside the nucellus lies the oval, multicellular embryo sac, which is the female gametophyte (Fig. 1.6).

Diagram

Fig. 1.6 – Longitudinal section of an anatropous ovule; label funicle, hilum, placenta, outer integument, inner integument, micropyle, nucellus, chalaza, embryo sac, and within the embryo sac the egg, synergids, secondary nucleus and antipodals.

Do Not Mix Up

Ovary vs Ovule (The ovary is the whole hollow basal part of the carpel; after fertilization it becomes the fruit. The ovule is a small body inside the ovary; after fertilization it becomes the seed. One ovary can contain one or many ovules.)

Structure and Function

nucellus of the ovule (The nucellus is a mass of food-rich parenchyma → it surrounds and cushions the embryo sac → it supplies nourishment to the developing embryo sac and megaspore. A built-in food and protection layer around the female gametophyte.)

Real-World Application

Whether an ovary is uniovulate or multiovulate decides how many seeds a fruit will hold. A mango, being uniovulate, carries a single large seed (the stone), whereas a tomato or a papaya, being multiovulate, is packed with many seeds. So the seed count you see when you cut open a fruit traces straight back to the number of ovules in its ovary.

1.5 Megasporogenesis

On the male side you saw microsporogenesis produce pollen grains. The female side has a matching process called megasporogenesis — the formation of megaspores from a megaspore mother cell.

The megaspore mother cell (MMC) is a single large, diploid (2n) cell that becomes distinct within the nucellus. It lies more or less in the centre of the nucellus but is shifted towards the micropylar end of the ovule.

Definition

Megasporogenesis is the formation of haploid megaspores from a diploid megaspore mother cell (MMC) through meiosis.

The MMC divides by meiosis to form a linear tetrad of four haploid megaspores arranged in a row. Of these four, the upper three (towards the micropyle) degenerate and abort, while the lowest one, nearest the centre of the nucellus, survives. This surviving cell is the functional megaspore, and it becomes the very first cell of the female gametophyte.

This is a striking contrast with the male side, where all four microspores of a tetrad survive and become pollen grains.

Compare Table

Development of the Female Gametophyte

The functional megaspore now develops into the female gametophyte, also called the embryo sac. It does this by three successive free nuclear mitotic divisions — that is, the nucleus divides three times without any wall forming between the daughter nuclei. The whole sequence is shown in FC 1.5 and Fig. 1.7.

Flowchart

FC 1.5 – Development of the female gametophyte: Megaspore mother cell (2n) → meiosis → linear tetrad of four haploid megaspores → three degenerate, one functional megaspore → 1st, 2nd and 3rd free-nuclear mitotic divisions → 8 nuclei (four at each pole) → one nucleus from each pole moves to the centre (the two polar nuclei) → organises into a 7-celled, 8-nucleate embryo sac (egg apparatus: 1 egg + 2 synergids at micropylar end; 3 antipodals at chalazal end; large central cell with 2 polar nuclei) → the two polar nuclei fuse → diploid secondary (definitive) nucleus

Diagram

Fig. 1.7 – Development of the female gametophyte from the megaspore mother cell through meiosis and three mitotic divisions to the mature embryo sac; label megaspore mother cell, four haploid megaspores, degenerated cells, mitosis I/II/III, egg, two synergids, three antipodals, two polar nuclei and secondary nucleus.

Three mitotic divisions of one nucleus give a total of eight nuclei, four gathered at each pole of the sac. Then one nucleus from each pole moves towards the centre; these two are the polar nuclei.

The three nuclei at the micropylar end organise into the egg apparatus. This consists of one large, central, haploid egg cell and two supporting haploid synergid cells. Each synergid bears hair-like projections called the filiform apparatus, which help guide the incoming pollen tube towards the egg.

At the opposite (chalazal) end sit three cells called the antipodal cells. In the middle, the two polar nuclei of the large central cell fuse, just before fertilization, to form a diploid secondary nucleus (also called the definitive nucleus).

Structure and Function

filiform apparatus of the synergid (The synergids carry finger-like extensions of wall and cytoplasm at the micropylar end → these secrete chemicals that attract the pollen tube → the pollen tube is steered straight to the egg apparatus. A chemical signpost pointing the way to the egg.)

The finished embryo sac is described as a seven-celled, eight-nucleate structure. This method, in which the whole embryo sac develops from a single functional megaspore, is called monosporic development (“mono” meaning one). Because the female gametophyte forms and stays inside the megaspore wall, its development is said to be endosporous. The female gametophyte is colourless, endosporic, and remains hidden within the ovule, which itself is enclosed by the ovary.

Do Not Mix Up

Synergids vs Antipodals (Both are groups of three haploid cells, which is why students confuse them. But the synergids (two, not three) plus the egg sit at the micropylar end and form the egg apparatus; the antipodals (three) sit at the chalazal end. Position is the key: synergids/egg towards the micropyle, antipodals towards the chalaza.)

Memory Aid

what makes up the embryo sac (Remember “3 – 2 – 1 – 2”: 3 antipodals, 2 synergids, 1 egg, and 2 polar nuclei — that is 7 cells and 8 nuclei in all.)

1.6 Pollination

There is a basic problem in plant reproduction: the gametes are non-motile and they are produced at two different places — the male gametes in the pollen of the anther, the egg deep inside the ovule. Somehow the pollen must reach the stigma before fertilization can even be attempted. Since the pollen grains cannot move on their own, flowering plants have evolved ways to have them carried, using non-living agents such as wind and water, and living agents such as insects, birds, bats and snails — often rewarding these visitors with food and, in return, exploiting their movement.

The actual transfer of pollen grains from the anther to the stigma of a flower is called pollination. It is an absolute prerequisite for fertilization, because the gametes cannot travel to meet each other by themselves.

Definition

Pollination is the transfer of pollen grains from the anther to the stigma of a flower.

Self-Pollination Versus Cross-Pollination

Pollination comes in two broad forms depending on where the pollen comes from. Self-pollination occurs within a single flower, or between two flowers of the same plant; it leads to inbreeding (selfing). Cross-pollination is the transfer of pollen from the anther of one flower to the stigma of a flower on a different plant of the same species.

On the basis of the source of the pollen, pollination is divided into three types, summarised in FC 1.6.

Flowchart

FC 1.6 – Types of pollination by source: Pollination → Self-pollination → Autogamy (within the same flower) and Geitonogamy (between two flowers of the same plant, needs an agent but genetically like selfing); Cross-pollination → Xenogamy (between flowers of two different plants of the same species, genetically variable)

Types of Pollination on the Basis of Source

Autogamy (self-pollination). This is pollination in which a bisexual flower (one with both anther and stigma) is pollinated by its own pollen grains. Because the pollen and egg come from the same flower, the offspring are genetically identical to the parent, as in pea and Clitoria.

Geitonogamy. This is the transfer of pollen to the stigma of a different flower borne on the same plant. In its working it resembles cross-pollination, because it needs a pollinating agent to carry the pollen from one flower to another. But since both flowers belong to the same plant, it cannot bring about genetic variation, so it is genetically equivalent to self-pollination and is only of ecological significance, as in Cucurbita maxima.

Xenogamy (cross-pollination / outbreeding). This is true cross-pollination, in which pollen from a flower of one plant is deposited, with the help of a pollinating agent, on the stigma of a flower of a different plant of the same species. It brings together genes from two different plants and so generates genetically varied offspring.

Compare Table

Do Not Mix Up

Geitonogamy vs Xenogamy (Both need a pollinator, so they look like cross-pollination. The difference is the plant: geitonogamy is between two flowers of the same plant, so it produces no new variation; xenogamy is between two different plants, so it does. Geitonogamy is genetically self-pollination wearing the costume of cross-pollination.)

Adaptations That Favour Self-Pollination

Flowers that rely on autogamy carry several structural adaptations that make selfing possible without any external agent. Such devices, called contrivances, include bisexuality, homogamy and cleistogamy.

When a flower opens to expose its sex organs, it is described as chasmogamous. Homogamy is the condition in which the anther and stigma of a flower mature at the same time, so that a flower’s own pollen is ripe exactly when its stigma is receptive. In cleistogamy, some flowers are self-pollinated even before they open; in certain plants these cleistogamous flowers are borne underground and never open at all, as in Commelina benghalensis. Interestingly, plants such as Viola and Commelina can bear both chasmogamous and cleistogamous flowers on the same plant.

Agents of Pollination

The vast majority of flowering plants depend on the transfer of pollen for reproduction; virtually all seed plants must be pollinated. Most of the world’s food and fibre crops rely on pollinators. The agents responsible for pollination fall into two main categories, shown in FC 1.7: abiotic (non-living) agents and biotic (living) agents. Living agents are especially important — about 80% of plants need moving creatures such as insects, birds, bats and snails to carry their pollen, and in doing so these pollinators also help sustain whole ecosystems.

Flowchart

FC 1.7 – Agents of pollination: Pollination agents → Abiotic (non-living) → Anemophily (wind) and Hydrophily (water → Hypohydrophily below the surface, Epihydrophily on the surface); Biotic (living) → Entomophily (insects), Ornithophily (birds), Chiropterophily (bats)

Real-World Application

Because roughly four out of five flowering plants rely on animal pollinators, the health of these pollinators directly affects our food supply. Bees, for instance, pollinate a large share of fruit, vegetable and oilseed crops. When pollinator numbers fall — due to habitat loss or pesticides — crop yields drop, which is why protecting bees and other pollinators is treated as an agricultural priority, not just a conservation ideal.

Pollination by Wind (Anemophily)

Wind pollination, or anemophily, is the abiotic method used by most of our important cereal crops — wheat, rice, corn, rye, barley and oats — and also by palms. Because wind is a blind, wasteful carrier, anemophilous flowers show a very distinctive set of adaptations (Fig. 1.8): the flowers are small, inconspicuous, colourless, and lack nectar and fragrance; the pollen grains are light, dry and produced in enormous numbers to offset heavy wastage; the stigma is large and feathery to catch drifting pollen; and the stamens are exserted on long filaments with versatile anthers, so that both stamens and stigmas are freely exposed to the air currents.

Diagram

Fig. 1.8 – A wind-pollinated maize plant; label the male inflorescence (tassel), female inflorescence (ear), styles (silks), grain (kernel) and prop roots, showing exposed stamens and feathery stigmas.

Structure and Function

feathery stigma of a wind-pollinated flower (The stigma is branched and feathery → this gives it a very large, net-like surface area → drifting pollen grains carried at random by the wind are far more likely to strike and stick to it. Turning a chancy delivery into a catch.)

Real-World Application

The pollen of wind-pollinated plants is the pollen most often behind hay fever. In people who are sensitive to it, breathing in this abundant airborne pollen triggers an over-reaction of the immune system (hypersensitivity), producing the sneezing, runny nose and itchy eyes of seasonal allergy.

Pollination by Water (Hydrophily)

Water pollination, or hydrophily, is quite rare, found in only about 30 genera of aquatic monocots such as Vallisneria, Zostera and Ceratophyllum. Hydrophilous flowers are small and inconspicuous; their perianth and other floral parts are unwettable; the pollen grains are long and unwettable owing to a coat of mucilage; and nectar and fragrance are absent.

Hydrophily is of two types. In hypohydrophily, pollination happens below the water surface. The pollen grains are heavier than water, so they sink and are caught by the stigmas of female flowers — for example in Zostera (sea grass), whose pollen grains are long, ribbon-like and lack an exine. In epihydrophily, the pollen grains float on the water surface and drift to the stigmas of female flowers. Vallisneria, a submerged dioecious freshwater plant, is a classic example: its female flowers rise temporarily to the surface to be pollinated, while the male flowers float on the water (Fig. 1.9). The pollen floats because its specific gravity equals that of water.

Diagram

Fig. 1.9 – Male and female plants of Vallisneria at the water surface, showing female flowers reaching the surface and male flowers floating; label water level, male flower and female flower.

Not every aquatic plant uses water for pollination — some are wind-pollinated (anemophilous), such as Potamogeton and Halogaris, and some are insect-pollinated (entomophilous), such as lotus, water hyacinth and waterlily.

Pollination by Insects (Entomophily)

Insect pollination, or entomophily, is a biotic method seen in Rose, Jasmine, Cestrum and many others. Entomophilous flowers are large, showy and often brightly coloured; they produce a sweet odour and bear nectar glands to reward insect visitors; their stigma is rough with hairs or sticky with a mucilaginous secretion; and their pollen grains are spiny and coated with a yellow, sticky substance called pollen-kit. Some flowers even have special mechanical devices to press pollen onto the visiting insect, such as the lever (turn-pipe) mechanism of Salvia (Fig. 1.10).

Diagram

Fig. 1.10 – Lever mechanism in Salvia: a bee entering the flower tips a lever-like stamen so that the anther dusts pollen onto the insect’s back.

Compare Table

Structure and Function

spiny pollen and pollen-kit in insect-pollinated flowers (The pollen grains are spiny and coated with sticky pollen-kit → this makes them cling firmly to the body of a visiting insect → the insect carries them from flower to flower and rubs them onto the next stigma. Stickiness turns an insect’s body into a pollen courier.)

Side Note

In biotic pollination, plants are adapted to attract the particular pollinators they depend on — this is called a pollination contrivance. Over long periods, plants and their pollinators have co-evolved, each shaping features in the other so that they interact successfully. The matching sets of features (flower colour, shape, scent, reward, and the pollinator’s body and behaviour) are together called pollination syndromes.

Side Note

Bumblebees are important early-season pollinators. You may see them in early spring searching for a place to start a nest and raise a colony. If you find a bumblebee nest, it is best to leave it alone — the nest lasts only a single season — and to help spread awareness of why we need to protect bees.

Pollination by Birds (Ornithophily)

Bird pollination, or ornithophily, is carried out by only a few specialised birds, usually small and long-beaked, such as sunbirds and hummingbirds. Bird-pollinated plants include Bombax, Callistemon (bottle brush) and Butea (Fig. 1.11). Their flowers are usually brightly coloured, large and showy; they secrete profuse, dilute nectar; their pollen grains are sticky and spiny; and they are generally without fragrance, since birds have a poor sense of smell.

Diagram

Fig. 1.11 – Ornithophily: a long-beaked sunbird or hummingbird probing a brightly coloured, showy flower for nectar and transferring pollen.

Pollination by Bats (Chiropterophily)

Bat pollination, or chiropterophily, allows pollen to be carried over long distances — sometimes several kilometres. Bat-pollinated flowers are dull-coloured but strongly fragrant (suiting a night-flying, keen-smelling visitor); they secrete abundant nectar; and they produce large amounts of edible pollen. Examples include Anthocephalous (kadamb tree), Adansonia (baobab tree, or gorakh chinch) and Kigelia (sausage tree).

Memory Aid

names of pollinating agents (The suffix -phily means “love of”. Link each to its agent: Anemophily = wind, Hydrophily = water, Entomophily = insects, Ornithophily = birds, Chiropterophily = bats. Recall the animals with “Insects Bring Bats” for the three biotic ones.)

1.7 Outbreeding Devices (Contrivances)

In the last section you saw that cross-pollination (xenogamy) is what brings genetic variation. Many plants take this further and actively discourage or prevent self-pollination, so as to force cross-pollination and boost genetic diversity. This diversity matters because it is the raw material for evolution by natural selection.

The reason plants avoid continual selfing is that it leads to inbreeding depression — a gradual loss of vigour and fertility over generations. To escape this, plants have evolved a wide variety of sexual strategies, together called outbreeding devices or contrivances.

Definition

Outbreeding devices are structural, timing or genetic mechanisms in flowers that discourage or prevent self-pollination and thereby promote cross-pollination.

The main outbreeding devices are summarised in FC 1.8.

Flowchart

FC 1.8 – Outbreeding devices in plants: Outbreeding devices → Unisexuality (monoecious / dioecious) · Dichogamy (protandry / protogyny) · Prepotency · Heterostyly · Herkogamy · Self-incompatibility

Common Outbreeding Devices

Unisexuality. Here the plant bears either male or female flowers, a condition also called dioecism. Because a unisexual flower has only one sex, self-pollination within a flower is impossible. A plant may be monoecious, carrying both male and female flowers on the same plant (as in maize), or dioecious, carrying male and female flowers on separate plants (as in mulberry and papaya).

Compare Table

Dichogamy. In this device the anthers and stigmas of a bisexual flower mature at different times, so the flower’s own pollen is never ready at the same moment as its own stigma. Dichogamy is of two types. In protandry, the androecium (anthers) matures earlier than the gynoecium, as in the disc florets of sunflower. In protogyny, the gynoecium (stigma) matures earlier than the androecium, as in Gloriosa.

Do Not Mix Up

Protandry vs Protogyny (Read the ending: protandry → the androecium (male) matures first; protogyny → the gynoecium (female) matures first. Both are forms of dichogamy that stagger the timing to block selfing.)

Prepotency. In some plants the pollen grains of other flowers germinate faster on the stigma than the flower’s own pollen, so the foreign pollen wins the race to fertilize, as in apple. This gives cross-pollen a competitive advantage.

Heterostyly (heteromorphy). In plants such as Primula (primrose), there are two or three forms of flower in which the stigmas and anthers are placed at different levels (heterostyly and heteroanthy). Because pollen shed from an anther at one level is best deposited on a stigma at the same level — and that matching stigma belongs to a different flower form — self-pollination within a single flower is discouraged and cross-pollination between forms is favoured.

Herkogamy. This is a mechanical device in a bisexual flower, where a natural physical barrier separates the two sex organs and prevents the flower’s own pollen from touching its own stigma. In Calotropis, for example, the pentangular stigma is positioned above the level of the anthers (whose pollen is packed as pollinia).

Structure and Function

herkogamy in Calotropis (The stigma sits physically raised above the anthers → the flower’s own pollinia cannot easily reach up to its own stigma → self-pollination is mechanically blocked, so pollen carried in by an insect from another flower is favoured. A simple change of position acts as a barrier.)

Self-incompatibility (self-sterility). This is a genetic mechanism that prevents a flower’s own pollen from germinating on its own stigma, so fertilization by self-pollen is inhibited, as in tobacco and Thea. Unlike the earlier devices, this one works at the biochemical level rather than through structure or timing.

Real-World Application

Plant breeders make deliberate use of these principles. In breeding programmes for crops such as wheat and rice, the plants are hand-pollinated so that pollen is transferred only between selected varieties, ensuring controlled cross-pollination and the desired combination of traits in the offspring.

1.8 Pollen–Pistil Interaction

Once a pollen grain lands on a stigma, a whole conversation begins between the pollen and the pistil. This is the pollen–pistil interaction — the interaction of pollen grains with the sporophytic tissue of the stigma. It begins with pollination and ends with fertilization, covering every event from the deposition of the pollen grain on the stigma to the entry of the pollen tube into the ovule (specifically, into a synergid).

Pollination alone does not guarantee that the right pollen has arrived — very often the wrong type of pollen, from another species, also lands on the stigma. The pistil, however, has the ability to recognise and accept only the compatible pollen of its own species, and to discard the wrong type. This recognition is controlled by special proteins, and the process amounts to pollen recognition followed by either the promotion or the inhibition of that pollen.

Definition

Pollen–pistil interaction is the sequence of events, controlled by recognition proteins, from the landing of pollen on the stigma to the entry of the pollen tube into the ovule, by which the pistil accepts compatible pollen and rejects incompatible pollen.

From Stigma to Embryo Sac

The stigmatic surface refuses wrong or incompatible pollen. A physiological mechanism ensures that only intraspecific (same-species) pollen germinates successfully. A compatible pollen grain absorbs water and nutrients from the stigma, germinates, and pushes out a pollen tube. The growth of this tube down through the style is guided by specific chemicals.

Importantly, the stigmatic surface supplies the essential requirements for germination — water, nutrients and chemical cues — that the pollen grain itself lacks. Finally the pollen tube is driven through the ovule and reaches the embryo sac. Its tip enters one of the synergids and then bursts, releasing its contents (the two male gametes) close to the egg. The full sequence, including the accept/reject decision, is shown in FC 1.9.

Flowchart

FC 1.9 – Pollen–pistil interaction: Pollen lands on stigma → recognition by proteins → if incompatible → pollen inhibited and rejected; if compatible → pollen absorbs water and nutrients → germinates → pollen tube grows down the style (guided by chemicals) → enters ovule → enters one synergid → tip ruptures → releases two male gametes near the egg

Trace It

a compatible pollen grain from stigma to egg 1. The pollen grain lands on the stigma and its recognition proteins match — it is accepted. 2. It absorbs water and nutrients from the moist stigmatic surface and germinates. 3. A pollen tube emerges through a germ-pore and grows down through the style, steered by chemical signals. 4. The tube reaches the ovule and is guided by the filiform apparatus of the synergids. 5. The tube tip enters one synergid and ruptures, releasing the two male gametes right beside the egg and the central cell, ready for fertilization.

Because many compatible pollen grains germinate together, an intense competition develops among them, and only the pollen tubes that grow fastest reach the ovules to bring about fertilization.

Structure and Function

the stigmatic surface (The stigma is moist and rich in nutrients and recognition chemicals → it supplies exactly the water, food and signals a pollen grain needs to germinate (things the pollen itself does not carry) → only pollen the stigma accepts can start growing here. The stigma is both a landing pad and a screening gate.)

In Vitro Pollen Germination

Pollen–pistil interaction is central to sexual reproduction and seed formation, but pollen can also be made to germinate outside the plant, in a synthetic medium (in vitro). Adding sucrose to the medium induces pollen germination and pollen-tube growth, and adding boric acid further helps and speeds up germination. Such artificial germination lets scientists study pollen without waiting for it to land on a real stigma.

Artificial Hybridization

Artificial hybridization is one of the major approaches used in crop improvement. The aim is to cross two selected parents so that only the desired pollen fertilizes the chosen female parent. To achieve this, only the desired pollen grains are hand-pollinated onto the stigma, and this is accomplished through two procedures: emasculation and bagging.

Real-World Application

In a plant-breeding programme, if the female parent has bisexual flowers, the breeder first performs emasculation — removing the anthers from the flower bud before they mature, so the flower cannot pollinate itself. The emasculated flower is then covered with a bag (bagging) to keep out unwanted pollen from the air or insects. When the stigma is receptive, the breeder dusts it with pollen collected from the chosen male parent and bags it again. This careful control is how new, improved varieties of crops are created.

1.9 Double Fertilization

Double fertilization is a complex fertilization mechanism found in flowering (angiospermic) plants. In it, both male gametes of a single pollen grain are used — one to fertilize the egg, the other to fuse with the secondary nucleus. No other group of plants does this.

Note that the discovery story of double fertilization, in Lilium and Fritillaria, was told in the chapter opener.

Definition

Double fertilization is the process, unique to angiosperms, in which one male gamete fuses with the egg (syngamy) to form a diploid zygote, while the second male gamete fuses with the diploid secondary nucleus (triple fusion) to form the triploid primary endosperm nucleus.

Entry of the Pollen Tube into the Ovule

You have already seen the pollen tube grow down the style. Once it reaches the ovary chamber, it enters the ovule. Its growth here is guided by chemicals secreted by the synergids.

The tube usually enters the ovule through the micropyle, and this route is called porogamy. In some cases, however, it enters through the chalaza — this is chalazogamy. In still other plants it forces its way in by piercing the integuments, which is mesogamy. All three routes are compared in Fig. 1.12. Whichever route it takes, the tube finally penetrates the embryo sac at its micropylar end.

Diagram

Fig. 1.12 – Three routes of entry of the pollen tube into the ovule: (A) porogamy through the micropyle, (B) chalazogamy through the chalaza, (C) mesogamy through the integuments; label pollen tube, micropyle, chalaza, integuments, embryo sac, egg cell, synergid, polar nuclei, antipodals, nucellus, funicle.

Memory Aid

three routes of pollen tube entry (Name the doorway: Porogamy → through the pore (micropyle); Chalazogamy → through the chalaza; Mesogamy → through the middle, piercing the integuments.)

Release of the Male Gametes

The pollen tube, carrying the male gametes, penetrates one of the synergids. The watery contents of that synergid are absorbed by the pollen tube, which then ruptures and releases its contents — including the two non-motile male gametes.

Notice the fundamental point here: the male gametes cannot swim. They are carried passively through the hollow pollen tube all the way to the egg. This delivery of non-motile gametes by means of a pollen tube is called siphonogamy, and it is what ensures that fertilization can happen at all in a land plant, without any water for the gametes to swim through.

Structure and Function

the pollen tube (The pollen tube is a long, hollow, growing outgrowth of the pollen grain → it forms a continuous protected channel from the stigma to the embryo sac → the two non-motile male gametes are carried down it and delivered right beside the egg. A pipeline that removes the need for gametes to swim.)

Syngamy and Triple Fusion

Two separate fusion events now take place in the embryo sac. Together they make up sexual reproduction in angiospermic flowering plants.

Syngamy is the fusion of one haploid male gamete with the haploid female gamete (the egg) to produce a diploid zygote.

Triple fusion is the fusion of the second haploid male gamete with the diploid secondary nucleus, producing the primary endosperm nucleus (PEN), which is triploid. It is called triple fusion because three nuclei are involved — one male gamete and the two polar nuclei that made up the secondary nucleus.

The triploid PEN develops into the triploid endosperm, while the zygote develops into an embryo. Syngamy is described as a type of generative fertilization, whereas triple fusion is a type of vegetative fertilization. The events are shown together in Fig. 1.13.

Diagram

Fig. 1.13 – Double fertilization: a pollen grain on the stigma with its pollen tube growing down the style into the ovule, showing syngamy (egg + male gamete) and triple fusion (secondary nucleus + male gamete); label stigma, style, ovary, ovule, pollen grain, pollen tube, synergids, micropyle, syngamy and triple fusion.

Because both male gametes take part — one in syngamy and the other in triple fusion — the whole process is described as double fertilization.

Compare Table

Trace It

the two male gametes of one pollen grain 1. The pollen tube reaches the embryo sac and enters a synergid. 2. The tube tip ruptures and releases both non-motile male gametes. 3. Male gamete 1 moves to the egg and fuses with it — syngamy — giving a diploid zygote (2n). 4. Male gamete 2 moves to the central cell and fuses with the diploid secondary nucleus — triple fusion — giving the triploid PEN (3n). 5. The zygote grows into the embryo; the PEN divides to form the nutritive endosperm that feeds it.

Flowchart

FC 1.10 – Double fertilization: Pollen tube enters synergid → ruptures → releases 2 male gametes → branch 1: male gamete + egg (n + n) → syngamy → zygote (2n) → embryo; branch 2: male gamete + secondary nucleus (n + 2n) → triple fusion → primary endosperm nucleus, PEN (3n) → endosperm

Significance of Double Fertilization

Double fertilization matters for several reasons. It is a unique feature of angiosperms, and it ensures that the parent plant invests a seed with a food store only if the egg has actually been fertilized — no wasted food on an unfertilized ovule. The diploid zygote develops into an embryo which then grows into a new plant, and the triploid PEN develops into the nutritive endosperm tissue that feeds that embryo. Double fertilization also restores the diploid condition, through the fusion of the haploid male gamete with the haploid female gamete in syngamy. Finally, it helps to avoid polyembryony (the formation of more than one embryo in a seed).

Curiosity Corner

Q1. Why do flowering plants use two male gametes for one act of fertilization, when a single one seems enough?

A. Because the second gamete is not spent on the embryo at all — it fuses with the secondary nucleus in triple fusion to create the triploid primary endosperm nucleus, which grows into the endosperm, the embryo’s food store. This is the elegance of double fertilization: the plant builds a food supply only after the egg has been successfully fertilized, so no energy is wasted stocking an ovule that would never have become a seed.

1.10 Development of Endosperm

With fertilization complete, the post-fertilization events begin. The first of these is the building of the embryo’s food store.

The triploid primary endosperm nucleus (PEN), formed by triple fusion, now divides repeatedly by mitosis to form a nutritive tissue called the endosperm. Inside the ovule, the embryo and the endosperm are seen to develop side by side, at the same time — a sensible arrangement, since the food store must be ready as the embryo grows.

Definition

Endosperm is the nutritive tissue formed by the repeated mitotic divisions of the triploid primary endosperm nucleus (PEN); it nourishes the developing embryo.

Meanwhile the other cells of the embryo sac — the synergids and the antipodals, which have served their purpose — become disorganized sooner or later. It is the formation of the triploid endosperm nucleus that triggers the wave of cell division leading to endosperm formation.

Types of Endosperm

On the basis of the mode of development — essentially, whether a cell wall is laid down after each nuclear division or not — there are three types of endosperm: nuclear type, cellular type and helobial type. All three are shown in Fig. 1.14.

Diagram

Fig. 1.14 – Types of endosperm: (i) Nuclear, (ii) Cellular and (iii) Helobial, each shown as a longitudinal view of the developing seed; label zygote/oospore, embryo and endosperm in each.

Flowchart

FC 1.11 – Types of endosperm by mode of development: Primary endosperm nucleus (3n) → if free nuclear divisions with no wall formation → Nuclear type (e.g. wheat, sunflower, coconut); if wall formation immediately after every division → Cellular type (e.g. Balsam, Petunia, Adoxa); if first division followed by a transverse wall, then free nuclear divisions in each chamber → Helobial type (e.g. Asphodelus)

Nuclear Type

This is the most common type, found in 161 angiospermic families. Here the primary endosperm nucleus divides mitotically again and again without any wall formation, producing a large number of free nuclei. A big central vacuole then appears in the cell, pushing all these nuclei out towards the periphery.

Later, walls do develop between the nuclei, so a multicellular endosperm is formed. In several cases, however, this wall formation stays incomplete — as in wheat, sunflower and coconut. Coconut is a good illustration: it has multicellular endosperm in the outer part, but free nuclear as well as vacuolated endosperm in the centre.

Real-World Application

Cut open a tender coconut and you meet nuclear endosperm directly. The coconut water is the free nuclear, vacuolated endosperm — nuclei suspended in liquid, with no walls between them. The soft white kernel you scoop out is the same endosperm after walls have formed around the nuclei, making it cellular. So the “water” and the “malai” of a coconut are two stages of one tissue.

Side Note

What is the kernel that you eat in a tender coconut? It is endosperm — the cellular, wall-formed part of the same nuclear endosperm whose liquid portion you drink as coconut water.

Cellular Type

In some plants, each division of the triploid primary endospermic nucleus is immediately followed by wall formation. As a result the endosperm is cellular right from the very beginning — there is never a free nuclear stage. This type is mostly seen in 72 families of dicots, in members such as Balsam, Petunia and Adoxa.

Helobial Type

This type occurs in the order Helobiales of the monocotyledons. Here the first division of the primary endosperm nucleus is followed by a transverse wall, which divides the cell unequally into two chambers. The smaller cell is the chalazal cell and the larger one is the micropylar cell.

The nuclei in each chamber then divide by free nuclear divisions, and afterwards walls develop between the nuclei in the micropylar chamber. Because it starts with a wall (like the cellular type) but then goes through a free nuclear phase (like the nuclear type), the helobial type is intermediate between the cellular and nuclear types, as in Asphodelus.

Compare Table

Do Not Mix Up

Nuclear vs Cellular endosperm (The single test is wall formation. In the nuclear type the nucleus divides many times with no walls, giving free nuclei. In the cellular type a wall follows every division, so the tissue is cellular from the very first division. The helobial type simply mixes the two.)

Side Note

Mosaic endosperm is endosperm that contains tissues of two different types. In plants like corn, the endosperm carries patches of two different colours, so that it forms a sort of mosaic pattern.

1.11 Development of Embryo

The process by which the zygote develops into an embryo is called embryogenesis. The embryo develops at the micropylar end of the embryo sac — the same end at which the pollen tube delivered the male gametes.

Definition

Embryogenesis is the process of development of the zygote into an embryo.

Note the timing carefully: the growth of the embryo is triggered only after a certain amount of endosperm has already been formed. This is the food-store-first arrangement you met in the previous section. Once fertilization is over and some endosperm is in place, embryonic development begins.

Development of the Dicot Embryo

The zygote first divides to form a two-celled proembryo. These two cells are not equal in size or in fate. The larger cell, lying towards the micropyle, is called the basal or suspensor initial cell. The smaller cell, lying towards the chalaza, is the terminal or embryonal initial cell (Fig. 1.15).

Diagram

Fig. 1.15 – Development of the dicot embryo as in Capsella: stages from the oospore and two-celled proembryo through the octant and heart-shaped stages to the mature horse-shoe-shaped embryo; label embryonal initial, suspensor initial, suspensor, hypophysis, embryonal mass, endosperm, antipodal tissue, cotyledons, hypocotyl, radicle, root-cap and plumule.

The suspensor initial cell divides transversely, in one plane only, to produce a filamentous suspensor of 6–10 cells. The first cell of this suspensor, at the micropylar end, becomes swollen and works as a haustorium — an absorbing organ. The lowermost cell of the suspensor is called the hypophysis. The suspensor as a whole has a clear job: it pushes the developing embryo deeper into the endosperm, right into its food supply.

Structure and Function

the suspensor (The suspensor is a file of 6–10 cells with a swollen haustorial cell at its tip → the haustorium absorbs nutrients while the file elongates → the embryo is pushed down into the nutritive endosperm. It acts as both an anchor-and-pusher and a feeding tube for the young embryo.)

Meanwhile the embryonal initial cell undergoes three successive mitotic divisions to form an octant — a group of eight cells. The planes of these divisions are at right angles to each other, which is what gives a neat cube-like block of eight.

The octant then divides its work between two tiers. The lower tier of four cells gives rise to the hypocotyl and the radicle (the future root), while the upper tier of four cells forms the plumule (the future shoot) and the one or two cotyledons (seed leaves). The hypophysis, by further division, gives rise to part of the radicle and to the root cap.

Flowchart

FC 1.12 – Development of the dicot embryo: Zygote → two-celled proembryo → basal/suspensor initial cell (towards micropyle) → suspensor of 6–10 cells (first cell = haustorium; lowermost cell = hypophysis → part of radicle + root cap); terminal/embryonal initial cell (towards chalaza) → three mitotic divisions → octant (8 cells) → lower tier of 4 cells → hypocotyl + radicle; upper tier of 4 cells → plumule + cotyledons → heart-shaped embryo → horse-shoe-shaped mature embryo

Subsequently, the cells in the upper tier of the octant divide in several planes so that the embryo becomes heart shaped, and this stage then forms the two lateral cotyledons with a terminal plumule between them. Further enlargement of the hypocotyl and the cotyledons makes the embryo curve, so that it finally appears horse-shoe shaped.

Development of the Monocot Embryo

Embryo development is similar in both dicots and monocots up to the octant stage. The difference appears only afterwards (Fig. 1.16).

Diagram

Fig. 1.16 – Development of the monocot (grass) embryo: stages from oospore and suspensor initial through to the mature embryo; label embryonal initial, suspensor initial, suspensor, plumule, radicle, cotyledon, scutellum, coleoptile, coleorhiza, shoot apex, epiblast and root cap.

In the monocot embryo, the single cotyledon occupies a terminal position, and the plumule is pushed to one side, into a lateral position. This single, shield-shaped cotyledon has a special name: the scutellum.

The monocot embryo also has two protective sheaths that dicots lack. The protective sheath covering the plumule is the coleoptile, and the one covering the radicle is the coleorhiza.

Compare Table

Structure and Function

coleoptile of the monocot embryo (The coleoptile is a tough, hollow, pointed sheath enclosing the plumule → it takes the friction as the seedling forces its way up through the soil → the delicate shoot tip inside is protected until it reaches the light, when the shoot pierces through. A hard hat for the emerging shoot.)

Real-World Application

The next time you see wheat or maize seedlings pushing up in a field, look at the pale, pointed spike that first breaks the soil — that is the coleoptile, not the leaf. The green leaves emerge only after the coleoptile has cleared the surface, having shielded them through the abrasive soil.

Finally, with the embryo formed, the last transformation of the whole reproductive process takes place: the ovule is transformed into the seed, and the ovary into the fruit.

1.12 Seed and Fruit Development

The goal of reproduction, in every living organism including plants, is to create offspring for the next generation. One of the chief ways a plant does this is by forming seeds. But seeds and fruits do not appear on their own — the flowers must first be pollinated, for only then can fertilization, and therefore seed and fruit formation, take place.

Seed development is initiated by fertilization. The integuments of the fertilized ovule persist and are transformed into the seed coat of the mature seed. The whole set of changes is traced in FC 1.13.

Flowchart

FC 1.13 – Fate of floral parts after fertilization: Ovule → Seed; Integuments → Seed coat (outer testa + inner tegmen); Zygote → Embryo; Primary endosperm nucleus → Endosperm; Nucellus (if it persists) → Perisperm; Ovary → Fruit; Ovary wall → Pericarp

Structure and Coverings of the Seed

The seed coat sometimes consists of two distinct coverings: a typical outer seed coat, the testa, and an inner, thin, membranous tegmen.

The nucellus, which you met inside the ovule, does not always vanish. In some genera, such as black pepper and beet, it persists as a thin, papery layer called the perisperm.

Do Not Mix Up

Endosperm vs Perisperm (Endosperm is the nutritive tissue formed after fertilization, from the triploid PEN. Perisperm is the leftover nucellus — unused ovule tissue that persists as a thin papery layer, as in black pepper and beet. Different origin entirely: one is a product of triple fusion, the other is a remnant of the mother plant’s tissue.)

Endospermic and Non-Endospermic Seeds

Seeds differ in how much of the endosperm survives to maturity.

In some seeds, the food reserves in the endosperm are only partially used up during the development of the embryo. In such seeds the endosperm remains conspicuous and fills a greater part of the seed. The resulting seed is called endospermic or albuminous, as in castor, coconut and maize (Fig. 1.18).

In other seeds, the embryo absorbs the food reserve from the endosperm completely during its developmental stages. The endosperm therefore disappears (disorganizes) in the mature seed, and the resulting seed is called non-endospermic or ex-albuminous, as in pea and bean (Fig. 1.17).

Diagram

Fig. 1.17 – Bean seed (dicot), a non-endospermic seed cut open; label seed coat, cotyledon, embryo, plumule, hypocotyl and radicle.

Diagram

Fig. 1.18 – Vertical section of a maize grain (monocot), an endospermic seed; label seed coat and fruit-wall, aleurone layer, endosperm, scutellum, coleoptile, plumule, radicle and coleorhiza.

Compare Table

In some non-endospermic seeds the cotyledons act as the food store, while in others they become the first photosynthetic organs of the seedling. Meanwhile the micropyle persists as a small pore in the seed coat, and it allows the entry of water and oxygen when the seed is soaked.

Structure and Function

the micropyle in a mature seed (The micropyle survives as a tiny pore in the otherwise tough seed coat → water and oxygen enter through it when the seed is soaked → the seed imbibes water, the embryo revives its metabolism, and germination can begin. The pollen tube’s old doorway becomes the seedling’s water inlet.)

Real-World Application

This is why cooks soak pulses and beans overnight before cooking. Water enters mainly through the micropyle, the seed swells, and the tissues soften. It is also why farmers soak seeds before sowing — imbibition through the micropyle is the first step towards germination.

Fruit Development

Fruit development is triggered by hormones produced by the developing seeds — a neat link, because it means a fruit normally grows only where seeds are actually forming.

As mentioned earlier, after fertilization the zygote is formed, and the ovary begins to differentiate into the fruit. The wall of the ovary develops into the pericarp. The pericarp is basically three-layered, and these layers become distinct in a fleshy fruit like mango or coconut.

Zoom In / Out

from ovule to fruit (Zoom in: inside the fruit lie the seeds, each formed from an ovule; inside each seed is the embryo, made of radicle, plumule and cotyledons. Zoom out: the fruit itself was the ovary, one part of a carpel of a single flower on the parent plant.)

Significance of Seed and Fruit Formation

Seeds and fruits are not decorative extras — each has a definite role. Fruits provide nourishment to the developing seeds, and they protect the seeds while they are still immature. Seeds themselves serve as the important propagating organs (units) of the plant. And both seeds and fruits develop special devices for their dispersal, thereby helping in the distribution of the species to new places.

Try Yourself

Help to rebuild a natural ecosystem. Mix seeds and potting soil together with dry clay. Mould the mixture with water into small balls and allow them to dry in the sun. Throw these seed balls at places suitable for germination.

Seed Dormancy

Even a perfectly formed, mature and viable seed does not always sprout at once. Dormancy is a temporary state of metabolic arrest that helps organisms survive adverse environmental conditions. Any structural or physiological adaptive mechanism for such survival is called dormancy.

Mature and viable seeds will not germinate even when favourable conditions are present, and during this dormant period they are dispersed to different places. Viable seeds germinate only after the dormancy period is complete.

Definition

Dormancy is a temporary state of metabolic arrest in which a mature, viable seed does not germinate even under favourable conditions, and which helps the seed survive adverse environmental conditions.

Side Note

How long do seeds stay viable, and can old seeds still grow? Some remarkably old mature seeds have indeed grown into viable plants — Lupinus arcticus has germinated after about 10,000 years, and Phoenix dactylifera after about 2000 years. At the other extreme, some seeds are short-lived, as in Citrus. Some tiny seeds are easy to disperse, as in Striga, orchids and Orobancha.

1.13 Apomixis

You have now followed the full sexual route — pollination, fertilization, seed and fruit. But some plants take a shortcut. Apomixis is the phenomenon of formation of embryo(s) by an asexual method of reproduction, without the formation of gametes and without the act of fertilization.

Another way to describe it is as an unusual form of sexual reproduction in which there is no meiosis and no syngamy. Yet the outward result looks perfectly normal: the embryo develops in the ovule, and the ovule develops into a seed. A seed is produced — but it carries an embryo made without any fusion of gametes.

Definition

Apomixis is the formation of an embryo, and hence a seed, by an asexual method — without gamete formation, meiosis or fertilization.

Curiosity Corner

Q5. Some plants produce seeds without any fertilization taking place at all — how do they manage it?

A. Through apomixis. Instead of an egg being fertilized by a male gamete, an embryo is formed directly from a cell of the ovule — from the egg without fertilization (parthenogenesis), from another gametophyte cell (apogamy), or from diploid nucellus or integument tissue. Because meiosis and syngamy are skipped, the seed still forms and looks ordinary, but the embryo inside is genetically identical to the mother plant.

Two Basic Terms

Two related terms describe which cell gives rise to the embryo-like structure.

Apogamy is the term used when a gametophyte organ or cell produces an embryo-like structure without fertilization.

Apospory is the term used when a diploid sporophyte cell produces a diploid gametophyte without undergoing meiosis, as in orange and mango. Since meiosis is skipped, the gametophyte formed is unreduced — that is, diploid rather than haploid.

Do Not Mix Up

Apogamy vs Apospory (Apogamy = an embryo forms from a gametophyte cell, skipping fertilization. Apospory = a gametophyte forms from a sporophyte cell, skipping meiosis. One skips fusion; the other skips reduction division.)

Main Categories of Apomixis

The main categories of apomixis are recurrent apomixis, non-recurrent apomixis and adventive embryony, summarised in FC 1.14.

Flowchart

FC 1.14 – Categories of apomixis: Apomixis → Recurrent apomixis (embryo sac is unreduced/diploid; via diplospory from the diploid MMC, e.g. Taraxacum, or via apospory from nucellar cells) · Non-recurrent apomixis (MMC undergoes normal meiosis → haploid embryo sac → embryo from egg by parthenogenesis or from another haploid cell by apogamy, e.g. Nicotiana; plants generally sterile) · Adventive embryony (embryo from somatic nucellus or integuments alongside the normal zygotic embryo, e.g. mango, orange, lemon → polyembryony)

Recurrent Apomixis

In this type, the embryo sac generally arises either from an archesporial cell or from some other part of the nucellus — and crucially, without the reduction of chromosome number.

In diplospory, the unreduced embryo sac is derived directly from the diploid megaspore mother cell, as in Taraxacum. In apospory, the nucellar cells give rise to the apomictic embryo sac.

Because the embryo sac is unreduced, the embryo it forms is diploid and genetically identical to the mother plant, generation after generation — which is why it is called recurrent.

Non-Recurrent Apomixis

Here the megaspore mother cell undergoes its usual meiotic division, so a haploid embryo sac is formed as normal. What is unusual is what happens next: the embryo arises either from the egg without fertilization, by parthenogenesis, or from some other haploid cell of the gametophyte, through apogamy.

Because the embryo comes from a haploid cell, plants produced by this method are generally sterile and do not reproduce sexually, as in Nicotiana.

Compare Table

Adventive Embryony

In this type, embryos may develop from the somatic (body) tissues of the ovule — from the nucellus or the integuments — along with the normal zygotic embryo. It is common in mango, orange and lemon. Because more than one embryo is now present in the same seed, this gives rise to a condition called polyembryony, which is taken up in a later section.

Why Apomixis Matters

Genetically identical plants can be produced effectively and rapidly by apomixis. This gives it real practical value: it combines the convenience of a seed (easy to store, transport and sow) with the genetic uniformity of a clone.

Real-World Application

Hybrid crop varieties give excellent yields, but their advantage is lost in the next generation, because the seed from a hybrid segregates and the offspring are no longer uniform. Farmers must therefore buy fresh hybrid seed every season. If apomixis could be introduced into such hybrids, each plant would produce seeds carrying the hybrid genotype unchanged, and a farmer could keep and re-sow her own seed year after year without losing the hybrid vigour. This is an active goal of plant-breeding research.

Side Note

The seed mother Rahibai Soma Popere, a farmer from Maharashtra, has become widely known for conserving native seed varieties. Find out how she has saved over 80 varieties of indigenous seeds, and why preserving such traditional varieties matters for crop diversity and food security.

1.14 Parthenocarpy

Apomixis, which you have just studied, produces a seed without fertilization. Parthenocarpy does something related but distinct: it produces a fruit without fertilization. The term was coined by Noll (1902).

Definition

Parthenocarpy is the condition in which a fruit develops without the process of fertilization; the fruit resembles a normally produced fruit but is seedless.

Parthenocarpy occurs naturally in some varieties of pineapple, banana and papaya.

How a Fruit Grows Without Fertilization

Recall from Section 1.12 that fruit development is normally triggered by hormones produced by the developing seeds. In a parthenocarpic plant there are no developing seeds — so where does the hormonal signal come from?

It seems that in these plants the placental tissue in the unfertilized ovary itself produces the auxin IAA (Indole-3 Acetic Acid), and it is this auxin that is responsible for the enlargement of the ovary into a fruit. In other words, the plant supplies the growth signal directly, instead of waiting for seeds to supply it.

The fruit that results resembles a normally produced fruit in every visible way, but it is seedless.

Flowchart

FC 1.15 – Normal fruit versus parthenocarpic fruit: Normal route: pollination → fertilization → seeds develop → seeds release hormones → ovary enlarges → fruit with seeds. Parthenocarpic route: no fertilization → placental tissue of unfertilized ovary produces auxin (IAA) → ovary enlarges → seedless fruit

Do Not Mix Up

Parthenogenesis vs Parthenocarpy (Parthenogenesis = development of an embryo directly from an egg cell without fertilization (a kind of apogamy). Parthenocarpy = development of a fruit without fertilization, giving a seedless fruit. One concerns the embryo, the other the fruit.)

Curiosity Corner

Q2. A banana has no seeds inside it, yet it still grows into a full fruit — how is that possible?

A. The banana is a parthenocarpic fruit. Its ovary develops into a fruit without fertilization, so no seeds are ever formed. The placental tissue of the unfertilized ovary produces the auxin IAA, which by itself makes the ovary enlarge into the familiar fleshy fruit. The fruit therefore looks and tastes normal, but it is seedless — which is exactly why edible bananas must be propagated vegetatively rather than from seed.

Real-World Application

Seedless fruits are commercially valuable because consumers prefer them and processing is easier. Besides the naturally parthenocarpic pineapple, banana and papaya, growers can induce parthenocarpy artificially. This is done by spraying gibberellins, by delaying pollination, or by using foreign pollen — treatments that trick the ovary into enlarging without producing seeds. Seedless grapes, tomatoes and brinjals are grown using such approaches.

Side Note

Three related terms are worth keeping straight. Parthenogenesis is the development of an embryo directly from an egg cell or a male gamete, and it is a kind of apogamy. Agamospermy is the condition in which plants produce seeds, but the embryo inside is produced without meiosis and syngamy. Parthenocarpy can be induced artificially — by spraying gibberellins, delaying pollination, or using foreign pollens.

Exam Corner

Q. What do bananas and figs have in common?

A. Both are fruits that you eat without meeting a hard, mature seed inside. The cultivated banana is parthenocarpic, so it forms no seeds at all. In the common edible fig, the little gritty specks are not true seeds developed in the usual way either — the fig is an unusual fruit whose flowers line the inside of a hollow receptacle, and cultivated varieties can set fruit without fertilization. In both, we enjoy a fruit that grows without producing the seeds a fruit is normally “meant” to protect.

1.15 Polyembryony

You met polyembryony briefly at the end of the section on apomixis, where adventive embryony gave rise to it. Here we look at it properly.

Polyembryony is the development of more than one embryo inside a single seed. Since each embryo can grow into a plant, this results in the emergence of multiple seedlings from one seed.

Definition

Polyembryony is the occurrence of more than one embryo inside a single seed.

It was first noticed by Leeuwenhoek (1719) in the seeds of the Citrus genus.

Real Incident / Discovery

Antonie van Leeuwenhoek, the Dutch pioneer of microscopy, was in the habit of examining ordinary things very closely — and in 1719 he turned that habit on citrus seeds. Where he expected a single embryo in each seed, he found more than one. It was the first recorded observation of polyembryony, and it came, characteristically, not from a grand experiment but from someone taking a careful second look at something everybody had assumed they already understood.

Curiosity Corner

Q3. How can a single orange seed sometimes give rise to several seedlings instead of one?

A. Because Citrus seeds show polyembryony — more than one embryo forms inside a single seed. In Citrus this happens by adventive polyembryony: besides the normal zygotic embryo formed by fertilization, additional embryos develop directly from the diploid cells of the nucellus and the integuments of the ovule. When such a seed germinates, each embryo grows out, so one seed yields several seedlings.

Where the Extra Embryos Come From

The additional embryos result from the differentiation and development of various maternal and zygotic tissues associated with the ovule of the seed. In other words, some extra embryos come from the mother plant’s own tissue, and some from the zygote itself.

Polyembryony may be true or false, depending on whether the many embryos arise in the same embryo sac or in different embryo sacs within the same ovule.

Do Not Mix Up

True vs False polyembryony (The test is how many embryo sacs. True polyembryony: the several embryos arise within the same embryo sac. False polyembryony: they arise in different embryo sacs of the same ovule.)

Types of Polyembryony

Two important types are distinguished by the tissue that produces the extra embryos, as shown in FC 1.16.

Flowchart

FC 1.16 – Types of polyembryony: Polyembryony → Adventive polyembryony (embryo develops directly from the diploid cells of the nucellus and integuments, e.g. Citrus) · Cleavage polyembryony (the zygote/proembryo divides or cleaves into many units → each unit develops into an embryo)

In adventive polyembryony, an embryo develops directly from the diploid cells of the nucellus and the integuments, as in Citrus. Such an embryo is not a product of fertilization at all, so it is genetically identical to the mother plant.

In cleavage polyembryony, the zygote proembryo sometimes divides (cleaves) into many parts or units, and each unit then develops into a separate embryo. Here all the embryos come from the same zygote, so they are genetically identical to one another.

Significance of Polyembryony

Polyembryony has a clear survival value: with several embryos in one seed, the chances of survival of the new plants are increased, because even if one embryo fails, another may establish itself.

Nucellar adventive polyembryony is of great significance in horticulture. Because nucellar embryos come from the mother’s own tissue and not from fertilization, the seedlings raised from them are genetically uniform, parental-type plants.

Real-World Application

Citrus growers exploit exactly this. In a polyembryonic orange or lemon seed, most of the seedlings are nucellar, so they are true-to-type clones of the parent tree — carrying the same fruit quality — and they are also free of many virus infections that would otherwise pass through vegetative cuttings. This gives nurseries a way to raise large numbers of uniform, healthy rootstocks and plants from seed.

Side Note

Genetically uniform, parental-type seedlings are obtained from nucellar embryos — which is why nucellar polyembryony is so useful for maintaining a chosen variety unchanged.

Compare Table

Try Yourself

Prepare a chart of natural vegetative propagation in flowering plants, showing the organ (root, stem or leaf), the specific part involved, and the name of the plant for each example.

1.16 Summary

Reproduction produces young ones like the parents; it continues the species and maintains the continuity of life. It is of two types — asexual and sexual.
Asexual reproduction involves no fusion of gametes. A single parent gives rise to genetically and morphologically identical offspring called clones.
Modes of asexual reproduction include fragmentation (Spirogyra), budding (Protosiphon, yeast), spore formation by motile zoospores (Chlamydomonas), binary fission (Chlorella, diatoms, Chlamydomonas), conidia formation (Penicillium) and gemma formation (Marchantia).
Vegetative reproduction uses vegetative parts of the plant, so the new plants are genetically identical to the parent. Natural examples occur through root (sweet potato, Asparagus, Dahlia), leaf (Bryophyllum, Kalanchoe, Begonia) and stem (rhizome of turmeric, tuber of potato, bulb of onion).
Artificial methods used in agriculture and horticulture are cutting (rose, Bougainvillea, Sansvieria, blackberry), grafting (scion joined to stock; bud grafting in apple, pear, rose) and tissue culture / micropropagation.
Sexual reproduction involves fusion of two compatible haploid gametes. Its two major events are meiosis and the fusion of gametes, forming a diploid zygote and producing genetically variable offspring — the raw material for survival and evolution. Its sequence has three stages: pre-fertilization, fertilization and post-fertilization.
The flower is the reproductive structure. Its four whorls are calyx, corolla, androecium and gynoecium. In angiosperms the diploid sporophyte is dominant and the gametophytes are much reduced, developing within the flower.
The androecium is made of stamens (filament, connective, anther). The anther is dithecous and tetrasporangiate (four pollen sacs). Its wall has four layers — epidermis, endothecium, middle layer and tapetum — the tapetum being the innermost nutritive layer.
Microsporogenesis: each diploid microspore mother cell divides by meiosis to give a tetrad of haploid microspores (pollen grains).
A pollen grain has a two-layered wall, the sporoderm: the thick outer exine of sporopollenin (with thin germ-pores) and the inner intine of cellulose and pectin. Sporopollenin makes pollen highly resistant and well preserved as fossils.
Male gametophyte: the pollen grain divides mitotically into a large vegetative cell and a small generative cell. The generative cell later divides to form two non-motile male gametes. Pollen is usually shed at the two-celled stage.
The gynoecium (pistil) is made of carpels — free (apocarpous, Michelia) or fused (syncarpous, brinjal). A carpel has ovary, style and stigma. Ovaries may be uniovulate (paddy, wheat, mango) or multiovulate (tomato, lady’s finger).
The anatropous ovule (commonest in angiosperms) has the micropyle turned down beside the funiculus. Its parts are placenta, funiculus, hilum, nucellus, two integuments, micropyle, chalaza and the embryo sac.
Megasporogenesis: the diploid megaspore mother cell divides by meiosis to give a linear tetrad of four haploid megaspores; three abort and one functional megaspore survives.
Female gametophyte: the functional megaspore undergoes three free-nuclear mitotic divisions to give eight nuclei, organised into a 7-celled, 8-nucleate embryo sac — egg + two synergids (with filiform apparatus) at the micropylar end, three antipodals at the chalazal end, and two polar nuclei that fuse into the diploid secondary nucleus. This is monosporic, endosporous development.
Pollination is the transfer of pollen from anther to stigma. By source it is autogamy (same flower; pea, Clitoria), geitonogamy (different flower, same plant; Cucurbita maxima — needs an agent but gives no variation) or xenogamy (different plant, same species — gives variation).
Devices favouring self-pollination include bisexuality, homogamy and cleistogamy; open flowers are chasmogamous (Viola, Commelina bear both types).
Pollinating agents are abiotic — anemophily (wind; wheat, rice, maize) and hydrophily (water; hypohydrophily in Zostera, epihydrophily in Vallisneria) — and biotic — entomophily (insects; rose, jasmine, Cestrum, lever mechanism in Salvia), ornithophily (birds; Bombax, Callistemon, Butea) and chiropterophily (bats; Anthocephalous, Adansonia, Kigelia). Each type of flower shows matching adaptations.
Outbreeding devices prevent self-pollination and avoid inbreeding depression: unisexuality (monoecious maize; dioecious mulberry, papaya), dichogamy (protandry in sunflower disc florets; protogyny in Gloriosa), prepotency (apple), heterostyly (Primula), herkogamy (Calotropis) and self-incompatibility (tobacco, Thea).
Pollen–pistil interaction runs from pollen landing on the stigma to pollen-tube entry into a synergid. Recognition proteins let the pistil accept compatible pollen and reject incompatible pollen. Pollen germinates in vitro in a medium with sucrose; boric acid accelerates germination. Artificial hybridization uses emasculation and bagging.
Double fertilization (discovered by Nawaschin in Lilium and Fritillaria) is unique to angiosperms. The pollen tube enters the ovule by porogamy (micropyle), chalazogamy (chalaza) or mesogamy (integuments), and delivers two non-motile gametes through the tube (siphonogamy).
Syngamy: male gamete + egg → diploid zygote → embryo. Triple fusion: second male gamete + diploid secondary nucleus → triploid primary endosperm nucleus (PEN) → endosperm. Its significance: food is stored only if the egg is fertilized, the diploid condition is restored, and polyembryony is avoided.
Endosperm forms by repeated mitosis of the PEN. Its three types are nuclear (commonest, 161 families; wheat, sunflower, coconut), cellular (72 dicot families; Balsam, Petunia, Adoxa) and helobial (intermediate; Asphodelus). Mosaic endosperm shows patches of two types, as in corn.
Embryogenesis: the zygote forms a two-celled proembryo — a suspensor initial (giving a 6–10 celled suspensor with a haustorium and a hypophysis) and an embryonal initial (giving the octant). The lower tier forms hypocotyl and radicle, the upper tier forms plumule and cotyledons; the embryo passes through heart-shaped to horse-shoe-shaped stages (Capsella).
Dicot and monocot embryos are alike up to the octant stage. The monocot embryo has one terminal, shield-shaped cotyledon (scutellum), a lateral plumule, and the sheaths coleoptile and coleorhiza.
After fertilization, the ovule becomes the seed and the ovary becomes the fruit; integuments become the seed coat (testa and tegmen), the ovary wall becomes the three-layered pericarp, and a persistent nucellus becomes the perisperm (black pepper, beet).
Seeds are endospermic/albuminous (castor, coconut, maize) or non-endospermic/ex-albuminous (pea, bean). Fruit development is triggered by hormones from developing seeds.
Seeds and fruits nourish and protect the developing seed, serve as propagating units, and aid dispersal. Dormancy is a temporary metabolic arrest that lets viable seeds survive adverse conditions and germinate only after the dormant period.
Apomixis forms embryos and seeds asexually, without gametes, meiosis or fertilization. Types: recurrent (diplospory in Taraxacum; apospory), non-recurrent (parthenogenesis/apogamy from a haploid sac; Nicotiana, generally sterile) and adventive embryony (mango, orange, lemon). Apogamy and apospory are the related terms.
Parthenocarpy (term coined by Noll, 1902) is fruit development without fertilization, giving seedless fruits — naturally in pineapple, banana and papaya; auxin IAA from the placental tissue of the unfertilized ovary enlarges the ovary.
Polyembryony is more than one embryo in a seed, first noticed by Leeuwenhoek (1719) in Citrus. It is true or false (same or different embryo sacs), and includes adventive polyembryony (from nucellus and integuments, Citrus) and cleavage polyembryony. Nucellar adventive polyembryony is valuable in horticulture.

The chapter’s whole sequence of sexual reproduction is condensed in FC 1.17.

Flowchart

FC 1.17 – Overall sequence of sexual reproduction in angiosperms: Flower → Anther → microsporogenesis → pollen grain (male gametophyte) → two male gametes; Ovary → Ovule → megasporogenesis → functional megaspore → embryo sac (female gametophyte) → egg + secondary nucleus; Pollination → pollen–pistil interaction → pollen tube growth → double fertilization → branch A: syngamy → zygote (2n) → embryo; branch B: triple fusion → PEN (3n) → endosperm; then Ovule → Seed and Ovary → Fruit

1.17 Points to Ponder

Why does a gardener so often choose to propagate plants asexually rather than raise them from seed? Think about what is gained — and what is quietly lost — when every new plant is a clone of the parent.
Why do plants such as Viola and Commelina bother to produce both chasmogamous (open) and cleistogamous (closed) flowers on the same plant? What is a plant hedging against by keeping both options open?
Plants and their pollinators are said to have co-evolved, each shaping features in the other. What would happen to a flower whose single specialised pollinator disappeared — and what does this suggest about how tightly we should link a crop to one pollinator?
Bumblebees look for nesting sites early in the year, and each nest lasts only a single season. Why should we take the trouble to leave such nests undisturbed, and what would a landscape without wild pollinators look like for agriculture?
Continued self-pollination leads to inbreeding depression, yet some plants still self-pollinate deliberately, and cleistogamous flowers make selfing certain. Under what conditions might the guarantee of some seed be worth more to a plant than genetic variation?
Seeds of Lupinus arcticus have germinated after roughly 10,000 years, while some Citrus seeds are short-lived. What must a seed be able to do — and to avoid doing — in order to stay alive that long? How long do you think a seed can stay viable, and can old seeds still grow?
Why are some seeds of Citrus described as polyembryonic, and what practical use does a horticulturist make of this fact when raising new plants?
Bananas and figs are both eaten without meeting a hard, mature seed. What do they have in common, and what does the absence of seeds imply about how such plants must be propagated by farmers?
Double fertilization ensures that a plant invests food in a seed only if the egg has actually been fertilized. Why might such “conditional investment” be an advantage worth evolving — and can you think of other places in biology where an organism withholds resources until a commitment is made?
If apomixis could be introduced into hybrid crop varieties, farmers could save and re-sow their own seed each season without losing hybrid vigour. What might be the wider consequences — for farmers, for seed companies, and for crop genetic diversity — if this became widespread?
Collect information about the seed mother Rahibai Soma Popere. How has she conserved over 80 varieties of native seeds, and why does preserving traditional varieties matter in an age of uniform, commercially bred crops?

1.18 Objective Questions

Insect pollinated flowers usually possess …………….
1. Sticky pollens with rough surface
2. Large quantities of pollens
3. Dry pollens with smooth surface
4. Light coloured pollens
In ovule, meiosis occurs in …………….
1. Integument
2. Nucellus
3. Megaspore
4. Megaspore mother cell
The ploidy level is NOT the same in …………….
1. Integuments and nucellus
2. Root tip and shoot tip
3. Secondary nucleus and endosperm
4. Antipodals and synergids
Which of the following types require a pollinator but the result is genetically similar to autogamy?
1. Geitonogamy
2. Xenogamy
3. Apogamy
4. Cleistogamy
If the diploid chromosome number in a flowering plant is 12, then which one of the following will have 6 chromosomes?
1. Endosperm
2. Leaf cells
3. Cotyledons
4. Synergids
In angiosperms, endosperm is formed by / due to …………….
1. Free nuclear divisions of megaspore
2. Polar nuclei
3. Polar nuclei and male gamete
4. Synergids and male gamete
Point out the odd one …………….
1. Nucellus
2. Embryo sac
3. Micropyle
4. Pollen grain
Fill in the blanks (using the given diagram of a flower).
1. The ……………. collects the pollen grains.
2. The male whorl, called the ……………., produces ……………. .
3. The pollen grains represent the ……………. .
4. The ……………. contains the egg or ovum.
5. ……………. takes place when one male gamete and the egg fuse together. The fertilised egg grows into a seed from which new plants can grow.
6. The ……………. is the base of the flower to which other floral parts are attached.
7. ……………. is the transfer of pollen grains from the anther of the flower to the stigma of the same or a different flower.
8. Once the pollen reaches the stigma, the pollen tube traverses down the ……………. to the ovary where fertilisation occurs.
9. The ……………. are coloured to attract the insects that carry the pollen. Some flowers also produce ……………. or ……………. that attracts insects.
10. The whorl ……………. is green and protects the flower until it opens.
Match the column.

Column – I (Structure before seed formation)	Column – II (Structure after seed formation)
a. Funiculus	i. Hilum
b. Scar of ovule	ii. Tegmen
c. Zygote	iii. Testa
d. Inner integument	iv. Stalk of seed
	v. Embryo

a – v, b – i, c – ii, d – iv
a – iii, b – iv, c – i, d – v
a – iv, b – i, c – v, d – ii
a – iv, b – v, c – iii, d – ii

1.19 Check Your Concepts

Why are the offspring produced by asexual reproduction called clones? Explain why this is an advantage in a stable environment but a risk in a changing one.
Distinguish between budding (as in yeast) and bud grafting (as in apple). Why is it wrong to treat them as the same process?
Arrange the four layers of the anther wall in sequence, from the outermost to the innermost, and state the specific function of the outermost and the innermost layer.
Assertion (A): The anther of a flowering plant dehisces to release its pollen. Reason (R): The cells of the endothecium possess fibrous thickenings. Is the reason a correct explanation of the assertion? Justify your answer.
Why can a pollen grain survive as a fossil for millions of years, while the tapetum of the same anther leaves no trace? Explain in terms of the substances involved.
What happens if the tapetum of an anther degenerates prematurely, while the microspore mother cells are still dividing? Predict the effect on pollen formation and explain your reasoning.
All four products of meiosis survive in microsporogenesis, but only one survives in megasporogenesis. Explain this difference and suggest why it makes biological sense for the female side.
A mature embryo sac is described as seven-celled and eight-nucleate. Identify each of the seven cells with its position, and explain how the number of nuclei can exceed the number of cells.
Sequence the following events of female gametophyte development in the correct order: three free-nuclear mitotic divisions; formation of the functional megaspore; fusion of the polar nuclei; meiosis of the megaspore mother cell; degeneration of three megaspores; organisation of the egg apparatus.
Geitonogamy involves a pollinating agent, just as xenogamy does. Why, then, is geitonogamy said to be genetically equivalent to self-pollination?
Match the following flowers with the pollinating agent they are adapted to, and give one adaptation in each case: (i) small, colourless flower with feathery stigma; (ii) dull-coloured, strongly fragrant flower with abundant nectar; (iii) brightly coloured, showy flower with dilute nectar and no fragrance; (iv) submerged flower with long, mucilage-coated pollen.
Why are the flowers of wind-pollinated plants generally without nectar, fragrance and bright colour, while insect-pollinated flowers possess all three? Explain the underlying cause-and-effect.
Assertion (A): Ornithophilous flowers are usually without fragrance. Reason (R): Birds have a poor sense of smell. Evaluate whether R correctly explains A.
What happens if a plant that normally shows protandry is grown in isolation, with no other plant of its species nearby? Explain the likely outcome for seed set.
A plant is dioecious. Can it show autogamy? Can it show geitonogamy? Justify each answer.
Explain how self-incompatibility differs, in its underlying mechanism, from herkogamy and dichogamy — even though all three prevent self-pollination.
Trace the path of a compatible pollen grain from the moment it lands on the stigma to the moment its male gametes are released, naming each structure it passes through or interacts with.
Why does a stigma reject the pollen of a different species even though that pollen may be perfectly healthy and viable? What ensures this selectivity?
Explain why the embryo is diploid while the endosperm is triploid, tracing the ploidy of every nucleus involved in each fusion.
Double fertilization is said to ensure that a plant “invests food in a seed only if the egg is fertilized.” Explain how the mechanism achieves this, and why such an arrangement is advantageous to the plant.
Distinguish between porogamy, chalazogamy and mesogamy. In each case, name the structure through which the pollen tube enters the ovule.
What happens if the second male gamete fails to fuse with the secondary nucleus, while syngamy occurs normally? Predict the fate of the resulting seed and explain why.
Compare nuclear, cellular and helobial endosperm on the basis of wall formation. Why is the helobial type described as intermediate between the other two?
Coconut water and coconut kernel are both endosperm, yet they look completely different. Explain this in terms of the mode of endosperm development.
Distinguish between endosperm and perisperm on the basis of their origin, and name one plant in which perisperm is present.
Sequence the following stages of dicot embryo development in the correct order: octant; heart-shaped embryo; two-celled proembryo; horse-shoe-shaped embryo; zygote; formation of the suspensor.
What is the function of the suspensor in a developing dicot embryo, and how is its structure suited to that function?
Draw and label a diagram of a V.S. of a maize grain, and identify which of the labelled parts are absent in a bean seed.
A pea seed has no endosperm at maturity, yet triple fusion certainly took place in its ovule. Explain this apparent contradiction.
What happens if a seed lacking dormancy is dispersed just before a long dry season? Explain why dormancy is an adaptive advantage rather than a defect.
Distinguish between apomixis and parthenocarpy. Which one gives a seedless fruit, and which one gives a seed without fertilization?
Assertion (A): Plants raised by non-recurrent apomixis are generally sterile. Reason (R): The embryo in non-recurrent apomixis develops from a haploid cell. Evaluate whether R correctly explains A.
How could the introduction of apomixis into hybrid crop varieties benefit farmers? Explain the connection between apomixis and hybrid vigour.
A single Citrus seed produces five seedlings, four of which are identical to the parent tree and one of which is not. Explain this observation using your knowledge of adventive polyembryony.
Distinguish between true and false polyembryony, and between adventive and cleavage polyembryony, stating clearly the criterion used in each case.
Match the following structures before seed formation with what they become after fertilization, and state the ploidy of each product where applicable: ovule; ovary; zygote; secondary nucleus; integuments; ovary wall.
Name the part of the gynoecium that determines the compatible nature of the pollen grain. (30–40 words)
How many haploid cells are present in a mature embryo sac? (30–40 words)
Even though each pollen grain has 2 male gametes, why are at least 20 pollen grains required to fertilize 20 ovules in a particular carpel? (30–40 words)
Define megasporogenesis. (30–40 words)
What is hydrophily? (30–40 words)
Name the layer which supplies nourishment to the developing pollen grains. (30–40 words)
Define parthenocarpy. (30–40 words)
Are pollination and fertilization necessary in apomixis? (30–40 words)
Name the parts of the pistil which develop into fruits and seeds. (30–40 words)
What is the function of the filiform apparatus? (30–40 words)
How can polyembryony be commercially exploited? (80–100 words)
Pollination and seed formation are very crucial for fruit formation. Justify the statement. (80–100 words)
Incompatibility is a natural barrier in the fusion of gametes. How will you explain this statement? (80–100 words)
Describe three devices by which cross pollination is encouraged in angiosperms by avoiding self pollination. (80–100 words)
Describe the process of double fertilization. (150–200 words)
Explain the stages involved in the maturation of a microspore into the male gametophyte. (150–200 words)
Explain the development of the dicot embryo. (150–200 words)
Draw a labelled diagram of the L.S. of an anatropous ovule, list the components of the embryo sac, and mention their fate after fertilization. (150–200 words)
Label the parts of the seed in the given diagram.