Morphology of Flowering Plants: Class 11 Biology · Chapter 5

Biology began as careful looking. Long before anyone could measure photosynthesis or read a gene, naturalists filled notebooks with what they saw by eye, then through hand lenses and microscopes. That descriptive tradition, once called natural history, can seem like list-making, but it supplied many of the questions that physiology and evolutionary biology later answered. Nobody can ask why a leaf base is swollen until somebody has noticed the swelling. This unit (Chapters 5 to 7) describes the structure of plants and animals in that spirit, beginning with flowering plants.

Walk past a sabzi cart and you are looking at a botany lesson. The radish is a root, the potato a stem, spinach is leaves, the cauliflower head is a packed mass of flower buds, and brinjal, bhindi and tomato are fruits. Flowering plants (angiosperms) vary enormously in size and shape, yet almost all of them are assembled from the same few organs: roots, stems, leaves, flowers and fruits. The outward form of these organs is the plant's morphology.

To compare thousands of species, botanists need a shared vocabulary. A ‘whorled’ leaf arrangement or an ‘inferior ovary’ must mean exactly the same thing to a botanist in Shillong and one in Kochi. The same vocabulary lets us describe how organs vary as plants adapt to their surroundings, for climbing, storing food or defending themselves. This chapter builds that vocabulary organ by organ; Chapter 6 then looks inside the same organs at their tissues.

Whether it is a tulsi plant in a pot or a peepal tree in the village square, the plan is the same. Below the soil is the root system. Above it is the shoot system: the stem with its leaves and, in season, flowers and fruits (Fig. 5.1). The stem is marked off into nodes, where leaves are attached, and internodes, the bare lengths of stem between them. Buds sit at the stem tip and in the angle between each leaf and the stem.

5.1 The Root

When a seed germinates, the first organ to emerge is the radicle, the embryo's root. In most dicots the radicle simply keeps growing down as the primary root, and branches spread out from it: secondary roots, tertiary roots off those, and so on. A root system dominated by one main root like this is a tap root system (मूसला जड़); a mustard or gram seedling shows it well.

Monocots do it differently. In them the root formed by the radicle withers early, and its work is taken over by dozens of roots of roughly equal thickness sprouting from the base of the stem. The result is a fibrous root system, the tangled mop you lift with a clump of wheat or rice. Some plants go a step further and grow roots from parts other than the radicle: from stem nodes, from branches, even from leaves. Such roots are adventitious (अपस्थानिक जड़). The pillar roots hanging from banyan branches, the aerial roots of Monstera, the roots of lawn grass and the roots on a money-plant cutting kept in a bottle of water are all adventitious (Fig. 5.2).

Whatever its pattern, a root system does four jobs:

• It absorbs water and dissolved minerals from the soil.
• It anchors the plant firmly in the ground.
• It stores reserve food in many plants, which is why we eat radish and carrot.
• It makes plant growth regulators, chemical signals that travel to other parts of the plant.

5.1.1 Regions of the Root

Grow a gram seed on damp cotton and look at its root tip with a hand lens. Within the last centimetre or so you can map four zones (Fig. 5.3). The apex is covered by the root cap, a cone of loosely packed cells. As the root forces its way between soil particles, the cap takes the scraping and shields the delicate tissue behind it.

Just behind the cap sits the root's cell factory, the region of meristematic activity. Packed with small cells that have thin walls and dense cytoplasm, it keeps dividing, adding new cells to the root and replacing cap cells as they wear away. The cells it leaves behind stop dividing and start to stretch, quickly growing many times longer: this region of elongation is what lengthens the root and drives the tip forward. Further back, in the region of maturation, the stretched cells settle into their final, specialised forms. Some of its surface (epidermal) cells push out root hairs, extremely fine single-celled threads through which the root takes in most of its water and minerals.

5.2 The Stem

Is a potato a root? It grows underground and is swollen with stored food, just like a radish. But look closely and you find ‘eyes’ scattered over it, each a bud sitting in a little scar. Buds, and the nodes that bear them, are found only on stems, and a potato is exactly that: an underground stem.

The stem is the upward-growing part of the plant's axis, carrying the branches together with their leaves, flowers and fruits; its starting point is the plumule, the shoot end of a germinating embryo. Two features give a stem away. It is divided into nodes, where leaves are attached, and internodes, and it carries buds: a terminal bud at each tip and an axillary bud in each leaf axil. A young stem is usually green; with age, many stems turn woody and dark brown.

Two simple experiments bring out the difference in behaviour (Fig. 5.4). Put a potted seedling on a sunny windowsill and within a day or two its shoot leans towards the glass: stems are positively phototropic. Lay the pot on its side in a dark cupboard and the shoot turns to grow upward while the root turns to grow downward. The root follows gravity; the stem grows against it.

The stem's main job is to hold leaves, flowers and fruits where they work best: leaves in the light, flowers within reach of pollinators, fruits where their seeds can spread. It is also the plant's transport corridor, carrying water and minerals up from the roots and photosynthates (the sugars made in leaves) to wherever they are needed. Some stems do extra work as well. Potato and sugarcane stems store food. Others hold the plant up or help to defend it, and many can grow into new plants without seeds: farmers raise sugarcane from pieces of stem, not from seed.

5.3 The Leaf

Leaves are the plant's food factories, its chief organs of photosynthesis. Each leaf grows out sideways from the stem at a node, usually as a thin, flat blade. Leaves begin in the outer tissues of the shoot tip (their origin is exogenous), and new ones keep forming there, so the youngest leaves are always nearest the tip and the oldest further down: they develop in acropetal order. In the angle between a leaf and the stem, its axil, sits an axillary bud that may later grow into a branch.

Pick a hibiscus leaf and trace it from the stem outwards (Fig. 5.5). Where it joins the stem is the leaf base; next comes the stalk, or petiole; and then the flat green lamina, or blade.

The leaf base takes different forms in different groups. Hibiscus and rose have a pair of tiny leaf-like outgrowths, stipules, beside it. In grasses, maize and other monocots the base is broadened into a sheath that clasps the stem, sometimes all the way round. Many legumes have a swollen, cushion-like base, the pulvinus; in the touch-me-not plant (Mimosa pudica) it is the pulvinus that folds the leaf when you touch it.

Why have a petiole at all? A stalk lifts the blade clear of the stem and turns it to the light. Peepal leaves show a second benefit: their long, thin, bendy petioles let them tremble in the slightest breeze, and the movement cools the blade and keeps fresh air flowing over it.

In the lamina, the thickest vein usually runs down the middle as the midrib, with veins and finer veinlets branching from it. Like the ribs of an umbrella they keep the blade spread and stiff, and like pipes they carry water and minerals in and food out. Leaf blades also differ from species to species in shape, edge, tip, surface and how deeply they are cut, and each variant has its botanical name.

5.3.1 Venation

The pattern of veins in a lamina is its venation (शिराविन्यास). Hold a hibiscus leaf up to the light and you see veins dividing and rejoining into a fine mesh: this is reticulate venation, typical of dicots. In a leaf of maize, wheat or bamboo the veins run side by side along the blade without forming a mesh: this is parallel venation, found in most monocots.

5.3.2 Types of Leaves

Run a finger along the edge of a leaf blade. If you can follow the margin all the way round without ever reaching the midrib, however deep the notches, the leaf is simple: mango, peepal, hibiscus and even the deeply lobed papaya leaf are simple. If some cuts reach the midrib itself and split the blade into separate pieces, the leaflets, the leaf is compound (संयुक्त पत्ती).

This creates a classic trap, because a compound leaf can look just like a twig carrying several simple leaves. The test is the bud (Fig. 5.6). Every leaf, simple or compound, has a bud in the axil of its petiole. The leaflets of a compound leaf never have buds in their axils. A twig, by contrast, has a bud in the axil of every leaf it carries.

Compound leaves follow two plans. Neem, tamarind and curry-leaf leaves are pinnately compound: their leaflets are lined up, often in pairs, on either side of a central stalk called the rachis, which is the leaf's midrib in a new form. Silk cotton (semal) and the potted umbrella plant (Schefflera) have palmately compound leaves: every leaflet springs from the same spot at the tip of the petiole, like fingers spreading from a palm.

5.3.3 Phyllotaxy

Look down on a plant from above and you will notice that its leaves rarely shade one another completely. That is partly the work of phyllotaxy (पर्णविन्यास), the pattern in which leaves are arranged on a stem or branch. There are three main patterns (Fig. 5.7):

• Alternate: just one leaf per node, each new leaf turned to a different side from the one below, as in china rose (hibiscus), mustard, sunflower and mango.
• Opposite: two leaves per node, set exactly across the stem from each other, as in Calotropis (aak), guava and tulsi.
• Whorled: three or more leaves per node, ringing the stem, as in Alstonia (saptaparni) and oleander.

5.4 The Inflorescence

Compare a flower with a twig. A twig is an axis with nodes, internodes and leaves. A flower is the same plan squashed: its shoot apical meristem has switched to being a floral meristem, the internodes never stretch, and so the nodes sit crowded together on a short axis. At those crowded nodes the apex makes sepals, petals, stamens and carpels, one ring after another, where a twig would make leaves. Because the apex is used up in making the flower, a shoot tip that turns into a flower bears just that one, solitary flower.

Most plants, however, carry several flowers on a special flowering axis. The arrangement of flowers on this floral axis is the inflorescence (पुष्पक्रम). One question sorts inflorescences into two major types: does the tip of the main axis keep growing, or does it end in a flower?

In a racemose inflorescence the main axis keeps growing at its tip and bears flowers on its sides. The oldest flowers are therefore at the base and the youngest (still buds) near the tip, an acropetal succession. A mustard or radish stalk in a winter field shows it well: pods below, open flowers in the middle, buds on top.

In a cymose inflorescence the main axis ends in a flower, so its growth is limited. Further flowers come from side branches below the terminal one, and each of these may also end in a flower. The oldest flower is at the top and younger ones open below it, a basipetal order (Fig. 5.8). Jasmine and makoi (Solanum nigrum) bear their flowers in cymes.

5.5 The Flower

Everything about a flower serves one purpose, sexual reproduction. Pull one apart from the outside in and its parts come away in rings, or whorls (Fig. 5.9). First come the green sepals, together the calyx; then the coloured petals, the corolla; then the stamens, the androecium; and at the very centre one or more carpels, the gynoecium. All four rings sit on the thalamus (receptacle), the swollen top of the flower stalk or pedicel. Only the inner two whorls make gametes, so they are the reproductive whorls; calyx and corolla, which shield the bud and draw in pollinators, are the accessory whorls. In a few flowers, such as the lily, sepals and petals look alike; the two outer whorls are then called together the perianth, and its members tepals.

A flower carrying both stamens and carpels is bisexual; a flower that has stamens but no carpels, or carpels but no stamens, is unisexual. Mustard and brinjal flowers are bisexual, while the separate male and female flowers on a cucumber vine or a maize plant are unisexual.

Symmetry, merosity and bracts

Imagine slicing a flower vertically through its centre. If every such cut, whatever its angle, gives two mirror-image halves, the flower has radial symmetry and is actinomorphic (त्रिज्यसममित); mustard, datura and chilli flowers are examples. If just one cut, the one running down the middle from top to bottom, gives matching halves, the flower has bilateral symmetry and is zygomorphic, as in pea, bean, gulmohur and Cassia. If no cut through the centre gives similar halves, the flower is asymmetric (irregular), as in canna (Fig. 5.10).

Flowers are also described by the number of members in each whorl. A trimerous flower has its parts in threes or multiples of three, which is typical of monocots; tetramerous and pentamerous flowers have them in fours and fives, common in dicots. Finally, many flowers grow in the axil of a small, reduced leaf at the base of the pedicel, the bract. Such flowers are bracteate; flowers without bracts are ebracteate.

Position of the ovary on the thalamus

The last descriptor depends on where the ovary sits compared with the other three whorls. Picture three shapes of thalamus (Fig. 5.11):

• Hypogynous (जायांगधर): the ovary sits at the very top of a domed or flat thalamus, and sepals, petals and stamens all arise beneath it. The ovary is superior. Examples: mustard, china rose, brinjal.
• Perigynous: the thalamus is shaped like a cup or saucer with the ovary in its hollow; sepals, petals and stamens grow from the cup's rim, roughly level with the ovary. The ovary is half-inferior. Examples: plum, peach, rose.
• Epigynous: the thalamus wraps right round the ovary and becomes fused to it, leaving sepals, petals and stamens to emerge from the top, above the ovary. The ovary is inferior. Examples: guava, cucumber and the ray florets of sunflower.

5.5.1 Parts of a Flower

Now take the four whorls one at a time, from the outside in. Many of the terms come in pairs built from Greek roots: gamo- means united and poly- means many, here in the sense of separate; -sepalous and -petalous refer to sepals and petals.

5.5.1.1 Calyx

Look at a rosebud before it opens: the green flaps clasped around it are sepals, and together they form the calyx, the flower's outermost whorl. Their green, leaf-like tissue makes them good wrappers for the developing bud. When the sepals are joined into a cup or tube, as in datura, the calyx is gamosepalous; when each is separate, as in mustard, it is polysepalous.

5.5.1.2 Corolla

Inside the calyx are the petals, together the corolla. Their bright colours, patterns and scents are advertising aimed at bees, butterflies and other pollinators. Petals too may be joined (gamopetalous: datura, brinjal) or separate (polypetalous: mustard, china rose). Corollas come in a wide range of shapes; four to recognise are tubular, bell-shaped, funnel-shaped (morning glory) and wheel-shaped, opening flat like the brinjal flower.

Aestivation (पुष्पदल विन्यास) is the way the sepals or petals of one whorl lie in relation to each other in the bud. It shows best if you slice a mature bud straight across and look at the cut face (Fig. 5.12):

• Valvate: edge meets edge, like the planks of a barrel, with no overlap at all. Calotropis (aak).
• Twisted: every member has one edge lying over its neighbour and the other edge tucked under the next, all turning the same way, like the blades of a pinwheel. China rose, lady's finger (bhindi), cotton.
• Imbricate: members overlap with no single direction of turning; typically one is wholly outside, one wholly inside, and the rest mixed. Cassia, gulmohur.
• Vexillary (papilionaceous): in pea and bean the five petals are of three kinds. One large posterior petal, the standard (vexillum), covers the two side wings; the wings in turn cover the two smallest, anterior petals that together make the keel.

5.5.1.3 Androecium

Each stamen, a male reproductive organ, looks like a tiny matchstick: a thin stalk, the filament, carrying a swollen head, the anther, where pollen is made. Cut an anther across and you usually see two lobes, each hollowed into two chambers called pollen sacs, four in all (Fig. 5.13). The androecium is the flower's full set of stamens. A stamen that makes no pollen is a sterile staminode.

Stamens are often joined, either to another whorl or to each other (Table 5.2). Joined to the corolla they are epipetalous (दललग्न), as in brinjal; joined to the perianth, epiphyllous, as in lily. Joined to each other by their filaments, they form bundles: monoadelphous (one bundle, also spelt monadelphous), diadelphous (two) or polyadelphous (many). Stamens left separate are polyandrous. Even filament length can vary inside one flower: a mustard flower has four tall stamens and two short ones, and Salvia is another standard example.

5.5.1.4 Gynoecium

At the centre of the flower is the gynoecium, the female part, built from one or more carpels. Think of a carpel as a flask: the swollen base is the ovary, the neck is the style, and the lip at the top is the stigma, where pollen lands and is held. Inside the ovary the ovules are attached to a cushion of tissue called the placenta.

Flowers with several carpels keep them either apart, each with its own ovary (apocarpous: lotus, rose), or fused into one compound ovary (syncarpous: mustard, tomato). Whatever the arrangement, the ovary is the future fruit and its ovules the future seeds; fertilisation sets both changes going.

Placentation (बीजांडन्यास) describes how the ovules are attached inside the ovary. Cut a pea pod, a lemon and a mustard pod across and you meet three different patterns (Fig. 5.14). Botanists name six types (marginal, axile, parietal, basal, central and free central); these five are the ones you must be able to describe:

• Marginal: a single-carpel ovary, like a pea pod, in which the ovules sit in two rows on a ridge-shaped placenta running along one seam, the ventral suture. Pea.
• Axile: partitions divide the ovary into several chambers (multilocular), and the ovules hang from the central column where the partitions meet. China rose, tomato, lemon.
• Parietal: the ovules are borne on the ovary wall itself. There is only one chamber to begin with, but a false septum grows across and divides it in two. Mustard, Argemone.
• Free central: a column rises from the floor of a single, unpartitioned chamber and carries the ovules. Dianthus, primrose.
• Basal: just one ovule, attached to a placenta at the floor of the ovary. Sunflower, marigold.

5.6 The Fruit

Only flowering plants make fruits. Once fertilisation has happened, the ovary enlarges and ripens into a fruit while the ovules inside become seeds. The ovary wall becomes the fruit wall, the pericarp (फलभित्ति). Occasionally an ovary develops into a fruit with no fertilisation at all; such a parthenocarpic fruit is usually seedless, like the bananas sold in every market.

Pericarps come dry, like a groundnut shell, or fleshy. A thick fleshy pericarp usually shows three layers: the skin is the epicarp, the middle the mesocarp and the innermost layer the endocarp.

Mango and coconut belong to one fruit type, the drupe. A drupe typically develops from a superior ovary with a single carpel (monocarpellary) and holds a single seed (Fig. 5.15). Bite into a mango and you meet the layers in turn: a thin skin (epicarp), the juicy flesh (mesocarp) and, around the seed, the hard stone (endocarp). In a coconut the same middle layer is a thick mass of fibre, the coir that is spun into ropes and woven into doormats.

5.7 The Seed

Each fertilised ovule ripens into a seed, a young plant packed for travel. The young plant, the embryo, is wrapped in a protective seed coat. It is built around an embryonal axis: one end, the radicle, will become the root and the other, the plumule, the shoot. Attached to the axis are one or two cotyledons (seed leaves): one in cereals such as rice and maize, two in pulses such as gram, moong and pea.

5.7.1 Structure of a Dicotyledonous Seed

Soak a few chana (gram) or rajma seeds overnight and you can take one apart with a fingernail (Fig. 5.16). On the curved inner edge of a rajma seed you will find a pale scar, the hilum: this is where the seed was joined to the fruit by its stalk. Beside it is a pore so small that you may need the hand lens, the micropyle, through which water enters as the seed soaks. Both lie on the seed coat, which has two layers, an outer testa and an inner tegmen.

Slip off the coat and most of what remains is two plump cotyledons, the seed's food store. Prise them apart and you find the tiny embryonal axis lying where they meet. One end of it, the radicle, will become the root; the other end, the plumule, will become the shoot, its first leaves already folded inside.

Seeds differ in where they keep their food. In castor the endosperm (भ्रूणपोष), a food-storing tissue that forms after double fertilisation, is still present in the ripe seed; such seeds are endospermic. In bean, gram and pea the growing embryo has used up the endosperm and moved its food into the cotyledons; these seeds are non-endospermous.

5.7.2 Structure of Monocotyledonous Seed

Turn now to a monocot. Endosperm is the rule in monocot seeds, with exceptions such as orchids, whose dust-like seeds have none. A maize grain shows the pattern clearly (Fig. 5.17). Strictly, the grain is a one-seeded fruit: its thin seed coat is fused with the fruit wall into a single papery layer. Most of the grain is a bulky endosperm full of stored starch. The outermost cells of the endosperm, just inside the fruit wall, form a protein-rich layer, the aleurone layer.

The embryo is small, tucked into a hollow beside the endosperm near the narrow end of the grain. Its single cotyledon is a broad shield, the scutellum, pressed flat against the endosperm so that it can draw food from it. The short embryonal axis carries the plumule at one end and the radicle at the other, each wrapped in a protective sheath: the coleoptile over the plumule, the coleorhiza over the radicle.

5.8 Semi-technical Description of a Typical Flowering Plant

Botanists describe a plant in a fixed order, in brief phrases of standard terms, so that any two descriptions can be compared line by line. A good semi-technical description reads almost like a form being filled in:

1. Habit: herb, shrub or tree.
2. Vegetative characters: root, then stem, then leaves.
3. Floral characters: the inflorescence, then the flower and each whorl from calyx to gynoecium.
4. Fruit and seed.
5. Floral formula and floral diagram, which squeeze the floral description into symbols and a drawing.

The symbols are standard (Table 5.4). Read a formula from left to right like a sentence. A subscript gives the number of members in a whorl, a ‘+’ separates two whorls of the same organ, and ∞ means ‘many’. So ${\mathrm{A}}_{2+4}$ means two stamens in an outer whorl and four in an inner whorl.

A floral diagram is the picture version: a plan of the flower as if it were sliced across just above the thalamus. A dot at the top marks the mother axis, the stem that bears the flower, so you know which side faced it. Moving inwards ring by ring, you meet calyx, corolla, androecium and finally the gynoecium at the centre. At one glance you can count the members of each whorl, see which members alternate with which, and spot the overlaps, fusions and attachments.

Fig. 5.18 builds the floral diagram of mustard (family Brassicaceae) one whorl at a time. Its floral formula is ⊕ ⚥ ${\mathrm{K}}_{2+2}{\mathrm{C}}_4{\mathrm{A}}_{2+4}{\underset{‾}{\mathrm{G}}}_{(2)}$ and it reads: actinomorphic and bisexual; four free sepals in two whorls of two; four free petals; six free stamens, two outer and four inner; two fused carpels in a superior ovary.

5.9 Solanaceae

Now put the vocabulary to work on a real family. Solanaceae is a large family with a nickname, the potato family, and a wide range: its members grow wild or are farmed from the tropics through the subtropics into temperate lands. They fill an Indian kitchen: potato, tomato, brinjal and chilli. Next time you hold a brinjal or a tomato, look at the green cap on top. That is the calyx, which in this family stays on (persists) while the fruit ripens. Here is the family's semi-technical description, in the standard order.

Now condense it. The floral formula of Solanaceae is ⊕ ⚥ ${\mathrm{K}}_{(5)}\overgroup{{\mathrm{C}}_{(5)}{\mathrm{A}}_5}{\underset{‾}{\mathrm{G}}}_{(2)}$ and its floral diagram is drawn in Fig. 5.20. Check that you can read every piece: why do the 5s of K and C have brackets while the 5 of A does not, and what does the arc over C and A record?

Few families matter more to farmers and cooks. Its members provide:

• Food: potato, tomato, brinjal, capsicum (shimla mirch) and cape gooseberry (rasbhari).
• Spice: chilli.
• Medicine: belladonna (Atropa) and ashwagandha (Withania).
• Fumigatory: tobacco, smoked or chewed for its nicotine, an addictive and harmful substance.
• Ornamentals: petunia and raat ki rani (night-blooming jasmine, Cestrum).