Chapter 3: Kingdom Plantae
When miners cut through a coal seam, they sometimes split a lump of coal and find something surprising pressed inside it — the delicate, feathery outline of a fern frond, every leaflet sharp and clear. The plant died hundreds of millions of years ago, yet its shape survived in the rock.
Those imprints are the reason geologists call the late Palaeozoic era the age of Pteridophytes. At that time, much of the land was covered by dense, swampy forests of tree-sized ferns, horsetails and their relatives. There were no flowers anywhere on Earth, no fruits, and no grasses. Generation after generation of these plants sank into waterlogged mud, were buried, compressed and slowly turned into coal. The electricity that lights a classroom today is often released by burning the buried bodies of ancient spore-producing plants.
Look at the plant world now and the range is astonishing. A single microscopic cell of Chlorella is a plant. So is a seaweed more than sixty metres long, a moss cushion barely a centimetre high on a wet wall, a fern on a school compound, a Cycas in a garden, and a sunflower in a field. All of them are placed in one kingdom — Plantae.
The obvious question is: on what basis do biologists sort such wildly different organisms into orderly groups?
Diagram
Fig. 3.1 – Opening illustration: a single panoramic scene showing the diversity of Kingdom Plantae side by side — microscopic algae in a water drop (inset), a large seaweed in shallow sea water, a moss cushion on a damp rock, a fern with pinnate fronds, a Cycas tree, and a flowering sunflower; label each with its group name (Thallophyta, Bryophyta, Pteridophyta, Gymnospermae, Angiospermae)
As you work through this chapter, you will be able to answer each of these questions yourself.
3.1 Kingdom Plantae
In the earlier chapter you studied how living organisms are classified and why a scientific system of grouping is needed at all. Kingdom Plantae was one of the kingdoms placed in that system. We now open that kingdom and look inside it.
What Separates One Plant Group from Another
Kingdom Plantae is further classified on the basis of characters such as the absence or presence of seeds, the presence or absence of vascular tissues, and the degree of differentiation of the plant body.
Each of these three characters tells us something real about the plant’s way of life. Vascular tissues are the conducting tissues — xylem, which carries water and minerals, and phloem, which carries food. A plant that has them can grow tall and can move water upward from soil to shoot; a plant without them must stay small and close to moisture.
“Differentiation of the plant body” simply asks whether the body is divided into true root, stem and leaves, or whether it is one undivided body with no such parts. A seed, meanwhile, is a protected, food-loaded package containing a young plant, while a spore is a single reproductive cell that must land in a favourable spot to grow.
The Two Sub-Kingdoms
On the basis of these characters, Kingdom Plantae is divided into two large sub-kingdoms.
Phanerogamae are commonly called seed-producing plants. They produce special reproductive structures that are visible — the word comes from phaneros, meaning visible. A flower or a cone is exactly such a visible reproductive structure.
Cryptogamae are spore-producing plants. They do not produce seeds and they do not produce flowers. They do reproduce sexually, by gametes, but their sex organs are concealed — the name comes from kryptos, meaning hidden, and gamos, meaning marriage.
Reading the Classification Chart
The whole arrangement of Kingdom Plantae can now be set out as a branching hierarchy, shown in FC 3.1. Read it from the top downwards, and notice that at each level a single character does the splitting: first seeds, then vascular tissue, then the number of cotyledons.
Flowchart
FC 3.1 – Classification of Kingdom Plantae: Kingdom Plantae → branches into Cryptogamae and Phanerogamae; Cryptogamae → branches into Non-Vascular Plants and Vascular Plants; Non-Vascular Plants → Thallophyta and Bryophyta; Thallophyta → Algae; Vascular Plants (under Cryptogamae) → Pteridophyta; Phanerogamae → Vascular Plants → Gymnospermae and Angiospermae; Angiospermae → Dicotyledonae and Monocotyledonae
In examinations this chart is used constantly. A very common question type gives you a plant — pea, jowar or fern — and asks you to place it at its correct systematic position, or to draw its flow chart. If you can recall FC 3.1 and the character used at each branch point, such questions become straightforward.
The rest of this chapter examines each of these groups in turn: first the members of Cryptogamae, then the members of Phanerogamae, and finally the life cycle pattern that runs through all of them.
3.2 Salient Features of Major Plant Groups Under Cryptogamae
Cryptogamae, as you have just seen, are the spore-producing plants. Within this sub-kingdom there are three divisions — Thallophyta, Bryophyta and Pteridophyta — and they represent three successive steps in the plant kingdom’s move out of water and onto land. Read them in that order and the story becomes easy to follow.
A. Division: Thallophyta
Members of Thallophyta are mostly aquatic. A few grow on other plants as epiphytes — that is, they simply perch on another plant for support without drawing food from it. Some grow symbiotically, and some are epizoic, meaning they grow or live non-parasitically on the exterior of living organisms.
The aquatic members, the algae, grow in marine water as well as in fresh water. Most of them are free living, while some are symbiotic — living in a close partnership with another organism in which both partners gain something.
The plant body of a thallophyte is thalloid. A thalloid body is one that is not differentiated into root, stem and leaves; it is a single, simple body with no such parts. This single feature is the reason all these organisms are grouped together, however different their sizes may be.
Some are small, unicellular and microscopic, such as Chlorella, which is non-motile, and Chlamydomonas, which is motile and swims about using flagella. Others are multicellular and unbranched filamentous, like Spirogyra, where cells are joined end to end into a thread. Still others are branched and filamentous, like Chara. And Sargassum, a huge macroscopic sea weed which measures more than 60 metres in length, is also an alga.
General Features of the Algal Cell
The algal cell wall contains either polysaccharides like cellulose or glucose, or a variety of proteins, or both. Reserve food is stored in the form of starch and its other forms — that is, starch itself in some groups and starch-like or related compounds in others.
Reproduction takes place by vegetative, asexual and sexual ways. Vegetative reproduction is simply the breaking of the body into pieces that grow into new individuals, asexual reproduction takes place through spores, and sexual reproduction involves the fusion of gametes.
The life cycle shows the phenomenon of alternation of generation, with a dominant haploid and a reduced diploid phase. In other words, the haploid (\(n\)) body occupies most of the life cycle, while the diploid (\(2n\)) phase is brief. You will study this pattern fully in Section 3.4.
Algae are classified as per their pigments, such as chlorophyll, xanthophylls and phycobilin. Pigments are used as the basis of classification because a pigment tells us what wavelengths of light that alga can absorb, and therefore at what depth of water it can survive and photosynthesise. The three classes described below follow this pigment-based grouping, as summarised in FC 3.2.
Flowchart
FC 3.2 – Classification of algae on the basis of pigments: Algae → Chlorophyceae (green algae, chlorophyll a and b) → Phaeophyceae (brown algae, chlorophyll a and c, fucoxanthin) → Rhodophyceae (red algae, chlorophyll a and d, phycoerythrin)
a. Chlorophyceae (Green Algae)
These are mostly fresh-water forms, with a few brackish-water and marine members. Brackish water is water that is saltier than fresh water but less salty than sea water, as in an estuary.
The plant body is unicellular, colonial or filamentous. A colonial form is one in which many similar cells live together as a group but each cell still functions largely on its own. The cell wall contains cellulose.
Chloroplasts are of various shapes — discoid, plate-like, reticulate, cup-shaped, ribbon-shaped or spiral — and contain chlorophyll a and b. The stored food is in the form of true starch. Notice in Fig. 3.2 how differently the chloroplast is shaped in Chlamydomonas, in Chara and in Spirogyra; the shape is often the quickest way to identify a green alga under the microscope.
Diagram
Fig. 3.2 – Chlorophyceae: (i) Chlamydomonas cell labelled chloroplast, nucleus, pyrenoid (starch storage), mitochondria (amino acid synthesis), flagella; (ii) Chara showing gametophyte, oogonium (contains eggs), antheridium (contains sperm); (iii) Spirogyra filament labelled mucilage, cell wall, cell membrane, cytoplasm, cytoplasm strand, nucleus, spiral chloroplast, pyrenoid, vacuole
Pyrenoids are located on the chloroplast. A pyrenoid is a protein-rich body around which starch is deposited, so it acts as the storage centre of the cell.
Members of this class are rich in protein, and so are used as food — they have even been used by space travellers. Examples include Chlorella, Chlamydomonas, Spirogyra, Chara, Volvox and Ulothrix.
b. Phaeophyceae (Brown Algae)
The plant body is mostly marine and rarely fresh water. It may be simple branched or filamentous, as in Ectocarpus, or profusely branched, as in Petalonia.
The cell wall has cellulose, fucans and algin. Photosynthetic pigments like chlorophyll-a, chlorophyll-c and fucoxanthin are present. Fucoxanthin is a brown pigment, and because it is present in large amounts it masks the green of chlorophyll — which is why these algae look brown.
Mannitol, laminarin and starch are the stored food materials.
The body is usually differentiated into a holdfast, a stalk called the stipe, and a leaf-like photosynthetic organ called the frond, as shown in Fig. 3.3. Do remember that these are not true root, stem and leaf — the body is still a thallus, and these are only root-like, stem-like and leaf-like parts.
Diagram
Fig. 3.3 – Phaeophyceae: Laminaria thallus labelled holdfast, stipe and frond (lamina); alongside, habit sketches of Sargassum and Fucus
Many species of marine algae are used as food, for example Laminaria and Sargassum. Some species are used for the production of hydrocolloids — thick, jelly-forming substances extracted from the cell wall — for example Ectocarpus and Fucus.
c. Rhodophyceae (Red Algae)
The plant body of red algae is found in marine as well as fresh water, on the surface, in the deep sea and in brackish water. The plant body is thalloid.
Cells contain chlorophyll a, chlorophyll d and phycoerythrin. Phycoerythrin is the red pigment that gives the class its name and its colour.
The cell wall is made up of cellulose and pectin glued with other carbohydrates. Stored food is in the form of Floridean starch.
Commercially important agar-agar, which is used as a solidifying agent in tissue culture medium, is obtained from red algae. Examples are Chondrus, Batrachospermum, Porphyra, Gelidium, Gracillaria and Polysiphonia, some of which are shown in Fig. 3.4.
Diagram
Fig. 3.4 – Rhodophyceae: habit sketches of Batrachospermum, Gracillaria and Polysiphonia, showing the thalloid, branched red body
Questions on algae in competitive examinations are very often of the match-the-following type — an alga against its class, its pigment, its stored food or its cell wall material. The compare table above is exactly the information such questions draw upon.
B. Bryophyta
The name comes from bryon, meaning moss, and phyton, meaning plant.
Bryophytes are mostly terrestrial plants. They are found in moist, shady places. But they badly need water for fertilization and for the completion of their life cycle, and hence they are called ‘amphibious plants’. They include approximately 960 genera and about 25,000 species.
The word amphibious is used here in the same sense as for a frog: the body can live on land, but the reproductive stage cannot be completed without water. The male gamete of a bryophyte swims, so a film of water on the surface of the plant is essential for it to reach the egg.
The life cycle of bryophytes shows sporophytic and gametophytic stages. The vegetative plant body, which is thalloid or leafy, represents the gametophytic generation. The spore-producing capsule represents the sporophytic generation.
This is worth pausing on. The green cushion you see is the gametophyte — the haploid, gamete-producing plant. The slender stalk with a capsule at its tip, rising out of that cushion, is the sporophyte — the diploid, spore-producing plant. Both are present at once, one growing out of the other.
Bryophytes have root-like structures called rhizoids. Rhizoids are unicellular in liverworts, while they are multicellular in mosses. Rhizoids absorb water and minerals and also help in the fixation of the thallus to the substratum.
Bryophytes are divided into two groups: liverworts and mosses, as set out in FC 3.3.
Flowchart
FC 3.3 – Groups of Bryophyta: Bryophyta → Liverworts (Hepaticeae; includes Hornworts, Anthocerotae) → examples Riccia, Marchantia, Anthoceros; and Bryophyta → Mosses (Musci) → examples Funaria, Polytrichum, Sphagnum
a. Liverworts (Hepaticeae)
These are the lower members of Bryophyta and form a primitive group of bryophytes.
The gametophyte possesses a flat plant body called the thallus. The thallus is green, dorsiventral and prostrate, with unicellular rhizoids. Dorsiventral means the upper and lower surfaces are clearly different from each other, and prostrate means the body lies flat along the ground rather than standing up. Examples are Riccia and Marchantia.
Hornworts (Anthocerotae)
These members possess a flattened thallus. The thallus produces horny structures which are the sporophytes — and it is from these horn-like sporophytes that the name hornworts comes. An example is Anthoceros, shown along with the other bryophytes in Fig. 3.5.
b. Mosses (Musci)
These are the advanced members of Bryophyta, and they possess an erect plant body.
The gametophytic phase of the life cycle includes two stages, namely the protonema stage and the leafy stage. The protonema is prostrate, green, branched and filamentous, and is also called the juvenile gametophyte. It bears many buds.
The leafy stage is produced from each bud. Thus the protonema helps in vegetative propagation — one protonema bearing many buds gives rise to many leafy shoots, and a whole moss cushion may in this way arise from a single spore.
The leafy stage has an erect, slender, stem-like main axis called the cauloid, bearing spirally arranged leaf-like structures called phylloids. It is fixed in the soil by multicellular branched rhizoids. This stage bears the sex organs. Notice in Fig. 3.5 how the capsule, seta and foot together form the sporophyte, while the main axis with its rhizoids is the gametophyte.
Diagram
Fig. 3.5 – Bryophyta: Riccia (liverwort) thallus; Anthoceros (hornwort) with horn-like sporophytes; Funaria (moss) labelled capsule, seta and foot (marked as sporophyte) and main axis with rhizoids (marked as gametophyte)
Vegetative reproduction takes place by fragmentation and by budding in the secondary protonema. Examples of mosses are Funaria, Polytrichum and Sphagnum.
Flowchart
FC 3.4 – Development of the moss gametophyte: spore → germinates → protonema (juvenile gametophyte, filamentous) → bears buds → each bud grows into leafy stage (cauloid with phylloids, fixed by multicellular rhizoids) → leafy stage bears sex organs → gametes → fertilization
Economic Importance of Bryophytes
Some mosses provide food for herbivorous mammals, birds and other animals. Species of Sphagnum, a moss, provide peat, which is used as fuel.
Mosses are also used as packing material for the transport of living materials, because they have a significant water-holding capacity and keep the material moist during the journey.
Just like lichens, mosses are among the first living beings to grow on bare rocks. They decompose the rock to form soil and make it suitable for the growth of higher plants. This is why they matter far beyond their own size — they prepare the ground on which a forest can later stand.
Dense mats of mosses help in the prevention of soil erosion, and thus act as soil binders. The mat holds the surface layer of soil in place so that running rainwater cannot carry it away.
C. Pteridophyta
The name comes from pteron, meaning feather, and phyton, meaning plant — a reference to the feathery leaves of ferns.
Evolutionarily, pteridophytes are the first vascular and true land plants. Hence they are considered the first successful terrestrial plants with true roots, stem and leaves. These plants have a primitive conducting system, and they are the only Cryptogams with vascular tissues.
The late Palaeozoic era is regarded as the age of Pteridophytes. The group has about 400 genera and 11,000 species.
The plants consist of pinnate, that is feather-like, leaves. Leaves may be small, called microphylls, as in Selaginella, or large, called macrophylls, as in Nephrolepis, the fern shown in Fig. 3.6.
Diagram
Fig. 3.6 – Nephrolepis (fern): whole plant labelled frond, blade, rhizome and roots; alongside, the back side of a leaflet showing clusters of sporangia and the front side for comparison
Body and Life Cycle
Pteridophytes grow in moist and shady places. Pteridophytes show sporophytic and gametophytic stages in their life cycle. Ferns and horsetails are common examples. Some are aquatic, such as Azolla and Marsilea, some are xerophytic, that is adapted to dry habitats, such as Equisetum, and some are epiphytic, such as Lycopodium.
Pteridophytes show heteromorphic alternation of generations, in which the sporophyte is diploid, dominant, autotrophic and independent. Heteromorphic means the two generations do not look alike — one is large and leafy, the other small and inconspicuous. The large fern you see in a garden is the sporophyte.
The sporophyte is differentiated into root, stem and leaves. The primary root is short lived and is soon replaced by adventitious roots, while the stem may be aerial or underground. Leaves may be scaly, as in Equisetum, simple and sessile, as in Lycopodium, or large and pinnately compound, as in Nephrolepis and other ferns.
In these members, xylem consists of only tracheids and phloem consists of only sieve cells. This is what “primitive conducting system” means — the more efficient vessels of the xylem and the sieve tubes with companion cells of the phloem, which you will meet in the seed plants, are absent here.
Secondary growth is not seen in pteridophytes due to the absence of cambium. Cambium is the dividing layer of cells that adds new xylem and phloem year after year and so increases the girth of a stem. Without it, a pteridophyte stem cannot thicken.
Classification of Pteridophytes
Pteridophytes are classified into four groups, as shown in FC 3.5 — Psilopsida, Lycopsida, Sphenopsida and Pteropsida.
Flowchart
FC 3.5 – Classification of Pteridophyta: Pteridophyta → Psilopsida (Psilotum); → Lycopsida (Selaginella, Lycopodium); → Sphenopsida (Equisetum); → Pteropsida (Dryopteris, Pteris, Adiantum)
Economic Importance of Pteridophytes
Pteridophytes are used for medicinal purposes and as soil binders. Many varieties are grown as ornamental plants.
3.3 Salient Features of Major Plant Groups Under Phanerogamae
With Phanerogamae we move to the seed-producing plants. Everything that follows depends on one change: the fertilized egg is no longer released as a naked spore, but is packed inside a seed, along with stored food and a protective coat. That single innovation is what finally freed plants from the need for a film of water, and it is why these groups dominate the land today.
Phanerogamae are divided into two groups, Gymnospermae and Angiospermae, as shown in FC 3.6.
Flowchart
FC 3.6 – Classification of Phanerogamae: Phanerogamae → Gymnospermae (naked seeds, no fruit; e.g. Cycas, Pinus) and Angiospermae (seeds enclosed in fruit) → Angiospermae branches into Dicotyledonae (two cotyledons; e.g. Helianthus annuus) and Monocotyledonae (one cotyledon; e.g. Zea mays)
A. Gymnospermae
The name comes from gymnos, meaning naked, and sperma, meaning seed.
There are about 70 genera and 1000 living species of gymnosperms in the world. In India the group is represented by 16 genera and 53 species.
Most of the gymnosperms are evergreen shrubs or woody trees. They are a primitive group among the seed-bearing plants, producing naked seeds. They are vascular plants, predominantly having xylem with tracheids and phloem with sieve cells.
A naked seed means the seed is not enclosed in a fruit. The ovule sits exposed on the surface of a scale-like leaf, and after fertilization it becomes a seed while still lying open on that scale. There is no ovary wall around it to ripen into a fruit.
Root, Stem and Leaves
The plant body is a sporophyte. It is differentiated into root, stem and leaves. The root system is of the tap root type, in which one main root grows straight down and gives off smaller branches.
In some gymnosperms, roots form a symbiotic association with other life forms. The coralloid roots of Cycas show association with blue green algae, and the roots of Pinus show association with endophytic fungi called mycorrhizae. Notice in Fig. 3.7 how these coralloid roots grow upward and branch repeatedly like a piece of coral, and how a transverse section through one shows a distinct algal zone.
Diagram
Fig. 3.7 – Cycas plant details: (a) plant body labelled stem and foliage leaves; (b) coralloid roots; (c) transverse section of coralloid root labelled algal zone; (d) male cone bearing microsporophylls; (e) megasporophyll of Cycas
In gymnosperms the stem is mostly erect, aerial, solid and cylindrical. Secondary growth is seen in gymnosperms due to the presence of cambium — which is why so many of them become thick, woody trees. In Cycas the stem is usually unbranched, while in conifers it is branched, as you can see in the Pinus tree in Fig. 3.8.
Diagram
Fig. 3.8 – Pinus tree with cones: branched woody stem bearing needle-like foliage leaves and mature cones
The leaves are dimorphic, that is, of two different kinds on the same plant. The foliage leaves are green and simple, either needle-like or pinnately compound, and carry out photosynthesis. The scale leaves are small, membranous and brown, and serve to protect buds rather than to make food.
Spores are produced by the microsporophyll, which is the male structure, and the megasporophyll, which is the female structure.
Gymnosperms vary greatly in size. Sequoia sempervirens, commonly called the coast red wood of California, is the tallest living plant in the world, with a height of about 366 feet. Taxodium mucronatum has a girth of about 125 feet. At the other extreme, Zamia pygmaea is the smallest gymnosperm and is only about 25 cms.
Economic Importance of Gymnosperms
Cycas is grown as an ornamental plant. Pinus is used as a source of pine wood, turpentine oil and pine resin.
B. Angiospermae
The name comes from angios, meaning enclosed or vessel, and sperma, meaning seed.
Angiosperms are the most advanced group of flowering plants. In these plants the seeds are enclosed within the fruit, that is, the ovary. Angiosperms are a group of highly evolved plants, primarily adapted to the terrestrial habitat. They vary greatly in size.
Angiosperms show heteromorphic alternation of generation, in which the sporophyte is diploid, dominant, autotrophic and independent. The gametophytes, male as well as female, are haploid, reduced, parasitic and concealed within the sporophyte.
This is the end point of a trend you have been following through the whole chapter. In bryophytes the gametophyte was the plant you could see; in pteridophytes the sporophyte took over; and here the gametophyte has shrunk so far that it is only a few cells living inside the tissues of the sporophyte, drawing nourishment from it — which is why it is described as parasitic.
The Flower and Its Whorls
Angiosperms are heterosporous, meaning they produce two different kinds of spores.
Microspores, commonly called pollens, are formed in the microsporangia of the anthers. They develop in highly specialized microsporophylls, or stamens. Megaspores are formed in megasporangia, or ovules, borne on a highly specialized megasporophyll called the carpel.
Besides the essential whorls of microsporophylls, together called the androecium, and megasporophylls, together called the gynoecium, there are accessory whorls, namely the calyx made of sepals and the corolla made of petals, arranged together to form the flower.
The whorls are called essential or accessory for a clear reason. The androecium and gynoecium actually produce the spores and gametes, so without them no seed can form. The calyx and corolla protect the young bud and attract pollinators, which helps a great deal but is not indispensable. The arrangement is summarised in FC 3.7.
Flowchart
FC 3.7 – Whorls of a flower: Flower → Accessory whorls → Calyx (sepals) and Corolla (petals); Flower → Essential whorls → Androecium (microsporophylls / stamens → anther → microsporangia → microspores or pollen) and Gynoecium (megasporophyll / carpel → ovule or megasporangium → megaspores)
Angiosperms are subdivided into two classes.
a. Dicotyledonae
These plants have two cotyledons in their embryo. A cotyledon is the seed leaf, the part of the embryo that stores or absorbs food for the germinating seedling.
They have a tap root system and the stem is branched. Leaves show reticulate venation, in which the veins form a net-like pattern, while the flowers show tetra- or pentamerous symmetry, that is, floral parts in multiples of four or five.
Vascular bundles are conjoint, collateral and open type. Cambium is present between the xylem and the phloem for secondary growth. In dicots, secondary growth is commonly found. Examples are Helianthus annuus, the sunflower shown in Fig. 3.9, and Hibiscus rosa-sinensis, the China rose.
Diagram
Fig. 3.9 – Helianthus annuus (sunflower): whole plant labelled tap root, stem, leaves with reticulate venation, and the inflorescence labelled ray florets, disc florets and seeds
b. Monocotyledonae
These plants have a single cotyledon in their embryo. They have an adventitious root system, in which many roots of similar size arise from the base of the stem instead of from a single main root, and the stem is rarely branched.
Leaves generally have a sheathing leaf base, which wraps around the stem, and parallel venation, in which the veins run side by side along the leaf. The flowers are generally trimerous, with floral parts in multiples of three.
The vascular bundles are conjoint, collateral and closed type. In monocots, except in a few plants, secondary growth is absent. Examples are Zea mays, maize, shown in Fig. 3.10, and Sorghum vulgare, jowar.
Diagram
Fig. 3.10 – Zea mays (maize): whole plant labelled roots (adventitious), leaf blade, ear, corn grain, tassel and silks
An “open” bundle is one in which cambium lies between the xylem and the phloem; a “closed” bundle has no cambium between them. This one difference explains the whole of the next point: a dicot stem can go on adding new xylem and phloem year after year and thicken, while a monocot stem cannot.
This pair of classes is one of the most frequently tested topics in the chapter. Questions usually give you a single clue — a leaf with parallel veins, an embryo with two cotyledons, a stem that never thickens — and ask you to name the class, or to justify a statement such as “the girth of a maize plant does not increase”. Learning the table as a set of linked characters, rather than as isolated facts, is what makes such questions quick.
3.4 Plant Life Cycle and Alternation of Generations
The Two Generations
The life cycle of a plant includes two phases, or two distinct generations, namely the sporophyte, which is diploid (\(2n\)), and the gametophyte, which is haploid (\(n\)).
Diploid means each cell carries two sets of chromosomes, one contributed by each parent; haploid means each cell carries a single set. Keeping this in mind makes the whole cycle easy to follow, because every step in it is simply a device either to halve the chromosome number or to restore it.
Some special diploid cells of the sporophyte divide by meiosis to produce haploid cells. Meiosis is the reduction division, in which the chromosome number is halved. These haploid cells are the spores.
These haploid cells divide mitotically to give rise to the gametophyte. Mitosis makes exact copies, so every cell of the gametophyte body remains haploid — the body grows, but the chromosome number does not change.
The gametophyte produces male and female gametes separately. These gametes fuse during fertilization to produce a diploid zygote, and the diploid number is thereby restored.
The zygote divides by mitosis to form the diploid sporophyte. The sporophytic and gametophytic generations generally occur alternately in the life cycle of a plant, and this phenomenon is called alternation of generations. The full cycle is shown in Fig. 3.11 and set out step by step in FC 3.8.
Diagram
Fig. 3.11 – Alternation of generation: a circular diagram with the sporophyte phase and gametophyte phase marked as two halves; nodes in order — Sporophyte (\(2n\)) → Meiosis → Spores (\(1n\)) → Gametophyte (\(1n\)) → Gametes (\(1n\)) → Fertilization → Zygote (\(2n\)) → back to Sporophyte (\(2n\))
Flowchart
FC 3.8 – Alternation of generations: Sporophyte (\(2n\)) → meiosis in special diploid cells → spores (\(n\)) → mitosis → gametophyte (\(n\)) → produces male and female gametes (\(n\)) → fertilization → zygote (\(2n\)) → mitosis → back to sporophyte (\(2n\))
Alternation in the Different Plant Groups
Distinct alternation of these two generations is observed in bryophytes and pteridophytes. In these two groups both generations are visible as separate structures, so the alternation can actually be seen.
In gymnosperms and angiosperms, the gametophyte is much reduced and exists within the sporophyte. It has not disappeared — it has simply been reduced to a handful of cells hidden inside the tissues of the plant, as you saw in the previous section.
In algae, based upon the nature of the dominant phase in the life cycle, the cycle is called haplontic, diplontic or haplo-diplontic.
In bryophytes, the haploid gametophyte is dominant. It is a photosynthetic, independent, thalloid or erect phase. The sporophyte is short lived and multicellular, and depends totally or partially on the gametophyte for nutrition and anchorage.
In pteridophytes, on the other hand, the sporophyte is dominant, independent, and forms the vascular plant body. The haploid gametophyte is multicellular, independent and non-vascular; it is generally autotrophic and short lived, and it alternates with the sporophyte.
Types of Life Cycle
Depending on which phase is multicellular and which divisions occur where, three types of life cycle are recognised, as shown in Fig. 3.12.
Diagram
Fig. 3.12 – Types of life cycle: three parallel cycle diagrams labelled Diplontic, Haplontic and Haplo-Diplontic, with the haploid (\(n\)) region shaded above and the diploid (\(2n\)) region below; each showing the positions of mitosis, meiosis, gametes, spores and fertilization, with examples marked — In animals; Fungi and some algae; Plants and some algae
Diplontic: Here mitotic divisions occur only in diploid cells. Gametes formed through meiosis are haploid. The diploid zygote divides mitotically. In this process, production of a multicellular diploid organism, or production of many diploid single cells, takes place. Example: animals.
Haplontic: Here mitosis occurs in haploid cells. It results in the formation of single haploid cells or a multicellular haploid organism. These forms produce the gametes through mitosis. The zygote is formed after fertilization. This cell is the only diploid cell in the entire life cycle. Thus the same zygotic cell later undergoes meiosis. Example: some algae and fungi.
Haplo-diplontic: Here mitosis occurs in both diploid and haploid cells. These organisms pass through a phase in which they are multicellular and haploid — the gametophyte — and a phase in which they are multicellular and diploid — the sporophyte. Example: land plants and many algae.
Questions on this section are usually diagram-based. A common pattern gives you the alternation of generations cycle with the labels interchanged — meiosis and fertilization swapped, or the gametophyte and sporophyte phases exchanged — and asks you to correct it. The safe way to check any such diagram is to test two points: meiosis must lead to spores, and fertilization must lead to the zygote.
3.5 Summary
Kingdom Plantae is classified on the basis of the presence or absence of seeds, the presence or absence of vascular tissues, and the degree of differentiation of the plant body.
Phanerogamae are seed-producing plants whose reproductive structures are visible (phaneros = visible). Cryptogamae are spore-producing plants with neither seeds nor flowers; they reproduce sexually by gametes, but their sex organs are concealed (kryptos = hidden, gamos = marriage).
Cryptogamae include the non-vascular divisions Thallophyta and Bryophyta, and the vascular division Pteridophyta. Phanerogamae include Gymnospermae and Angiospermae, and Angiospermae is further divided into Dicotyledonae and Monocotyledonae.
Thallophytes are mostly aquatic; a few are epiphytic, symbiotic or epizoic. The plant body is thalloid, that is, not differentiated into root, stem and leaves. Forms range from unicellular Chlorella and Chlamydomonas to filamentous Spirogyra and Chara, and up to Sargassum, which measures more than 60 metres.
The algal cell wall contains polysaccharides such as cellulose or glucose, proteins, or both. Reserve food is starch and its other forms. Reproduction is vegetative, asexual and sexual, and the life cycle shows alternation of generation with a dominant haploid and a reduced diploid phase. Algae are classified on the basis of their pigments.
Chlorophyceae are mostly fresh-water green algae with cellulose walls, chlorophyll a and b, chloroplasts of many shapes bearing pyrenoids, and true starch as stored food. Examples: Chlorella, Chlamydomonas, Spirogyra, Chara, Volvox, Ulothrix.
Phaeophyceae are mostly marine brown algae with cellulose, fucans and algin in the wall, chlorophyll-a, chlorophyll-c and fucoxanthin as pigments, and mannitol, laminarin and starch as stored food. The body is differentiated into holdfast, stipe and frond. Examples: Ectocarpus, Petalonia, Laminaria, Sargassum, Fucus.
Rhodophyceae are thalloid red algae of marine, fresh and brackish water, with chlorophyll a, chlorophyll d and phycoerythrin, walls of cellulose and pectin, and Floridean starch as stored food. Agar-agar, used as a solidifying agent in tissue culture medium, is obtained from red algae. Examples: Chondrus, Batrachospermum, Porphyra, Gelidium, Gracillaria, Polysiphonia.
Bryophytes are mostly terrestrial plants of moist, shady places, but they need water for fertilization and for completing the life cycle, and so are called amphibious plants. They include approximately 960 genera and about 25,000 species.
In bryophytes the vegetative plant body, thalloid or leafy, is the gametophytic generation, while the spore-producing capsule is the sporophytic generation. Rhizoids absorb water and minerals and fix the thallus to the substratum; they are unicellular in liverworts and multicellular in mosses.
Liverworts are the primitive members, with a green, dorsiventral, prostrate thallus and unicellular rhizoids, as in Riccia and Marchantia. Hornworts have a flattened thallus bearing horn-like sporophytes, as in Anthoceros.
Mosses are the advanced members with an erect body. The gametophytic phase has a protonema stage — prostrate, green, branched, filamentous and bud-bearing, helping vegetative propagation — and a leafy stage with a cauloid axis, spirally arranged phylloids, multicellular branched rhizoids, and the sex organs. Vegetative reproduction occurs by fragmentation and by budding in the secondary protonema. Examples: Funaria, Polytrichum, Sphagnum.
Mosses provide food for herbivores and birds; Sphagnum provides peat used as fuel; mosses serve as packing material because of their water-holding capacity; like lichens they are among the first to grow on rocks and help form soil; and their dense mats act as soil binders and prevent erosion.
Pteridophytes are the first vascular and true land plants, the first successful terrestrial plants with true roots, stem and leaves, and the only Cryptogams with vascular tissues. The late Palaeozoic era is regarded as their age. The group has about 400 genera and 11,000 species.
Leaves are pinnate and may be microphylls, as in Selaginella, or macrophylls, as in Nephrolepis. Members may be aquatic (Azolla, Marsilea), xerophytic (Equisetum) or epiphytic (Lycopodium).
Pteridophytes show heteromorphic alternation of generations in which the sporophyte is diploid, dominant, autotrophic and independent. The primary root is short lived and is replaced by adventitious roots. Xylem has only tracheids and phloem only sieve cells, and secondary growth is absent because cambium is absent.
Pteridophytes are classified as Psilopsida (Psilotum), Lycopsida (Selaginella, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum). They are used medicinally, as soil binders and as ornamental plants.
Gymnosperms produce naked seeds (gymnos = naked, sperma = seed). There are about 70 genera and 1000 living species in the world, and 16 genera and 53 species in India. Most are evergreen shrubs or woody trees with xylem of tracheids and phloem with sieve cells.
The gymnosperm plant body is a sporophyte with a tap root system. Coralloid roots of Cycas associate with blue green algae, and roots of Pinus associate with endophytic fungi called mycorrhizae. The stem is erect, aerial, solid and cylindrical, unbranched in Cycas and branched in conifers, and shows secondary growth because cambium is present. Leaves are dimorphic — green foliage leaves and small brown membranous scale leaves. Spores are produced by microsporophylls (male) and megasporophylls (female).
Ginkgo biloba is called a living fossil. Sequoia sempervirens, the coast red wood of California, is the tallest living plant at about 366 feet; Taxodium mucronatum has a girth of about 125 feet; Zamia pygmaea is the smallest gymnosperm at about 25 cms. Cycas is grown as an ornamental plant, and Pinus yields pine wood, turpentine oil and pine resin.
Angiosperms are the most advanced group of flowering plants, with seeds enclosed within the fruit, that is, the ovary. They are highly evolved and primarily adapted to terrestrial habitats. Wolffia, at 1 mm, is the smallest angiosperm, while Eucalyptus grows to over 100 metres.
Angiosperms show heteromorphic alternation of generations in which the sporophyte is diploid, dominant, autotrophic and independent, while the male and female gametophytes are haploid, reduced, parasitic and concealed within the sporophyte.
Angiosperms are heterosporous. Microspores, or pollen, form in the microsporangia of anthers borne on microsporophylls or stamens; megaspores form in megasporangia, or ovules, borne on the megasporophyll called the carpel. The essential whorls, androecium and gynoecium, together with the accessory whorls, calyx and corolla, form the flower.
Dicotyledonae have two cotyledons, a tap root system, a branched stem, reticulate venation and tetra- or pentamerous flowers. Their vascular bundles are conjoint, collateral and open, with cambium between xylem and phloem, so secondary growth is common. Examples: Helianthus annuus, Hibiscus rosa-sinensis.
Monocotyledonae have one cotyledon, an adventitious root system, a rarely branched stem, sheathing leaf bases, parallel venation and generally trimerous flowers. Their vascular bundles are conjoint, collateral and closed, and secondary growth is absent except in a few plants. Examples: Zea mays, Sorghum vulgare.
The life cycle of a plant has two generations: the sporophyte (\(2n\)) and the gametophyte (\(n\)). Special diploid cells of the sporophyte undergo meiosis to form haploid cells, which divide mitotically into the gametophyte. The gametophyte forms male and female gametes, which fuse at fertilization to give a diploid zygote, and the zygote divides mitotically to form the sporophyte. This regular alternation is called alternation of generations.
Distinct alternation is seen in bryophytes and pteridophytes. In bryophytes the haploid gametophyte is dominant, photosynthetic and independent, while the sporophyte is short lived and depends totally or partially on the gametophyte for nutrition and anchorage. In pteridophytes the sporophyte is dominant, independent and vascular, while the multicellular, independent, non-vascular gametophyte is generally autotrophic and short lived. In gymnosperms and angiosperms the gametophyte is much reduced and lies within the sporophyte.
Three types of life cycle are recognised. In the diplontic type, mitosis occurs only in diploid cells and gametes are formed by meiosis — as in animals. In the haplontic type, mitosis occurs in haploid cells, gametes are formed by mitosis, and the zygote is the only diploid cell, which itself undergoes meiosis — as in some algae and fungi. In the haplo-diplontic type, mitosis occurs in both diploid and haploid cells, so both a multicellular gametophyte and a multicellular sporophyte exist — as in land plants and many algae.
The whole chapter can be read as one sequence of gains, condensed in FC 3.9.
Flowchart
FC 3.9 – Trend across the plant groups: Thallophyta (thalloid body, no vascular tissue, spores) → Bryophyta (land habit, rhizoids, gametophyte dominant, water still needed for fertilization) → Pteridophyta (true root, stem and leaves, vascular tissue, sporophyte dominant, still spore-bearing) → Gymnospermae (seed formed, but naked; gametophyte reduced within sporophyte) → Angiospermae (flower formed, seed enclosed in fruit, gametophyte most reduced)
3.6 Points to Ponder
Classification of Kingdom Plantae begins with the presence or absence of seeds, and only afterwards looks at vascular tissue. What would the chart look like if biologists had chosen vascular tissue as the very first character instead? Would any group change its place?
A giant seaweed more than 60 metres long and a single microscopic cell are placed in the same division. Does this tell us that size is unimportant in classification, or only that it is unimportant here? Can you think of a situation where size would be a useful character?
Algae are classified by their pigments, and different pigments suit different depths of water. If a new alga were discovered with a pigment combination not found in any of the three classes, how should biologists decide where to place it?
Bryophytes conquered land but never freed themselves from water for reproduction. Is it fair to call them a “successful” group, or has success in evolution to be measured in some other way?
Rhizoids of bryophytes and roots of pteridophytes both anchor the plant, but only one of them belongs to a plant with vascular tissue. What does this suggest about the order in which anchorage and conduction evolved?
The late Palaeozoic era is called the age of Pteridophytes, yet today ferns are a comparatively small group living mostly in moist, shady corners. What might have changed on Earth to reduce a once-dominant group to this position?
Pteridophytes lack cambium and so cannot thicken, yet some pteridophytes of the past grew to the size of trees. How might a plant achieve height without secondary growth?
Across the chapter, the gametophyte becomes smaller and more dependent as we move from bryophytes to angiosperms, while the sporophyte becomes larger and more independent. Why might a life on land favour this particular direction of change?
A gymnosperm seed lies exposed on a scale, while an angiosperm seed is enclosed within a fruit. List the advantages the enclosed seed gains. Are there any conditions in which an exposed seed might actually be the better arrangement?
Cycas houses blue green algae in its coralloid roots and Pinus houses fungi in its mycorrhizae. If a plant depends on another organism for a vital nutrient, is it still correct to describe that plant as fully autotrophic?
Monocots generally cannot increase their girth, yet monocots include some of the most widely grown crops in the world. Does the absence of secondary growth limit these plants, or does it suit them in some way?
In the haplontic life cycle, the zygote is the only diploid cell in the entire life of the organism, while in the diplontic cycle the gametes are the only haploid cells. Both survive perfectly well. What, then, is the real advantage of being multicellular in both phases, as land plants are?
Meiosis and fertilization occur once each in every life cycle, and they exactly cancel each other’s effect on chromosome number. What would happen to a species, generation after generation, if the position of meiosis in the cycle were shifted from spore formation to gamete formation?
3.7 Check Your Concepts
Kingdom Plantae is divided first on the basis of seeds and only later on the basis of vascular tissue. Explain, with an example, why a plant may possess vascular tissue and still be placed in Cryptogamae.
A student finds a green, thread-like plant body in a pond and reports it as a “water plant with no roots, stem or leaves”. Which division would you place it in, and which single character led you to that decision?
Why are algae classified on the basis of their pigments rather than on the basis of their size or shape? Support your answer with the habitat of red algae.
Arrange the following in the correct sequence of a moss life cycle: leafy stage, spore, gametes, protonema, zygote, fertilization, capsule.
What would happen to a moss plant if it were kept in a well-lit but completely dry chamber for one full season? Which stage of its life cycle would be blocked, and why?
Distinguish between a holdfast and a root, and between a rhizoid and a root. Which of these three structures absorbs water and minerals?
Name the pigment other than chlorophyll a present in each of the three classes of algae, and state one example of each class.
Agar-agar is used as a solidifying agent in tissue culture media. From which group of algae is it obtained, and what property of the substance makes it suitable for this purpose?
A single moss spore can give rise to an entire cushion of moss plants on a wall. Explain how, naming the stage responsible.
Differentiate between liverworts and mosses on the basis of: a. nature of the plant body; b. rhizoids; c. stages of the gametophytic phase.
Why is Anthoceros called a hornwort? Which generation forms the “horn”?
Mosses are described as pioneers on bare rock and also as soil binders. Explain both roles, and state how one leads to the other.
Pteridophytes are called vascular Cryptogams. Break this name into its two parts and justify each part separately.
What happens to the primary root of a pteridophyte as the plant matures, and by what is it replaced?
A pteridophyte stem never increases in girth, however old the plant becomes. Identify the tissue whose absence is responsible and explain its normal function.
Classify the following into microphyllous and macrophyllous pteridophytes: Selaginella, Nephrolepis. On what basis, other than size, are the two types of leaves distinguished?
Name the four groups of Pteridophyta and give one example of each. Which group includes the common ferns?
Why is a cone not called a flower, even though both are visible reproductive structures? State two structural differences.
The coralloid roots of Cycas contain an algal zone. What does the plant gain from this association, and what does the partner gain? Name the type of association.
What is meant by dimorphic leaves in gymnosperms? Give the function of each type.
Label the following on a diagram of a Laminaria thallus: holdfast, stipe, frond. Against each label, write the function it performs.
Label the following on a diagram of a Funaria plant: capsule, seta, foot, main axis, rhizoids. Mark which of these belong to the sporophyte and which to the gametophyte.
Why is Ginkgo biloba called a living fossil? Does the term describe the plant’s age or the plant’s group?
Angiosperm gametophytes are described as “parasitic”. On which structure are they parasitic, and why is this description used?
Distinguish between the essential whorls and the accessory whorls of a flower, naming the parts included in each and giving the function of each group.
A seed is cut open and found to contain a single cotyledon. Predict, with reasons, the type of root system, venation and vascular bundles you would expect in the mature plant.
Why can a mango tree thicken year after year while a jowar plant cannot? Identify the exact structural difference in the vascular bundles.
Arrange the following in the correct sequence of alternation of generations, beginning from the sporophyte: gametes, zygote, meiosis, spores, fertilization, gametophyte, sporophyte.
In a plant life cycle, meiosis occurs once and fertilization occurs once. What would happen to the chromosome number of a species over several generations if fertilization occurred but meiosis did not?
Compare the haplontic, diplontic and haplo-diplontic life cycles on the basis of: a. the phase that is multicellular; b. the mode of gamete formation; c. one example each.
In the haplontic life cycle, which is the only diploid cell, and what does it undergo immediately afterwards?
When we look at a moss cushion and at a fern frond, we are looking at two different generations. Name the generation in each case and state which one is dominant in that group.
Trace the trend in the dominance of the gametophyte from Bryophyta to Angiospermae, naming the dominant generation at each step, and suggest why life on land might favour this trend.
How would you place pea, jawar and fern at their proper systematic positions? Draw a flow chart for each.
Complete the following table.
| Groups of algae | Chlorophyceae | Phaeophyceae | Rhodophyceae |
|---|---|---|---|
| 1. Stored food | Starch | ||
| 2. Cell wall | Cellulose and algin | ||
| 3. Major pigments | Chl- a, d and Phycoerythrin |
Differentiate between Dicotyledonae and Monocotyledonae based on the following characters: a. Type of roots; b. Venation in the leaves; c. Symmetry of flower.
We observe that land becomes barren soon after the monsoon. But in the next monsoon it flourishes again with the same varieties we observed in the earlier season. How do you think this takes place?
Fern is a vascular plant. Yet it is not considered a Phanerogam. Why?
Chlamydomonas is microscopic whereas Sargassum is macroscopic, and both are algae. Which characters of these plants include them in one group?
Girth of a maize plant does not increase over a period of time. Justify.
Radha observed a plant in the rainy season on the compound wall of her school. The plant did not have true roots, but root-like structures were present. To which group may the plant belong?
Draw neat labelled diagrams of: A. Spirogyra; B. Chlamydomonas; C. Funaria; D. Nephrolepis; E. Haplontic and haplo-diplontic life cycle.
Identify the plant groups on the basis of the following features: A. Seed producing plants; B. Spore producing plants; C. Plant body undifferentiated into root, stem and leaves; D. Plants that need water for fertilization; E. First vascular plants.
Observe the following diagram. Correct it and write the information in your own words.
Diagram
Fig. 3.13 – An incorrectly labelled alternation of generation cycle for correction: circular diagram with the labels wrongly placed — Gametes (1n) and Meiosis on the left, Fertilization on the right, Zygote (2n) at the lower left, Sporophyte (2n) at the top, Spores (1n) at the bottom, with the “Gametophyte phase” and “Sporophyte phase” bands interchanged
3.8 Objective Questions
Which is the dominant phase in Pteridophytes?
- Capsule
- Gametophyte
- Sporophyte
- Embryo
The tallest living gymnosperm among the following is ……….
- Sequoia sempervirens
- Taxodium mucronatum
- Zamia pygmaea
- Ginkgo biloba
In Bryophytes ……….
- Sporophyte and gametophyte generations are independent
- Sporophyte is partially dependent upon gametophyte
- Gametophyte is dependent upon sporophyte
- Inconspicuous gametophyte is present
A characteristic of Angiosperm is ………
- Collateral vascular bundles
- Radial vascular bundles
- Seed formation
- Double fertilization
Angiosperms differ from Gymnosperms in having ……….
- Vessels in wood
- Autotrophic mode of nutrition
- Siphonogamy
- Enclosed seed
Assertion: Bryophytes are called amphibious plants. Reason: Bryophytes are found growing in water as well as on land. Examine both statements and state whether each is correct, and whether the reason correctly explains the assertion.
Assertion: Rhizoids in mosses are multicellular. Reason: Mosses are the advanced members of Bryophyta. State whether the assertion and reason are correct, and whether the reason explains the assertion.
Assertion: Gymnosperms show secondary growth. Reason: Cambium is present in the stems of gymnosperms. State whether both statements are correct and whether the reason explains the assertion.
Assertion: The gametophyte of angiosperms is much reduced. Reason: The angiosperm sporophyte is diploid, dominant, autotrophic and independent. State whether both statements are correct and whether the reason explains the assertion.
Match the following.
| Column I | Column II |
|---|---|
| a. Fucoxanthin | i. Red algae |
| b. Floridean starch | ii. Green algae |
| c. Pyrenoid with true starch | iii. Brown algae |
| d. Algin | iv. Cell wall material |