Chapter 9: Morphology of Flowering Plants
In the late 1700s, a German writer far better known for his poetry and plays spent his mornings walking among plants in his garden at Weimar. Johann Wolfgang von Goethe was held by a question most people never bother to ask: why should a petal, a sepal, a stamen and an ordinary green leaf look so completely different, when all of them sit on the same short stretch of stem?
He looked closely, season after season. He paid special attention to unusual flowers in which petals seemed caught halfway, part petal and part leaf. From such observations he reached a bold conclusion, published in his 1790 essay on the metamorphosis of plants: these parts are not separate inventions at all. They are versions of one basic organ — the leaf — reshaped for different duties. A flower, in this view, is not something entirely new. It is a shoot whose leaves have been redesigned.
Later botanists confirmed the heart of this idea. A thorn, a tendril, a fleshy onion scale, a swollen carrot, a breathing root poking out of mangrove mud — each is a familiar plant organ altered to solve one particular problem: climbing, defence, storage, or survival in difficult soil.
Morphology is the study of these designs. Learn to read a plant’s external form, and a garden stops being a mass of green and becomes a collection of solutions.
Diagram
Fig. 9.1 – Opening illustration: a central flowering plant with root, stem, leaf, flower and fruit labelled, surrounded by small insets of modified organs (carrot, potato, cactus pad, pea tendril, mangrove breathing root, onion bulb) each linked by an arrow to the normal organ it is derived from
This chapter will help you answer each of these questions.
9.1 Angiosperms
Our earth carries an enormous diversity of plants, from mosses no taller than a fingernail to forest trees. Among all of them, the flowering plants dominate the plant world today, mainly because they are so well adapted to the conditions in which they live.
These plants show considerable variation in their general external and internal characters with respect to their habitat — the kind of place where they naturally grow. Such variations are not accidental. They help the plant body carry out its different functions in that particular setting: a plant standing in water and a plant standing in dry sand simply cannot afford the same design.
You have studied a broad classification of kingdom Plantae. Within it, plants that bear seeds are grouped as phanerogams, and angiosperms are the flowering plants belonging to this group. What sets them apart is that their seeds are never naked — each seed develops enclosed inside a fruit that forms from the ovary after fertilisation.
Grouping Angiosperms by Habitat
Because habitat shapes form so strongly, angiosperms can be classified into different types on the basis of the habitat they grow in. The five habitat groups are set out in FC 9.1.
Flowchart
FC 9.1 – Angiosperm classification based on habitat: Angiosperms → branches into (i) Hydrophytes – aquatic habitat, e.g. Hydrilla; (ii) Xerophytes – regions with scanty or no rainfall like desert, e.g. Opuntia; (iii) Psammophytes – sandy soil, e.g. Elymus; (iv) Lithophytes – rock, e.g. Cladopus, Dalzellia, Couchidium; (v) Halophytes – saline soil, e.g. mangrove plants like Rhizophora
Hydrophytes are the plants growing in an aquatic habitat, such as Hydrilla. Xerophytes grow in regions with scanty rainfall or none at all, as in deserts; Opuntia is a familiar example. Psammophytes grow in sandy soil, such as Elymus, where the ground is loose and drains water almost at once.
Lithophytes manage to grow on rock, with almost no soil to hold them — Cladopus, Dalzellia and Couchidium belong here. Halophytes grow in saline soil; the mangrove plants such as Rhizophora are the best-known members of this group.
From Seed to New Plant
In angiosperms the seed does not germinate the moment it is shed. It germinates only under favourable environmental conditions — suitable moisture, temperature and air — and then produces a seedling, which grows on to develop into a new plant. This waiting is itself an adaptation: it prevents a young seedling from appearing in a season it could not survive.
The complete cycle is summarised in FC 9.2, and every structure named in it is studied in detail in the sections that follow.
Flowchart
FC 9.2 – Life cycle of an angiosperm: seed → favourable environmental conditions → germination → seedling → mature plant with root, stem and leaf → flower → fruit → seed (returns to the start)
9.2 Morphology
Morphology is the study of the external form and structure of an organism. In a flowering plant, morphology shows us two kinds of structures: the vegetative structures — root, stem and leaf — which keep the plant alive and growing, and the reproductive structures — flower, fruit and seed — which produce the next generation.
A typical angiospermic plant is therefore built in two halves, as shown in Fig. 9.2. Everything below the soil forms the root system — the primary root and its secondary roots. Everything above the soil forms the shoot system — stem, leaves, flowers and fruits.
Diagram
Fig. 9.2 – Typical angiospermic plant with flower, fruit, stem and leaf labelled under the bracket “shoot system”, and primary roots and secondary roots labelled under the bracket “root system”
This division is not merely a matter of position. The two systems face completely different surroundings — one dark, dense and damp, the other bright, airy and dry — and each has evolved a form suited to its own set of problems. Keep this contrast in mind as you study the organs one by one.
The Root
The root is the descending axis of the plant body. It grows downward, beneath the soil surface, in the direction of gravity. Roots are generally non-green, cylindrical, and — unlike stems — they carry no nodes and no internodes.
The direction in which a root grows is decided by its responses to stimuli, called tropisms. The root is positively geotropic (it grows towards gravity) and positively hydrotropic (it grows towards water), but negatively phototropic (it grows away from light) and negatively aerotropic (it grows away from air). Together these four responses drive the root away from the open air and deeper into the moist soil, which is exactly where its work lies.
Typical Root Structure
A typical root is not uniform along its length. Moving up from the tip, it passes through a series of distinct regions, each doing one job, as shown in Fig. 9.3.
Diagram
Fig. 9.3 – Regions of root, labelled from tip upwards: region of root cap, meristematic region, region of cell elongation, region of root hairs, region of maturation
Flowchart
FC 9.3 – Regions of a root from tip upwards: root cap → meristematic region (region of cell division) → region of elongation → region of root hairs (piliferous/absorption zone) → region of cell maturation or differentiation
A parenchymatous multicellular structure in the form of a cap, present over the young growing root apex, is known as the root cap. The cells of the root cap secrete mucilage, a slimy substance that lubricates the passage of the root through the soil, so the delicate tip is not damaged as it pushes between hard soil particles.
Cells of the root cap also show the presence of starch granules. These heavy granules settle under gravity and so help in graviperception — the sensing of the direction of gravity — which in turn guides the geotropic movement of the root.
Usually a single root cap is present in plants. But in plants like Pandanus, or screw pine, multiple root caps are present. In hydrophytes the root caps are replaced by root pockets, as in Pistia and Eichhornia.
Because the root cap covers the tip, the growing apex of the root is subterminal in position — that is, the actual growing point lies a little behind the extreme tip, not at it. The apex of the root is a growing point about 1 mm in length, protected by the root cap. This region is called the region of cell division or meristematic region.
The structure is developed by compactly arranged, thin-walled, actively dividing meristematic cells. These cells bring about the longitudinal growth of the root by continuously adding new cells.
It is followed by the region of elongation. This region of cells is present just above the zone of cell division. The cells here are newly formed and show rapid elongation, which brings about the increase in length of the root. The cells of this region also help in the absorption of mineral salts.
A region of root hairs, also called the absorption zone or piliferous zone, lies above it and is made up of numerous hair-like outgrowths. The epiblema, or piliferous layer, is the outermost layer of the root, and it produces tubular, elongated, unicellular structures known as root hairs.
Root hairs are in close contact with soil particles and greatly increase the surface area available for the absorption of water. Root hairs are short-lived or ephemeral, and are replaced after every 10 to 15 days as the root tip advances into fresh soil.
The region of cell maturation or differentiation forms the major portion of the root; most of the root’s length is developed by this region. The cells of this region are quite impermeable to water because of their thick-walled nature.
The cells here show differentiation and form different types of tissues. This region helps in the fixation of the plant and in the conduction of absorbed substances. Development of lateral roots also takes place from this region.
Functions of the Root
Roots carry out several functions, which can be categorised into primary and secondary functions. The primary functions of the root are fixation or anchorage of the plant body in the soil, absorption of water and minerals from the soil, and conduction of the absorbed materials up to the base of the stem.
Secondary functions are the extra duties some roots take on — storage of food, respiration in waterlogged soil, support, and even vegetative propagation. These extra jobs are the reason roots become modified, as you will see shortly.
Types of Root
On the basis of origin, roots can be classified as tap roots or true roots, and adventitious roots. “Origin” here means the part of the plant from which the root first arises.
Flowchart
FC 9.4 – Classification of roots by origin: Roots → (i) Tap root (true root) – develops from the radicle → primary root → secondary roots → tertiary roots (tap root system, dicots); (ii) Adventitious root – develops from any part other than the radicle, e.g. base of stem, nodes or leaves (fibrous root system, monocots)
The root which develops from the radicle of an embryo during seed germination is known as the tap root or true root. The main root is called the primary root; its branches of the first order are called secondary roots, whereas branches of the second order are called tertiary roots. Pea, Bean and Sunflower are examples.
The main root with all its branches constitutes the tap root system. The tap root system is commonly seen in dicotyledonous plants, and because the primary root grows deep, such plants can draw water from far below the surface.
A root that develops from any part other than the radicle is known as an adventitious root. Such roots may develop from the base of the stem, from nodes, or from leaves.
In monocots, the radicle is short-lived, and from the base of the stem a thick cluster of equal-sized roots arises. This is the adventitious root system. Maize, Wheat and Sugarcane are examples. It is also known as the fibrous root system, as the roots look like thin fibres.
The growth of these roots is superficial — they spread out near the surface rather than going deep. Adventitious roots in some plants are used for vegetative propagation, as in Euphorbia and Carapichea ipecacuanha (Ipecac). The two systems are compared in Fig. 9.4.
Diagram
Fig. 9.4 – Types of root: (a) tap root system with a thick primary root bearing secondary and tertiary branches; (b) adventitious (fibrous) root system with a cluster of equal-sized roots from the stem base
Modification of Root
When roots have to perform some special type of function in addition to, or instead of, their normal function, they develop structural changes. Such roots are called metamorphosed roots.
Read every modification that follows with one question in mind: which function has been added, and how has the shape changed to make that function possible?
Modifications of Tap Root
When a tap root stores food, it becomes swollen and fleshy and also develops a definite shape. The main or primary root is the main storage organ, but sometimes the hypocotyl part of the embryo axis also joins the main root in storing food.
In such roots, the secondary roots remain thin. The stem in such cases remains reduced and discoid, and the leaves are radicle leaves borne close to the ground. On the basis of shape, swollen tap roots are classified as fusiform, conical and napiform.
The fusiform root is swollen in the middle and tapers towards both ends, forming a spindle-shaped structure, as in Radish (Raphanus sativus). The conical root is broad at its morphological base and narrows down gradually towards its apex, as in Carrot (Daucus carota). In the napiform root, the base of the root is highly swollen, almost spherical in shape, and abruptly narrows down towards its apex, as in Beet (Beta vulgaris). The three shapes are compared in Fig. 9.5.
Diagram
Fig. 9.5 – Swollen tap roots: conical root (carrot), fusiform root (radish) and napiform root (beet) drawn side by side with the swollen region and tapering apex marked
Roots for Respiration
Halophytes are the plants which grow in saline swamps, marshy places and salt lakes. These plants produce a special kind of root called pneumatophores or breathing roots.
The main root system of these plants does not get sufficient air for respiration, as the soil is waterlogged and the spaces between soil particles are filled with water instead of air. Because of this, the mineral absorption of the plant also gets affected — an actively respiring root is needed to absorb minerals against a concentration gradient.
To overcome this problem, the underground roots develop special roots which are negatively geotropic, growing vertically upwards out of the mud. These roots are conical projections present around the main trunk of the plant, and are shown in Fig. 9.6.
Diagram
Fig. 9.6 – Respiratory roots: pneumatophores rising vertically out of waterlogged soil from horizontal underground roots, with lenticels marked on the exposed portion
The roots show the presence of lenticels, that is, minute pores for gaseous exchange. The name itself explains the structure: pneumatic means hollow and phore means stalk. Rhizophora, Avicennia, Sonneratia and Heritiera (ver. sundri) are examples.
Modifications of Adventitious Roots
Fibrous roots also show food storage like tap roots, but the main difference is that fibrous roots usually do not develop a definite shape. Because they arise in clusters rather than as one main axis, the stored food is distributed among several swollen roots.
These roots are further classified as simple tuberous, fasciculated tuberous, beaded and nodulose roots.
Simple tuberous roots become swollen and do not show a definite shape. They are produced singly. The roots arise from nodes over the stem and then penetrate into the soil, as in sweet potato or shakarkand (Ipomoea batatas).
A cluster of roots arising from one point, which becomes thick and fleshy due to storage of food, is known as a fasciculated tuberous root. These clusters are seen at the base of the stem, as in Dahlia and Asparagus. The types are shown together in Fig. 9.7.
Diagram
Fig. 9.7 – Tuberous roots: simple tuberous root (single swollen root), fasciculated tuberous root (cluster of swollen roots from one point) and moniliform root (swellings at regular intervals)
The beaded roots are also called moniliform roots. These roots show swellings at regular intervals, like the beads of a necklace, as in Spinacia oleracea (Indian Spinach).
The cluster of long slender roots which become enlarged at the tips, forming nodules, is known as nodulose roots, as in Arrow (Maranta root) and Amhaldi (Curcuma amada). Notice in Fig. 9.8 that the swelling here is confined to the very tip of each root.
Diagram
Fig. 9.8 – Nodulose root: slender adventitious roots with swollen, nodule-like tips, labelled “nodulose root”
Roots for Mechanical Support
Several adventitious roots take up the job of holding a heavy or weak shoot in place. These roots are usually thick, and they reach the ground from a part of the stem that is well above soil level.
Prop roots, also called columnar roots, arise from the horizontal branches of a tree like the Banyan tree (Ficus benghalensis) and grow vertically downwards till they penetrate the soil. These prop roots show secondary growth, become thick, and act like pillars to provide mechanical support to the heavy branches, as seen in Fig. 9.9.
Diagram
Fig. 9.9 – Banyan tree with numerous thick pillar-like prop roots descending from the horizontal branches into the soil
Stilt roots normally arise from a few lower nodes of a weak stem in some monocots, shrubs and small trees. They show obliquely downward growth, penetrating the soil, and provide mechanical support to the plant.
In the members of the family Poaceae, plants like Maize, Jowar and Sugarcane produce stilt roots in a whorl around the node, as shown in Fig. 9.10. These roots provide additional support to the plant body. In Screwpine or Pandanus (Kewada), stilt roots arise only from the lower surface of the obliquely growing stem for additional support, and these roots show multiple root caps.
Diagram
Fig. 9.10 – Maize plant roots: whorl of stilt roots arising obliquely from the lower nodes of the stem and entering the soil
Different climbers with weak stems produce roots at their nodes, by means of which they attach themselves to a support and thereby raise themselves above the ground. These are climbing roots, seen in Betel leaf or Pan, black pepper or Piper nigrum (Kali Mirch), and Pothos or money plant. Fig. 9.11 shows such roots gripping a wall.
Diagram
Fig. 9.11 – Climbing roots of Betel leaf: short roots arising at the nodes and clasping a vertical support, with “climbing roots” labelled
Clinging roots are tiny roots which develop along the internodes and show a disc at their tips, which exudes a sticky substance. This substance enables the plant to get attached to the walls of buildings. They do not damage the substratum, as they only stick to it and do not bore into it. English Ivy (Hedera helix) is an example, illustrated in Fig. 9.12.
Diagram
Fig. 9.12 – Clinging roots of English Ivy along the internodes, with adhesive discs pressed flat against a tree bark or wall
Plank roots, also called buttresses, often develop at the base of large trees and form plank-like extensions around the stem. Silk cotton and Pipal are examples. They are shown in Fig. 9.13.
Diagram
Fig. 9.13 – Plank roots or buttresses forming vertical, board-like extensions radiating from the base of a large tree trunk
Buoyant roots are roots developed at the nodes of aquatic herbs like Jussiaea repens. They become highly inflated and spongy, providing buoyancy and helping the plant to float, as in Fig. 9.14.
Diagram
Fig. 9.14 – Spongy buoyant roots at the nodes of an aquatic herb, shown swollen and floating at the water surface
Roots for Special Functions
Small epiphytic plants such as orchids growing on the branches of huge trees in dense rain forests are unable to obtain soil moisture, since they never touch the ground. They produce a specialised root to hang in the air.
These epiphytic roots are provided with a spongy, membranous, absorbent covering of the velamen tissue. The cells of velamen absorb moisture from the air. Velamen tissue is hygroscopic and shows porous walls. The roots may be silvery white or green, but are without a root cap, as seen in Fig. 9.15.
Diagram
Fig. 9.15 – Epiphytic roots of an orchid: hanging aerial roots covered with velamen labelled “epiphytic root (hanging root)”, along with clinging roots attaching the plant to the branch
Sucking roots or haustoria are specialised microscopic sucking roots developed by parasitic plants to absorb nourishment from the host. Viscum album is a partial parasite. It develops haustoria which penetrate into the xylem of the host plant for absorption of water and nutrients, while its own green leaves still make food.
In Cuscuta reflexa or Dodder (Amarvel), the haustoria penetrate the vascular strand and suck food from the phloem, and water and minerals from the xylem. Cuscuta is a leafless plant with a yellow stem. Since it has no chlorophyll of its own, it is a total parasite. Fig. 9.16 shows its coils wound around a host shoot.
Diagram
Fig. 9.16 – Sucking roots: yellow leafless stem of Cuscuta coiled around a host stem, with haustoria shown penetrating the host tissue in an enlarged inset
The Stem
The aerial part of the plant body is known as the shoot system, and the stem is the main axis of this shoot system. The stem is the ascending part of the plant body, which develops from the plumule of the embryo and bears reproductive units, that is, buds.
The stem is differentiated into nodes and internodes. A node is the point at which a leaf is attached, and an internode is the length of stem between two nodes. This simple feature is the surest way of telling a stem from a root.
In its tropisms the stem is the exact opposite of the root. It is usually positively phototropic, negatively geotropic and negatively hydrotropic — it climbs towards light and away from gravity and water, which carries its leaves into the open air where photosynthesis is possible.
The stem shows different types of buds — axillary, apical, accessory, and so on. At the nodes it produces dissimilar lateral organs such as leaves and flowers, and similar organs such as branches. The young stem is green and capable of photosynthesis. Fig. 9.17 shows these parts on a typical stem.
Diagram
Fig. 9.17 – Stem structure with apical bud, leaf, petiole, node, internode, axillary bud and stem labelled
The primary functions of the stem are to produce and support branches, leaves, flowers and fruits; to conduct water and minerals upwards; and to transport food to different plant parts.
Modification of Stem
The stem develops some modifications for additional or accessory functions. To perform such functions the stem shows different modifications, and these are grouped by where the modified stem lies: underground, sub-aerial, or aerial.
Underground Stems
In some herbaceous plants the stem develops below the soil surface, and is called an underground stem. The underground stem remains dormant during unfavourable conditions and, on the advent of favourable conditions, produces aerial shoots.
The underground stem is known to store food, to help in perennation — that is, surviving from one favourable season to the next — and to assist in vegetative propagation.
A rhizome is prostrate, dorsiventrally thickened and brownish in colour. It grows either horizontally or obliquely beneath the soil. The rhizome shows nodes and internodes, bears scale leaves, and carries terminal and axillary buds at the nodes, as labelled in Fig. 9.18.
Diagram
Fig. 9.18 – Rhizome of Ginger with internode, scale leaf, bud, node and adventitious roots labelled
The terminal bud, under favourable conditions, produces an aerial shoot which degenerates at the end of the favourable season. Growth of the rhizome then takes place with the lateral buds, and such growth is known as sympodial growth. Ginger (Zingiber officinale), Turmeric (Curcuma domestica) and Canna are examples.
In plants where the rhizome grows obliquely, the terminal bud itself brings about the growth of the rhizome. This is known as monopodial growth, as in Nymphea, Nelumbo (Lotus) and Pteris (Fern).
Stem tubers are special underground branches of the stem whose tips become irregularly swollen due to storage of food material, which is mostly starch. The presence of distinct nodes, but not internodes, classifies the tuber as a stem.
At the nodal part, scale leaves are present with axillary buds, commonly known as ‘eyes’. These ‘eyes’ can produce aerial shoots under favourable conditions, which is why tubers are propagated vegetatively. Potato (Solanum tuberosum) and Matalu (Helianthus tuberosus) are examples, and a sprouting potato is shown in Fig. 9.19.
Diagram
Fig. 9.19 – Tuber of potato showing ‘eyes’ with scale leaves and one eye sprouting into a green aerial shoot
A tuber has two distinct ends, namely the apical end and the basal end, called the rose end and the heel end respectively. The number of nodes and eyes is more towards the rose end.
A bulb is an underground spherical or pyriform (pear-shaped) stem. The stem itself is highly reduced and discoid — squeezed into a flat disc — and it bears a whorl of fleshy leaves.
The scale leaves or fleshy leaves show a concentric arrangement over the stem, and these store food material. Some outer scale leaves become thin and dry, forming a protective covering. The reduced stem produces adventitious roots at its base.
The bulb is of different types. A tunicated or layered bulb is made up of fleshy leaves arranged in a concentric manner with an outer dry scale leaf, as in Onion. In garlic the bulb is scaly, or compound tunicated; the fleshy scales, that is the cloves, are arranged in an overlapping pattern. Both are shown in Fig. 9.20.
Diagram
Fig. 9.20 – Bulbs: (a) scaly bulb, e.g. Garlic, showing overlapping cloves; (b) tunicate bulb, e.g. Onion, cut in half to show concentric fleshy scale leaves and the reduced discoid stem
A corm is a swollen underground spherical or subspherical internode of a vertically growing stem. It is a condensed structure with circular or ring-like nodes. The presence of axillary buds and scales is observed on it.
Adventitious buds are produced, which help in vegetative propagation, and adventitious roots are produced at the lower part of the stem. Colocasia (Arbi) and Amorphophallus (Zamikand or Elephant foot) are examples. Fig. 9.21 shows the old corm with a daughter corm developing beside it.
Diagram
Fig. 9.21 – Corm with soil surface, old corm, daughter corm, adventitious roots, node and underground stem labelled
Sub-Aerial Stems
Sub-aerial stems are generally weak or straggling stems growing over the ground, which need support for perpetuation. Sometimes these stems are found to grow beneath the soil surface as well. Thus they show contact with both air and soil.
Sub-aerial stems are meant for perennation and vegetative propagation. Scale leaves and axillary buds are present over the stem surface, and the axillary buds produce aerial shoots.
A trailer is a shoot that spreads over the ground without intervals. The branches are either flat, that is procumbent, or partly vertical, that is decumbent. Euphorbia and Tridax are examples, as shown in Fig. 9.22.
Diagram
Fig. 9.22 – Euphorbia: trailer with a weak stem spreading flat over the ground, with “ground” and “trailer weak stem” labelled
Runners are special narrow, prostrate or horizontal green branches which develop at the base of erect shoots, known as the crown. Runners spread in all directions to produce new crowns with a bunch of adventitious roots.
The presence of nodes with scale leaves and axillary buds is observed on a runner. Cynodon (Lawn grass), Centella (Hydrocotyl) and Oxalis are examples, illustrated in Fig. 9.23.
Diagram
Fig. 9.23 – Runner of Cynodon with internode, new plantlet, new root and adventitious roots labelled along a horizontal green branch
A stolon is a slender lateral branch arising from the base of the main axis. In some plants it is above the ground, as in wild strawberry. Primarily the stolon shows upward growth in the form of an ordinary branch, but when it bends and touches the ground, the terminal bud grows into a new shoot and produces adventitious roots, as in Mentha. Fig. 9.24 shows the daughter plant forming where the stolon meets the soil.
Diagram
Fig. 9.24 – Stolon: arching lateral branch from the crown of Mentha bending down to the soil and forming a daughter plant
A sucker is a non-green, runner-like branch of the stem which develops from the underground base of the shoot. It grows horizontally below the soil and finally comes above the soil surface to produce a new plant. A sucker can be termed an underground runner. Chrysanthemum and Banana are examples, as in Fig. 9.25.
Diagram
Fig. 9.25 – Sucker of Chrysanthemum: main stem with an underground horizontal branch emerging above the soil as a new shoot, with tap root and roots labelled
Offsets are one-internode-long runners in rosette plants, at ground or water level. An offset helps in vegetative propagation. Water hyacinth or Jal kumbhi (Eichhornia) and Pistia are examples, shown in Fig. 9.26.
Diagram
Fig. 9.26 – Offset: (a) Pistia with leaves, offset and roots; (b) Eichhornia with lamina, swollen petiole, spongy stem, offset and root pocket labelled
Aerial Modifications of Stem
The stem, or its vegetative part, may modify to carry out specialised functions above the ground. It develops various modifications for this purpose, and such modified stems are called metamorphosed stems. The different modifications can be discussed as follows.
A thorn is a modification of an apical or axillary bud. A thorn is hard, pointed and mostly straight, except in Bougainvillea, where it is curved and useful for climbing.
The thorn helps in reducing transpiration, since it replaces a leafy branch that would have lost water; and being hard and pointed, it provides a protection device against browsing or marauding animals. The apical bud develops into a thorn in Carrisa, whereas the axillary bud develops into a thorn in Duranta, Citrus and Bougainvillea, as seen in Fig. 9.27.
Diagram
Fig. 9.27 – Thorn: a hard pointed thorn arising in the axil of a leaf on a green stem, marked as a modified axillary bud
The modification of a stem into a leaf-like photosynthetic organ is known as a phylloclade. Being a stem, it possesses nodes and internodes. It is thick, fleshy and succulent, and contains mucilage for retaining water. Opuntia is the familiar flattened example; the phylloclade is cylindrical in Casuarina and ribbon-like in Muehlenbeckia. Fig. 9.28 shows the flattened green stem of a cactus.
Diagram
Fig. 9.28 – Phylloclade: flattened green succulent stem of Opuntia bearing spines at the areoles and a flower, with nodes marked
Cladodes are branches of limited growth, that is, one internode long, which perform the photosynthetic function. In such plants the true leaves are reduced to spines or scales. Asparagus is an example, shown in Fig. 9.29.
Diagram
Fig. 9.29 – Cladodes of Asparagus: needle-like one-internode-long green branches borne in the axil of a scaly leaf, with cladode and scaly leaf labelled
Cladophylls are leaf-like structures borne in the axil of a scale leaf. Such a structure has a floral bud and a scale leaf in the middle — that is, the upper half is leaf and the lower half is stem. Ruscus is the example, illustrated in Fig. 9.30.
Diagram
Fig. 9.30 – Cladophylls of Ruscus: flat leaf-like green structures each bearing a small flower and a scale leaf at its middle
Stem tendrils are thin, wiry, photosynthetic, leafless coiled structures. They give additional support to a developing plant, and tendrils have adhesive glands for fixation. Different buds may be the source of a tendril in different plants.
In Vitis quadrangularis the apical bud gets modified into a tendril, and further growth is carried out by the axillary bud; this branching pattern is termed dichotomous. In Passiflora the axillary bud gets modified into a tendril.
An extra axillary bud is one which grows outside the axil; this bud in Cucurbita gets modified into a tendril. Normally floral buds are destined to produce flowers, but in plants like Antigonon they produce tendrils instead.
In plants like Dioscorea, the axillary bud becomes fleshy and rounded due to the storage of food, and is called a bulbil. When a bulbil falls off, it produces a new plant and thus helps in vegetative propagation. Fig. 9.31 shows bulbils sitting in the leaf axils.
Diagram
Fig. 9.31 – Bulbils of Dioscorea: rounded fleshy axillary buds borne in the leaf axils of a twining stem
Flowchart
FC 9.5 – Modifications of the stem: Stem → (i) Underground → rhizome, stem tuber, bulb, corm; (ii) Sub-aerial → trailer, runner, stolon, sucker, offset; (iii) Aerial → thorn, phylloclade, cladode, cladophyll, stem tendril, bulbil
The Leaf
Leaves are the most important appendages of the plant, as they carry out photosynthesis and also help to remove the excess amount of water from the plant body by transpiration. A leaf develops from a leaf primordium, a small bulge produced by the shoot apex.
The leaf is a dorsiventrally flattened lateral appendage of the stem, and it is produced at the nodal region. It is thin, expanded and green due to the presence of photosynthetic pigments. It shows exogenous origin, that is, it arises from the outer tissues of the shoot apex.
The axil of a leaf shows the presence of an axillary bud. A leaf shows limited growth; it does not have an apical bud or a growing point, which is why a leaf reaches a certain size and then stops, while a stem can keep extending.
Typical Leaf Structure
A typical leaf shows the presence of three main parts: the leaf base or hypopodium, the petiole or mesopodium, and the leaf lamina, blade or epipodium. All three are labelled in Fig. 9.32.
Diagram
Fig. 9.32 – Structure of a typical leaf with apex, margin, vein, lamina (epipodium), midrib, axillary bud, petiole (mesopodium), stipules and leaf base (hypopodium) labelled
The point by which the leaf remains attached to the stem is known as the leaf base. The nature of the leaf base varies in different plants; it may be pulvinus, that is swollen, or sheathing, or ligulate.
In some plants, leaves possess a pair of lateral outgrowths called stipules. A leaf with stipules is said to be stipulate, and a leaf without stipules is exstipulate. Stipules are normally green protective structures that shield the young leaf while it is still folded in the bud.
The part of the leaf which connects the leaf lamina with the leaf base is known as the petiole. A leaf with a petiole is petiolate, and a leaf without a petiole is termed a sessile leaf. The petiole helps the lamina to get exposed to light, and also helps in conduction between the lamina and the stem.
The lamina or epipodium is the large, expanded, flat and green part of the leaf. The lamina surface plays an important role in photosynthesis, gaseous exchange and transpiration.
Anatomically, the leaf is either dorsiventral or isobilateral. A dorsiventral leaf has clearly different upper and lower surfaces and is common in dicots, while an isobilateral leaf has similar surfaces on both sides and is common in monocots.
Centric or cylindrical leaves are those in which both the surfaces of the leaf cannot be distinguished distinctly. The leaf lamina varies greatly in shape, margin and apex from one plant to another.
Leaf Venation
The arrangement of veins and veinlets in the leaf lamina is known as venation. Veins are responsible for the conduction of water and minerals as well as food. The structural framework of the lamina is developed by the veins, which act like the ribs of an umbrella and keep the thin blade spread out.
Broadly, venation may be reticulate, in which the veinlets form a net-like mesh and which is typical of dicots, or parallel, in which the main veins run side by side along the lamina and which is typical of monocots.
Types of Leaf
Based on the incision of the lamina, leaves are of two main types, namely simple and compound. The leaf with an entire lamina is called a simple leaf, and the leaf in which the lamina is divided into a number of leaflets is called a compound leaf.
Compound leaves are of two types.
In a pinnately compound leaf, the leaflets are present laterally on a common axis called the rachis, which represents the midrib of the leaf. The kinds of pinnately compound leaves are shown in Fig. 9.33 (a): paripinnate (Cassia), imparipinnate (Rosa), bipinnate (Caesalpinia), tripinnate (Moringa) and decompound (Coriandrum).
Diagram
Fig. 9.33 (a) – Types of compound leaf (pinnately compound): paripinnate (Cassia), imparipinnate (Rosa), bipinnate (Caesalpinia), tripinnate (Moringa) and decompound (Coriandrum), with leaflets and rachis marked
In a palmately compound leaf, all the leaflets are attached at the tip of the petiole, spreading out like the fingers of a palm. The types are shown in Fig. 9.33 (b): unifoliate (Citrus), bifoliate (Zorina), trifoliate (Oxalis), quadrifoliate (Marsilea) and multifoliate (Bombax).
Diagram
Fig. 9.33 (b) – Types of compound leaf (palmately compound): unifoliate (Citrus), bifoliate (Zorina), trifoliate (Oxalis), quadrifoliate (Marsilea) and multifoliate (Bombax), with leaflets and petiole marked
Modification of Leaves
Apart from photosynthesis, the leaf also performs transpiration, gaseous exchange and perception of light for flowering. However, leaves may undergo modifications to perform several other functions. According to the modification, there are different types of leaves, described below.
Sometimes the entire leaf is modified into spines, as in Opuntia; or the margin of the leaf becomes spiny, as in Agave; or the stipule modifies into a spine, as in Acacia. These leaf spines check the rate of transpiration by cutting down the exposed surface, or protect the plant from grazing. Zizyphus is another example, and leaf spines are shown in Fig. 9.34.
Diagram
Fig. 9.34 – Leaf spines: a twig bearing compound leaves with a pair of stipular spines at the base of each leaf
In some weak stems, for providing additional support, a leaf, a leaflet or another part modifies to produce a thin, green, wiry, coiled structure called a leaf tendril. It helps the plant in climbing.
Which part of the leaf becomes the tendril differs from plant to plant, as shown in Fig. 9.35: the whole leaf tendril, as in Lathyrus; the leaflet tendril, as in Pisum sativum; the leaf tip tendril, as in Gloriosa; and the stipular tendril, as in Smilax.
Diagram
Fig. 9.35 – Leaf tendrils: whole leaf tendril (Lathyrus), leaflet tendril (Pisum sativum), leaf tip tendril (Gloriosa) and stipular tendril (Smilax)
In plants like Bignonia unguis-cati (Cat’s nail), the terminal leaflets get modified into three stiff, curved and pointed hooks, which are used to cling over the bark of a tree. These leaf hooks are shown in Fig. 9.36.
Diagram
Fig. 9.36 – Leaf hooks of Bignonia unguis-cati: terminal leaflets modified into three stiff curved hooks gripping tree bark
When the petiole of a leaf becomes flat, green and leaf-like, it is called a phyllode. In Acacia auriculoformis the normal leaf is bipinnately compound and falls off soon; the petiole then modifies itself into a phyllode, as seen in Fig. 9.37. It is a xerophytic adaptation, since a narrow, tough phyllode loses far less water than a broad compound lamina.
Diagram
Fig. 9.37 – Phyllode of Acacia auriculoformis: flattened green leaf-like petioles borne on a twig, replacing the fallen bipinnate leaves
Flowchart
FC 9.6 – Modifications of the leaf: Leaf → (i) leaf spines (whole leaf, margin or stipule) → protection and reduced transpiration; (ii) leaf tendrils (whole leaf, leaflet, leaf tip or stipule) → climbing; (iii) leaf hooks (terminal leaflets) → clinging; (iv) phyllode (petiole) → photosynthesis in dry habitats
Phyllotaxy
Phyllotaxy is the arrangement of leaves on the stem and branches in a specific manner. It enables each leaf to get sufficient light, since a leaf shaded by the leaf above it contributes little to the plant.
The types are shown in Fig. 9.38. In alternate phyllotaxy, a single leaf arises from each node, as in Mango. In whorled phyllotaxy, many leaves arise from each node, as in Nerium.
Diagram
Fig. 9.38 – Types of phyllotaxy: alternate (Mango), whorled (Nerium), opposite decussate (Calotropis) and opposite superposed (Jamun), each showing the leaves at successive nodes
In opposite decussate phyllotaxy, a pair of leaves arises from each node and the consecutive pair is placed at right angles to it, as in Calotropis. In opposite superposed phyllotaxy, a pair of leaves arises from each node and the consecutive pair is arranged just above the previous one, as in Jamun.
Inflorescence
A specialised axis or branch over which flowers are produced or borne in a definite manner is known as an inflorescence. An inflorescence has two parts: the peduncle, which is the axis, and the flowers borne on it. There are basic types of inflorescence, described below.
In an inflorescence, all the flowers do not mature at the same time. Because of this, the chances of pollination increase, a large number of flowers can be pollinated in a single visit by a pollinator, and the massed flowers also make the plant more attractive from a distance.
In a racemose inflorescence, the growth of the peduncle is infinite or unlimited. The apical bud is free for continuous growth, because it is not used up in making a flower. The older flowers are therefore at the base and the youngest at the top.
The flowers are borne in acropetal succession, that is, from the base towards the apex. When the peduncle is disc-shaped, the order of opening is centripetal — from the outside towards the centre. A racemose inflorescence is shown in Fig. 9.39.
Diagram
Fig. 9.39 – Racemose inflorescence: an unlimited peduncle bearing mature flowers at the base and flower buds towards the apex, with peduncle, mature flower and flower bud labelled
In a cymose inflorescence, the growth of the peduncle is finite or limited, because the apical meristem terminates into a flower. The oldest flower is therefore at the apex.
The flowers are borne in basipetal succession, that is, from the apex downwards. If the peduncle is compressed from the top, the order of opening is centrifugal — from the centre towards the outside. A cymose inflorescence is shown in Fig. 9.40.
Diagram
Fig. 9.40 – Cymose inflorescence: a limited peduncle ending in the oldest mature flower at the apex, with younger flower buds borne below it
The Flower
A flower is a highly modified and condensed shoot meant for sexual reproduction. Every part of it — sepal, petal, stamen and carpel — is a modified leaf borne on an extremely shortened axis.
On the basis of position, a flower can be axillary, arising in the axil of a leaf, or terminal, ending a branch. In a typical flower the thalamus consists of four compactly arranged nodes and three highly condensed internodes. From each node of the thalamus, a circle or whorl of modified leaves is produced.
A flower may or may not show the presence of a bract at the base of the pedicel, or over the pedicel. Such a flower is said to be bracteate or ebracteate respectively. A flower with a pedicel is said to be a pedicellate flower, and one without a pedicel is called a sessile flower. The parts of a typical flower are shown in Fig. 9.41.
Diagram
Fig. 9.41 – Typical flower (diagrammatic), longitudinal section, with stigma, style, ovary (together labelled carpel or pistil), ovule, anther, filament, petal, sepal, thalamus and pedicel labelled
A flower with bilateral symmetry is called a zygomorphic flower, as in Sweet pea, where there is only one plane along which, if the flower is cut, the two halves are mirror images. A flower with radial symmetry, which can be cut along a diameter in any plane and still give mirror images, is called an actinomorphic flower, as in Hibiscus.
Terms Used to Describe Flowers
The following terms are used constantly in floral descriptions, and are worth learning together.
A complete flower shows the presence of all four floral whorls, while an incomplete flower shows the absence of any one of the floral whorls.
A perfect flower has both androecium and gynoecium present in it; it is also called a monoclinous or bisexual flower. An imperfect flower has any one reproductive whorl present in it; it is also called a diclinous flower or unisexual flower.
A unisexual flower can be either staminate (male) or pistillate (female); this condition is also called dicliny. A flower in which both reproductive whorls are absent is said to be a neuter flower, as in the ray florets of Tagetus (marigold).
In a monoecious plant, male and female reproductive flowers are borne on the same plant, as in Maize. In a dioecious plant, only one type of unisexual flower is present on a plant, as in date palm and papaya.
Insertion of Floral Whorls
The position and arrangement of the rest of the floral whorls with respect to the gynoecium on the thalamus is known as the insertion of floral whorls. In a typical flower the thalamus consists of four compactly arranged nodes and three internodes, and the shape of the thalamus decides the insertion of the floral whorls. The three conditions are compared in Fig. 9.42.
Diagram
Fig. 9.42 – Insertion of floral whorls: longitudinal sections of hypogynous, perigynous and epigynous flowers, showing the position of the ovary relative to the other whorls in each
In hypogyny, a convex or conical thalamus is present in the flower, so the ovary occupies the highest position while the other floral parts are below the ovary. Such an ovary is said to be superior, and the flower is called a hypogynous flower, as in Brinjal, Mustard and China rose. It is denoted as \(\underline{G}\) in the floral formula.
In perigyny, a cup-shaped or saucer-shaped thalamus is present in the flower, so the ovary and the other floral parts occupy about the same position. Such an ovary is said to be semi-superior or semi-inferior. All the floral whorls are at the rim of the thalamus. The flower is perigynous, as in Rose, Pea and Bean. It is denoted as \(\overline{G}\) in the floral formula.
In epigyny, a closed cup-like thalamus completely encloses the ovary and may show fusion with the wall of the ovary. The other floral parts then occupy a superior position and the ovary becomes inferior. Such a flower is said to be an epigynous flower, as in Sunflower and Guava. It is denoted as \(\overline{G}\) in the floral formula.
Floral Parts and Their Structure
All floral parts are developed from the thalamus, from its different nodes. From each node of the thalamus a circle or whorl of modified leaves is produced. The thalamus is also called the torus or receptacle. The thalamus is green in colour, hence it can perform the process of photosynthesis.
The calyx (K) is the outermost floral whorl, and its individual members are known as sepals. Sepals are usually green in colour and perform photosynthesis. If all the sepals are united, the condition is gamosepalous, and if they are free, the condition is called polysepalous. A gamosepalous calyx is found in China rose and a polysepalous calyx is found in Brassica.
The main function of the sepals is to protect the inner floral parts in the bud condition. Sometimes sepals become brightly coloured — petaloid sepals — and attract insects for pollination, as in Mussaenda.
The calyx, that is the sepals, may also modify into stiff, hairy structures called pappus. Such a persistent calyx helps in the wind dispersal of fruits, as in Tridax and Sonchus.
| Type of calyx | Nature of sepals | Example |
|---|---|---|
| Caducous | Sepals fall off as soon as the flower bud opens | Argemone (Poppy) |
| Deciduous | Sepals survive till fruit formation, that is, till the withering of petals | Lotus, Mustard |
| Persistent | Sepals remain even after fruit formation | Brinjal, Pea, etc. |
The corolla (C) is the second floral whorl from the outer side, and it is variously coloured. The individual member is called a petal. Petals may be sweet to taste, and may possess scent, odour, aroma or fragrance.
The condition in which the petals are free is said to be polypetalous, as in Rose, and if they are fused, it is called gamopetalous, as in Datura. The main function of the corolla is to attract different agencies for pollination.
Many times the calyx and corolla remain undifferentiated. Such a member is known as a tepal, and the whorl of tepals is known as the perianth (P).
If all the tepals are free, the condition is called polyphyllous, and if they are fused, the condition is called gamophyllous. A sepaloid perianth shows green tepals, while a petaloid perianth shows brightly coloured tepals, as in Lily, Amaranthus and Celosia.
The perianth protects the other floral whorls. Petaloid tepals help in pollination, and sepaloid tepals can perform photosynthesis.
Aestivation
Aestivation is the mode of arrangement of sepals, petals or tepals in a flower bud with respect to the members of the same whorl. The four types are described in Table 9.43.
Diagram
Table 9.43 – Arrangement of sepals, petals or tepals: four labelled pairs of drawings (side view of the bud and top view of the whorl) showing valvate, twisted, imbricate and vexillary aestivation
| Type | Arrangement of members | Example |
|---|---|---|
| Valvate | Margins of sepals or petals remain either in contact or lie close to each other, but do not overlap | Calyx of Datura, Calotropis |
| Twisted | One margin of each sepal or petal is directed inwards and is overlapped, while the other margin is directed outwards and overlaps the margin of the adjacent member | Corolla of China rose, Cotton |
| Imbricate | One of the sepals or petals is internal and is overlapped at both margins; one is external, that is completely outside the rest of the members; the remaining ones overlap and get overlapped | Calyx of Cassia, Bauhinia |
| Vexillary | Corolla is butterfly-shaped and consists of five petals: the outermost and largest is the standard or vexillum, two lateral partly overlapping petals are the wings, and two smaller inner fused petals form the boat-shaped keel | Corolla of Pisum sativum |
An epicalyx is an additional whorl of sepal-like structures formed by bracteoles, which occurs on the outside of the calyx. These are 5 to 8 in number. It is a characteristic feature of the family Malvaceae and is protective in function, as in Ladies finger.
Androecium
The androecium (A) is the third floral whorl from the outer side. The androecium is the male reproductive part of a flower, and its individual member is known as a stamen. If all the stamens are free, the condition is polyandrous.
Two terms are needed before going further. Cohesion is fusion between members of the same whorl, while adhesion is fusion between members of different whorls.
A typical stamen shows three different parts. The anther is terminal in position and produces pollen grains in its locules. It is usually bilobed, and each lobe contains two locules, hence the anther is a tetralocular or tetrasporangiate structure, as in Datura.
In some plants there is a single anther lobe having two locules. At other times an anther lobe has only one locule; it is then called monothecous, formed due to the fusion of two locules, as in Hibiscus.
The filament is the stalk of the stamen and bears the anther at its tip. It raises the anther to a proper height for easy dispersal of the pollen grains.
The connective is in continuation of the filament. It is similar to a midrib, and it connects the two anther lobes together and also connects the anther with the filament.
Cohesion of stamens occurs when stamens are united by their filaments while the anthers remain free; this condition is adelphy. The forms of adelphy are shown in Fig. 9.44: monoadelphous, diadelphous and polyadelphous, depending on whether the filaments are united into one bundle, two bundles, or many bundles.
Diagram
Fig. 9.44 – Cohesion of stamens: monoadelphous, diadelphous and polyadelphous conditions, together with syngenesious and syngeny, drawn side by side
Adhesion of stamens occurs when the stamens are united with the petals or tepals; they are then described as epipetalous, as in Datura, or epiphyllous, as in Lily.
When anthers are united and filaments are free, it is known as syngeny, as in Sunflower. Stamens fused by both filaments and anthers are in the synandrous condition, as in Cucurbita.
Gynoecium
The gynoecium (G) is the female reproductive part of the flower and is innermost in position. It is also known as the pistil. The individual member of the gynoecium is known as a carpel.
The number of carpels may be one to many. If all the carpels are fused, the condition is described as syncarpous, and if they are free, the condition is described as apocarpous. A polycarpellary gynoecium can be bicarpellary, with two carpels as in Datura; tricarpellary, with three carpels as in Cucurbita; pentacarpellary, with five carpels as in Hibiscus; and so on.
A typical carpel consists of three parts, namely stigma, style and ovary. The stigma is the terminal part of the carpel which receives pollen grains during pollination. It helps in the germination of the pollen grain. The stigma shows variation in structure to suit the pollinating agent — feathery in wind-pollinated flowers, sticky in insect-pollinated ones.
The style is a narrow, thread-like structure that connects the ovary with the stigma. The ovary is the basal, swollen, fertile part of the carpel. Ovules are produced in an ovary on a soft fertile tissue called the placenta.
Placentation
Placentation is the mode of arrangement of ovules on the placenta within the ovary. The five types are shown in Fig. 9.45.
Diagram
Fig. 9.45 – Types of placentation: transverse sections of ovaries showing marginal, axile, parietal, basal and free central placentation, with the placenta and ovules marked in each
In marginal placentation, the ovules are placed on the fused margins of a unilocular ovary, as in Pea and Bean. In axile placentation, the ovules are placed on the central axis of a multilocular ovary, as in China rose and Cotton.
In parietal placentation, the ovules are placed on the inner wall of the unilocular ovary of a multicarpellary syncarpous gynoecium, as in Papaya and Cucumber. In basal placentation, a single ovule is present at the base of a unilocular ovary, as in Sunflower, Rice and Wheat. In free central placentation, the ovules are borne on a central axis which is not attached to the ovary wall, as in Argemone, Dianthus and Celosia.
The Fruit
Angiosperms produce fruit after fertilisation, from the ovary. The wall of the ovary becomes the wall of the fruit, and the ovules inside it become the seeds.
Sometimes a fruit is produced from the ovary without fertilisation. Such fruits are called parthenocarpic fruits and the phenomenon is called parthenocarpy. Cultivated varieties of Banana and seedless Grapes are examples — and since no fertilisation has occurred, such fruits are seedless.
The fruit that develops only from the ovary is a true fruit, as in Mango, shown in Fig. 9.46. The fruit which develops from floral parts other than the ovary is called a false fruit or pseudocarp, as in Apple, shown in Fig. 9.47, where the fleshy edible part is the swollen thalamus.
Diagram
Fig. 9.46 – Mango: whole fruit and its vertical section showing epicarp, mesocarp and endocarp, with the stony endocarp (pit) labelled
Diagram
Fig. 9.47 – Apple: vertical and transverse sections showing thalamus forming the fleshy part, with seed, endocarp and mesocarp labelled
A typical true fruit has a protective wall, the pericarp, and one or more seeds. In fleshy fruits, the pericarp is further divided into an outer epicarp, a middle mesocarp and an inner endocarp, as in Mango.
Fruits can develop from one ovary of one flower. Such fruits are simple fruits. Simple fruits are further classified on the basis of their pericarp. Those having a thin pericarp are dry fruits, but those with a thick pericarp are fleshy fruits.
In dry fruits the pericarp becomes dry and thin. It breaks open, that is, it is dehiscent, at maturity. But in some others it does not break open, and is indehiscent. Achene (Mirabilis), Caryopsis (Maize) and Cypsela (Sunflower) are indehiscent fruits. Capsule (Lady’s finger) and legume (Pea) are dry dehiscent fruits.
Among fleshy fruits, the berry (Tomato) has a very soft pericarp, but the drupe (Coconut) has a stony endocarp.
Many ovaries of an apocarpous gynoecium can form one fruit. Such fruits are aggregate fruits. Aggregate fruits are a collection, or etario, of many varieties. Accordingly, they can be an etario of achenes (Strawberry), an etario of berries (Custard apple), or an etario of follicles (Calotropis).
Many ovaries of many flowers of one inflorescence can form one fruit. Such fruits are composite fruits. These fruits develop from one inflorescence. The one which develops from a hypanthodium inflorescence is a syconus (fig). Sorosis (Pineapple) develops from a catkin inflorescence.
Flowchart
FC 9.7 – Classification of fruits: Fruit → (i) Simple fruit (one ovary of one flower) → dry (thin pericarp) → dehiscent, e.g. capsule, legume; or indehiscent, e.g. achene, caryopsis, cypsela; and → fleshy (thick pericarp), e.g. berry, drupe; (ii) Aggregate fruit (many ovaries of one apocarpous gynoecium) → etario of achenes, etario of berries, etario of follicles; (iii) Composite fruit (many ovaries of many flowers of one inflorescence) → syconus, sorosis
The Seed
A seed is a reproductive unit that is developed from a fertilised, mature ovule. The seed is composed of a seed coat and an embryo, with or without endosperm. The parts are shown in Fig. 9.50.
Diagram
Fig. 9.50 – Structure of a seed in vertical section, with testa, tegmen, hilum, cotyledon, plumule, radicle, epicotyl, hypocotyl and endosperm labelled
The embryo contains the radicle, the plumule, and one or two cotyledons. The outermost covering of a seed is called the seed coat, which shows an outer layer called the testa and an inner layer called the tegmen.
The hilum is a scar on the seed coat through which the seed was attached to the fruit. The embryo of a seed is enclosed within the seed coat.
The embryonal axis consists of the radicle and the plumule. The part of the embryonal axis between the cotyledon and the plumule is the epicotyl, while the part between the cotyledons and the radicle is the hypocotyl.
The endosperm formed in the seed is a nutritive tissue. The food stored in the endosperm is partially or completely used during embryo formation. When it is partially used up, the resultant seed is endospermic, as in castor. When it is completely used up, the resultant seed is non-endospermic, as in pea, where the food has been transferred into the two thick cotyledons.
9.3 Study of Some Important Families
Families are groups of plants having very distinguished common characters. Two plants may look quite different in size and habit, yet if their flowers are built on the same plan — the same number of whorls, the same fusions, the same type of ovary and fruit — they are placed in the same family.
This is why family study in botany is really the study of the flower. Vegetative parts change quickly with habitat, but floral structure is conservative and therefore far more reliable for classification.
Family Fabaceae
The plants of this family are either trees, shrubs or herbs. The pea plant belongs to this family. The root shows root nodules — small swellings that house nitrogen-fixing bacteria, which is why members of this family enrich the soil they grow in. Pea is an erect climber.
The leaves are pinnately compound and are arranged in alternate phyllotaxy. The inflorescence is of the racemose-raceme type, so the peduncle keeps growing and the flowers open in acropetal succession.
The flowers are bisexual and zygomorphic. The calyx has five fused sepals, that is, it is gamosepalous, arranged in imbricate aestivation. The corolla has five free petals, that is, it is polypetalous, arranged in vexillary aestivation.
The petals are unequal in size. The largest petal is the vexillum, two free small petals are the wings, and two fused smallest petals form the keel. Fig. 9.48 shows these petals separated out, along with the habit of the plant.
Diagram
Fig. 9.48 – Pea plant details: habit sketch with tendrils, leaflets, flower and foliaceous stipule; L.S. of flower with stigma, style, ovary, petal and sepal labelled; and corolla opened out to show standard, wings and keel
The androecium has ten stamens arranged in the diadelphous [(9)+1] condition — nine filaments united into one bundle and the tenth standing free. The gynoecium is monocarpellary. The unilocular ovary is superior, with many ovules on a marginal placenta.
The ovary develops into the legume type of fruit, and the seeds are non-endospermic, since the food has been transferred into the cotyledons.
Floral Formula and Floral Diagram of Fabaceae
\[\% \; ⚥ \; K_{(5)} \; C_{1+2+(2)} \; A_{(9)+1} \; \underline{G}_{1}\]
Read the formula symbol by symbol. The sign \(\%\) shows that the flower is zygomorphic, and ⚥ shows that it is bisexual. \(K\) stands for calyx, \(C\) for corolla, \(A\) for androecium and \(G\) for gynoecium. A number in brackets means those members are fused, while a number without brackets means they are free. The line below \(G\) shows that the ovary is superior.
So \(K_{(5)}\) reads as five fused sepals, \(C_{1+2+(2)}\) as one standard plus two wings plus two fused keel petals, \(A_{(9)+1}\) as nine fused stamens plus one free stamen, and \(\underline{G}_{1}\) as one carpel with a superior ovary.
Diagram
Fig. 9.51 – Floral diagram of Fabaceae with the mother axis at the top, five sepals, the five petals in vexillary arrangement, ten stamens in the (9)+1 grouping, and a single carpel with marginal placentation at the centre
Family Solanaceae
The plants of this family are herbs, shrubs or small trees. The Datura plant belongs to this family. The root shows a tap root system. The stem is erect, woody and branched, and in some plants it is covered by hairy structures. In potato, the stem is an underground tuber.
The leaves are simple, arranged in alternate phyllotaxy, with reticulate venation. The inflorescence is of the cymose-solitary type. The flowers are solitary, bisexual and actinomorphic, as shown in Fig. 9.49.
Diagram
Fig. 9.49 – Datura plant: branch bearing a simple leaf, a large funnel-shaped solitary flower and a spiny fruit, with flower and fruit labelled
The calyx has five fused sepals, that is, it is gamosepalous, arranged in valvate aestivation, and it is persistent — the sepals remain even after the fruit has formed. The corolla has five fused petals, that is, it is gamopetalous, arranged in contorted aestivation, which is another name for the twisted arrangement you met in Table 9.43.
The androecium has five stamens which are free from one another but epipetalous, that is, they show adhesion by being attached to the petals. The gynoecium is bicarpellary and syncarpous.
The bilocular ovary is superior, with many ovules arranged in axile placentation on a swollen placenta. The ovary develops into a berry or a capsule type of fruit, and the seeds are endospermic.
Floral Formula and Floral Diagram of Solanaceae
\[\oplus \; ⚥ \; K_{(5)} \; C_{(5)} \; A_{5} \; \underline{G}_{(2)}\]
Here the sign \(\oplus\) shows that the flower is actinomorphic and ⚥ again shows that it is bisexual. \(K_{(5)}\) reads as five fused sepals, \(C_{(5)}\) as five fused petals, \(A_{5}\) as five free stamens, and \(\underline{G}_{(2)}\) as two fused carpels with a superior ovary.
Diagram
Fig. 9.52 – Floral diagram of Solanaceae with the mother axis at the top, five fused sepals, five fused petals in contorted arrangement, five epipetalous stamens, and a bicarpellary ovary showing axile placentation with the carpels placed obliquely
9.4 Summary
Angiosperms are seed-bearing flowering plants (phanerogams) in which the seeds develop enclosed within a fruit formed from the ovary. They dominate the plant world because they are well adapted to a wide range of conditions.
On the basis of habitat, angiosperms are classified as hydrophytes (aquatic, e.g. Hydrilla), xerophytes (scanty or no rainfall, e.g. Opuntia), psammophytes (sandy soil, e.g. Elymus), lithophytes (rock, e.g. Cladopus, Dalzellia, Couchidium) and halophytes (saline soil, e.g. Rhizophora). A seed germinates under favourable conditions to give a seedling, which grows into a new plant.
Morphologically, a plant shows vegetative structures — root, stem and leaf — and reproductive structures — flower, fruit and seed. The parts below the soil form the root system and those above it form the shoot system.
The root is the descending axis: positively geotropic and hydrotropic, negatively phototropic and aerotropic, non-green, without nodes and internodes.
From the tip upwards, a root shows the root cap, the meristematic region (region of cell division), the region of elongation, the region of root hairs (piliferous or absorption zone) and the region of cell maturation or differentiation. Root hairs are unicellular outgrowths of the epiblema and are replaced every 10 to 15 days.
Primary functions of the root are fixation, absorption of water and minerals, and conduction of absorbed materials to the stem base. Storage, respiration, support and propagation are secondary functions.
On the basis of origin, roots are tap roots (from the radicle, in dicots) or adventitious roots (from any part other than the radicle, fibrous, in monocots).
Tap roots modify for food storage into fusiform (radish), conical (carrot) and napiform (beet) forms, and into pneumatophores or breathing roots with lenticels in halophytes such as Rhizophora and Avicennia.
Adventitious roots modify for storage (simple tuberous, fasciculated tuberous, beaded or moniliform, nodulose), for mechanical support (prop, stilt, climbing, clinging, plank or buttress, buoyant) and for special functions (epiphytic roots with velamen, sucking roots or haustoria in parasites such as Cuscuta).
The stem is the ascending axis developing from the plumule. It bears nodes and internodes and different types of buds, is positively phototropic and negatively geotropic, and conducts water, minerals and food while supporting leaves, flowers and fruits.
Stems modify underground (rhizome, stem tuber, bulb, corm), sub-aerially (trailer, runner, stolon, sucker, offset) and aerially (thorn, phylloclade, cladode, cladophyll, stem tendril, bulbil). Presence of nodes, scale leaves and buds identifies a modified stem.
A typical leaf has a leaf base or hypopodium, a petiole or mesopodium and a lamina or epipodium. Leaves may be stipulate or exstipulate, petiolate or sessile, dorsiventral (dicots) or isobilateral (monocots).
Venation is the arrangement of veins and veinlets in the lamina; veins conduct water, minerals and food and give the lamina its framework.
On the basis of incision of the lamina, leaves are simple or compound; compound leaves are pinnately compound (leaflets on a rachis) or palmately compound (leaflets at the tip of the petiole).
Leaves modify into spines, tendrils, hooks and phyllodes. Phyllotaxy — the arrangement of leaves on the stem — may be alternate, whorled, opposite decussate or opposite superposed.
An inflorescence is a specialised axis bearing flowers, made of peduncle and flowers. Racemose inflorescence has an unlimited peduncle with acropetal succession and centripetal opening; cymose inflorescence has a limited peduncle with basipetal succession and centrifugal opening.
A flower is a highly modified condensed shoot for sexual reproduction. It may be bracteate or ebracteate, pedicellate or sessile, zygomorphic (bilateral symmetry) or actinomorphic (radial symmetry).
Depending on the shape of the thalamus, insertion of floral whorls is hypogynous (superior ovary), perigynous (semi-inferior ovary) or epigynous (inferior ovary).
The four whorls are calyx (sepals; gamosepalous or polysepalous), corolla (petals; gamopetalous or polypetalous), androecium (stamens, each with anther, filament and connective) and gynoecium (carpels, each with stigma, style and ovary). Undifferentiated calyx and corolla form the perianth of tepals.
Aestivation, the arrangement of sepals or petals in the bud, is valvate, twisted, imbricate or vexillary. Cohesion of stamens gives adelphy and syngeny; adhesion gives epipetalous and epiphyllous conditions.
Placentation, the arrangement of ovules on the placenta, is marginal, axile, parietal, basal or free central.
Fruits develop from the ovary after fertilisation; without fertilisation they are parthenocarpic. A true fruit develops only from the ovary (mango), a false fruit from other floral parts (apple). Fruits are simple (dry dehiscent, dry indehiscent or fleshy), aggregate or composite.
A seed develops from a fertilised mature ovule and consists of seed coat (testa and tegmen), embryo (radicle, plumule and cotyledons) and, in endospermic seeds such as castor, endosperm. Non-endospermic seeds such as pea store food in the cotyledons.
Family Fabaceae: root nodules, pinnately compound alternate leaves, racemose inflorescence, bisexual zygomorphic flowers, gamosepalous imbricate calyx, polypetalous vexillary corolla, diadelphous [(9)+1] androecium, monocarpellary superior ovary with marginal placentation, legume fruit, non-endospermic seeds — floral formula \(\% \; ⚥ \; K_{(5)} \; C_{1+2+(2)} \; A_{(9)+1} \; \underline{G}_{1}\).
Family Solanaceae: tap root, simple alternate leaves with reticulate venation, cymose-solitary inflorescence, bisexual actinomorphic flowers, gamosepalous valvate persistent calyx, gamopetalous contorted corolla, five free epipetalous stamens, bicarpellary syncarpous superior ovary with axile placentation, berry or capsule fruit, endospermic seeds — floral formula \(\oplus \; ⚥ \; K_{(5)} \; C_{(5)} \; A_{5} \; \underline{G}_{(2)}\).
Flowchart
FC 9.8 – The chapter at a glance: Angiosperm plant body → root system (regions → functions → tap or adventitious → modifications) and shoot system → stem (nodes and internodes → modifications: underground, sub-aerial, aerial) → leaf (leaf base, petiole, lamina → venation → simple or compound → modifications → phyllotaxy) → inflorescence (racemose or cymose) → flower (thalamus → calyx, corolla, androecium, gynoecium → aestivation, insertion, placentation) → fruit (simple, aggregate, composite) → seed (seed coat, embryo, endosperm) → germination back to a new plant
9.5 Points to Ponder
A potato and a sweet potato do the same job in the same place, yet one is a stem and the other a root. If function does not decide what an organ is, what does — and why do biologists insist on origin rather than usefulness?
The same plant organ turns up as a thorn in one species, a tendril in another and a fleshy storage body in a third. What does this tell you about how new structures arise in evolution — are they invented afresh, or borrowed and reshaped?
Root hairs are ephemeral and are replaced every 10 to 15 days. Why might it be an advantage for a plant to keep rebuilding an absorbing surface rather than making a permanent one?
Xerophytes reduce their leaves to spines and hand photosynthesis to the stem. What does the plant gain, what does it give up, and why is this trade acceptable in a desert but not in a rain forest?
A pneumatophore solves the problem of a waterlogged soil. Can you think of other situations in which a plant grows a structure in the “wrong” direction because the resource it needs lies the other way?
Cross-pollination requires an outsider — wind, water or an animal. In what sense is a brightly coloured, scented, nectar-bearing flower an advertisement, and what is the cost of running that advertisement?
Zygomorphic flowers usually work with one particular kind of pollinator, while actinomorphic flowers accept many. Which strategy would you expect to be safer in an unpredictable environment, and why?
An inferior ovary is enclosed within the thalamus and better protected, yet superior ovaries are extremely common. Why has evolution not settled on one arrangement?
Parthenocarpic fruits set without fertilisation and carry no seeds. Such a plant cannot reproduce sexually on its own. Is a seedless variety a success or a dead end — and for whom?
A leguminous plant houses nitrogen-fixing bacteria in its root nodules and gives them shelter and food in return. Where else in this chapter have you seen one organism living on or in another, and how would you distinguish such a partnership from parasitism?
Families are recognised mainly from floral characters rather than from leaves, stems or roots. Why should the reproductive parts of a plant be more reliable for classification than the vegetative parts?
If you were handed an unfamiliar plant with no flowers on it, how far could you go in identifying its family, and at what point would you have to wait for it to bloom?
9.6 Check Your Concepts
Why does a root grow downwards even when a seed is sown upside down? Name the tropisms responsible and state whether each is positive or negative.
Arrange the following regions of a root in correct sequence, starting from the tip: region of elongation, region of maturation, root cap, region of root hairs, meristematic region.
What would happen to the absorbing ability of a plant if the epiblema of its root failed to produce root hairs? Explain your reasoning.
Why do gardeners lift a seedling along with a ball of soil while transplanting it? Link your answer to a specific region of the root.
In hydrophytes, root caps are replaced by root pockets. Suggest a reason why a plant floating in water does not need a true root cap.
Distinguish between fusiform, conical and napiform roots on the basis of shape, and give one example of each.
A mangrove growing in waterlogged saline soil absorbs fewer minerals when its pneumatophores are buried under silt. Explain the connection between breathing roots and mineral absorption.
A student is given two underground swollen structures. What single feature would allow the student to decide which one is a stem and which one is a root? Justify your answer.
Prop roots and stilt roots both provide mechanical support. State two differences between them.
Why are the aerial roots of an orchid covered with velamen, and what would happen if such an orchid were planted in wet garden soil?
Viscum album is a partial parasite while Cuscuta reflexa is a total parasite. Explain the difference in terms of what each one draws from its host and why.
Why does a piece of potato without an eye fail to grow into a new plant, even though it contains plenty of stored starch?
Compare a rhizome, a corm and a bulb with respect to the direction of growth of the stem and the part in which food is stored.
Explain the difference between sympodial and monopodial growth of a rhizome, giving one example of each.
A lawn of Cynodon recovers quickly from a bare patch without any seed being sown. Explain how, naming the modification involved.
A flat green organ bears a small flower in its middle. Is it a leaf or a stem? Give a reason for your answer.
What is the functional advantage of a phylloclade to a desert plant? Mention two structural features that make it suitable.
Name the bud that is modified into a tendril in each of the following: Vitis quadrangularis, Passiflora, Cucurbita, Antigonon.
How would you distinguish a compound leaf from a leafy branch? State the single most reliable test.
Explain why the opposite decussate arrangement of leaves is more efficient for a plant than the opposite superposed arrangement.
In a racemose inflorescence the oldest flower is at the base, while in a cymose inflorescence it is at the apex. Explain how the behaviour of the apical bud produces this difference.
A flower can be cut into two mirror-image halves along only one plane. Name the type of symmetry, and suggest what this symmetry indicates about the flower’s pollinator.
Distinguish between a unisexual flower and a dioecious plant. Why must a papaya orchard contain more than one type of plant?
Draw and label a vertical section of a typical flower showing thalamus, pedicel, sepal, petal, filament, anther, ovary, style, stigma and ovule.
How does the shape of the thalamus decide whether a flower is hypogynous, perigynous or epigynous? State the position of the ovary in each case.
Differentiate between cohesion and adhesion of stamens, giving one example of each condition.
A floral formula shows \(A_5\) without brackets, yet the description says the stamens are epipetalous. Explain why there is no contradiction.
Both axile and free central placentation have ovules on a central axis. What single feature of the ovary allows you to tell them apart?
Why is an apple called a false fruit while a mango is a true fruit? Name the floral part that forms the edible portion of an apple.
What is parthenocarpy? Explain why parthenocarpic varieties must be propagated vegetatively.
Classify the following as simple, aggregate or composite fruits: pineapple, custard apple, tomato, fig, strawberry.
Castor seeds are endospermic and pea seeds are non-endospermic. Where is the reserve food stored in each, and why does the difference arise?
Arrange in correct sequence the events from pollination to the formation of a fruit: pollen tube grows through the style; fertilisation occurs; pollen lands on the stigma; ovary wall becomes pericarp; ovule matures into seed.
Write the floral formula of family Fabaceae and explain the meaning of each symbol used in it.
Compare Fabaceae and Solanaceae with respect to symmetry of the flower, cohesion of stamens, placentation and type of fruit.
Why are floral characters preferred over vegetative characters while classifying plants into families? Support your answer with one example from this chapter.
Two of the vegetables we consume are nothing but leaf bases. Which are they?
Opuntia has spines but Carissa has thorns. What is the difference between the two?
A teacher described Hibiscus as a solitary cyme. What does this mean?
Write notes on the following.
- Fusiform root
- Racemose inflorescence
- Fasciculated tuberous roots
- Region of cell maturation
- Rhizome
- Stolon
- Leaf venation
- Cymose inflorescence
- Perianth
- Vexillary aestivation
- Axile placentation
Identify the figures given in Fig. 9.53 and write down the types of leaf arrangement shown in each.
Diagram
Fig. 9.53 – Three leafy shoots drawn side by side, each showing a different phyllotaxy at the nodes, for the student to identify
- Observe the figures given in Fig. 9.55 and label the different parts.
Diagram
Fig. 9.55 – Two unlabelled outline figures with leader lines: a vertical section of a seed and a stamen, for the student to label
- Differentiate between the following, with diagrammatic representation.
- Racemose and cymose inflorescence
- Reticulate and parallel venation
- Tap root and adventitious roots
9.7 Case-Based Questions
- Students went on an excursion to a botanical garden and noted the following observations. Will you be able to help them understand these conditions?
- A wiry outgrowth was seen on a plant, arising from between the leaf and the stem.
- There was a green plant with a flat stem but no leaves. The entire plant was covered by soft spines.
- Many obliquely produced roots were given out from the lower nodes, apparently for extra support.
- Many plants in the marshy region had upwardly growing roots. They could be seen better during low tide.
- A plant had leaves with a long leaf apex, which was curling around a support.
- A plant was found growing on another plant. The teacher said it is not a parasite. It exhibited two types of roots.
- While having lunch, onion slices were served to them. The teacher asked which part of the plant they were eating.
- Students observed the large leaves of coconut and the small leaves of Mimosa. The teacher asked in what way they are similar.
- The teacher showed them a marigold flower and said it is not one flower. What did the teacher mean?
- Students cut open a papaya fruit and found all the seeds attached to the sides. The teacher enquired about the possible placentation of the papaya ovary.
9.8 Objective Questions
Which one of the following will grow better in a moist and shady region?
- Opuntia
- Orchid
- Mangrove
- Lotus
A particular plant had a pair of leaves at each node arranged in one plane. What is this arrangement called?
- Alternate phyllotaxy
- Decussate phyllotaxy
- Superposed phyllotaxy
- Whorled phyllotaxy
In a particular flower the insertion of floral whorls was such that the ovary was below the level of the other three whorls, but its stigma was taller than the other three whorls. What will you call such a flower?
- Hypogynous
- Perigynous
- Epigynous
- Half superior – half inferior
Beet and Arum both store food for perennation. Do these examples represent two different types?
- Beet is a stem but Arum is a root
- Beet is a root but Arum is a stem
- Beet is a stem but Arum is a leaf
- Beet is a stem but Arum is an inflorescence
Assertion: The growing apex of a root is subterminal in position. Reason: A parenchymatous root cap covers the extreme tip of the root. Decide whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Assertion: Adventitious roots are used for vegetative propagation in many plants. Reason: Adventitious roots arise from the radicle of the embryo. Comment on the truth of the assertion and the reason.
Assertion: All the flowers of an inflorescence do not mature at the same time. Reason: This increases the chances of pollination and keeps the plant attractive for a longer period. State whether both are true and whether the reason explains the assertion.
Match the following. Group A shows the five ovary sections in Fig. 9.54; match each with the correct type in Group B.
| Group A (the five sections shown in Fig. 9.54) | Group B |
|---|---|
| 1. | i. Marginal |
| 2. | ii. Basal |
| 3. | iii. Axile |
| 4. | iv. Free central |
| 5. | v. Parietal |
Diagram
Fig. 9.54 – Five transverse or vertical sections of ovaries numbered 1 to 5, each showing a different arrangement of ovules on the placenta, for matching
- Match the following.
| Group A | Group B |
|---|---|
| a. Lathyrus | i. Stipular tendril |
| b. Pisum sativum | ii. Whole leaf tendril |
| c. Gloriosa | iii. Leaf tip tendril |
| d. Smilax | iv. Leaflet tendril |