Morphology of Flowering Plants

Every flowering plant is built from the same small set of organs. Below the soil there is a root system; above the soil there is a shoot system made of stem, leaves, flowers, fruits and seeds. Morphology is the study of the outward form of these organs. The words you learn in this chapter come back in plant anatomy, in classification and in reproduction, so it pays to learn them carefully the first time rather than cramming them at the last minute.

The root is the descending part of the plant body. In most plants it develops from the radicle of the germinating seed. A root normally has no nodes, no internodes and no leaves, it is usually not green, and it grows towards gravity and towards moisture. These three negatives are the quickest way to check whether an underground structure is really a root.

There are three broad root systems. In a tap root system the radicle grows into one thick primary root that goes straight down, and this primary root gives off smaller secondary and tertiary branches. Mustard, gram and neem show this, and most dicotyledons follow this pattern. In a fibrous root system the primary root is short-lived and is soon replaced by a large bunch of slender roots of roughly equal thickness that come out from the base of the stem. Wheat is the standard example, and most monocotyledons are like this. Adventitious roots are roots that grow from any part of the plant other than the radicle, such as a stem, a node or a leaf. Grass, the banyan tree and Monstera all bear adventitious roots.

If you move along a young root from the tip upwards you can name four regions in order. The root cap is a thimble-shaped cover that protects the delicate growing tip as it is pushed through hard soil. Immediately behind it lies the region of meristematic activity, where small thin-walled cells with dense cytoplasm divide repeatedly. Above that is the region of elongation, where the newly formed cells lengthen and so push the tip further into the soil. Higher still is the region of maturation, where the cells differentiate; some epidermal cells here grow out as very fine root hairs, and it is these hairs that actually take in water and dissolved minerals from the soil.

Roots also get modified for jobs other than anchoring and absorption. For storage, the tap roots of carrot and turnip swell up with stored food, and so do the adventitious roots of sweet potato. For support, the banyan tree sends down thick prop roots from its large horizontal branches; these hanging roots reach the soil, thicken, and hold up branches that would otherwise break. Maize and sugarcane put out stilt roots from the lower nodes of the stem, which slant into the soil and stop tall shoots from toppling over. For respiration, plants such as Rhizophora that grow in swampy, oxygen-poor mud send up special roots called pneumatophores, which grow vertically upwards out of the mud and help the plant get oxygen.

The stem is the ascending part of the shoot and develops from the plumule of the germinating seed. Its defining features are nodes, the points where leaves are attached, and internodes, the lengths of stem between two nodes. A stem also bears buds: a terminal bud at the tip and axillary buds in the angle between a leaf and the stem. Young stems are usually green and later turn brown and woody. The stem spreads out branches so that leaves get light, holds up flowers and fruits, conducts water and food, and in many plants also stores food.

Keep the checklist of nodes, internodes, scale leaves and axillary buds ready, because stem modifications are designed to fool you. Many of them look nothing like a stem.

Underground stem modifications store food and also help the plant survive an unfavourable season, a property called perennation. A rhizome is a thick horizontal underground stem with clear nodes, internodes and scale leaves, as in ginger and turmeric. A tuber is the swollen tip of an underground branch, as in potato; the eyes of a potato are nodes and each eye carries a bud, which is why a cut piece of potato can grow into a new plant. A corm is a short, thick underground stem that grows vertically rather than sideways, and it is usually rounded with a flat base, as in Colocasia and zaminkand. A bulb, as in onion and garlic, is a very short disc-like stem covered by fleshy scale leaves in which the food is actually stored.

Sub-aerial modifications help the plant spread sideways and multiply vegetatively. A runner is a slender branch that creeps flat along the ground and roots at the nodes, as in grasses and strawberry. A stolon arches out from the base of the stem and then bends down to touch the soil, as in jasmine. An offset is a short thick runner of one internode found in water plants such as Pistia and Eichhornia. A sucker grows underground from the base of the stem and then comes up as a new shoot, as in banana, pineapple and chrysanthemum. In all of these, when the older connecting parts die, the rooted pieces become independent plants.

Aerial modifications serve climbing, defence or photosynthesis. Stem tendrils are slender, spirally coiled structures that develop from axillary buds and help weak stems climb; gourds such as cucumber, pumpkin and watermelon, and also grapevine, climb this way. Thorns are hard, woody, pointed structures formed from axillary buds, as in Citrus and Bougainvillea, and they protect the plant from browsing animals. In hot, dry places, where broad leaves would lose too much water, the stem itself takes over photosynthesis: in Opuntia the stem becomes flattened, green and fleshy, and in some Euphorbia species it becomes fleshy and cylindrical. Such a green, leaf-like stem that takes over the work of a leaf is called a phylloclade. Because these green stems carry out photosynthesis, the leaves in such plants are often reduced to spines.

One warning before you leave stem modifications: a tendril made from an axillary bud is a stem tendril, but a tendril formed by a leaf or by leaflets, as in pea, is a leaf tendril. The same distinction applies to spines and thorns.

A leaf is a flattened lateral outgrowth of the stem, borne at a node and usually carrying a bud in its axil. Leaves are produced by the shoot apical meristem in acropetal succession, which means the oldest leaf is lowest and the youngest is nearest the growing tip. The leaf is the main photosynthetic organ of the plant.

A typical leaf has three parts. The leaf base attaches the leaf to the stem and may bear two lateral outgrowths called stipules. In monocots the leaf base widens into a sheath that partly or wholly wraps the stem, which is why grass leaves seem to be rolled around the shoot. In many members of the pea family the leaf base is swollen and is called a pulvinus. The petiole is the stalk; a long, flexible petiole lets the flat blade flutter, which cools the leaf and brings fresh air to its surface. The lamina or leaf blade is the green expanded portion with veins and veinlets, and the thick central vein is called the midrib. The veins give the blade rigidity and act as channels for transport within the leaf.

Venation is the arrangement of veins and veinlets in the lamina. In reticulate venation the veinlets form an irregular network, and this is the usual pattern in dicot leaves. In parallel venation the veins run parallel to one another from base to tip, and this is the usual pattern in monocot leaves. Textbooks often quote exceptions in both directions, such as Smilax, a monocot with net-like venation, and Calophyllum, a dicot whose veins look parallel; treat these as exceptions to remember, not as the rule.

A simple leaf has an undivided lamina, or one in which the incisions do not reach up to the midrib. In a compound leaf the incisions cut right down to the midrib, so the lamina is broken into separate leaflets. A pinnately compound leaf has leaflets arranged on both sides of a common central axis, as in neem. A palmately compound leaf has all its leaflets attached at a single point at the tip of the petiole, as in silk cotton. Students often mistake a compound leaf for a branch carrying many small simple leaves. The test is simple and reliable: a bud is present in the axil of a whole leaf, whether simple or compound, but there is never a bud in the axil of a leaflet.

Phyllotaxy is the pattern in which leaves are arranged on the stem, and it is designed so that leaves shade one another as little as possible. In alternate phyllotaxy a single leaf arises at each node in an alternating manner, as in China rose, mustard and sunflower. In opposite phyllotaxy two leaves arise at each node and stand opposite each other, as in Calotropis and guava. In whorled phyllotaxy more than two leaves come out of one node and form a ring, as in Alstonia.

Leaves are also modified for special work. In pea, leaflets turn into thread-like tendrils that coil around a support. In Opuntia, leaves become sharp spines that cut water loss and keep animals away. In onion and garlic, fleshy leaves store food and water. In the insectivorous plants Nepenthes and Utricularia, leaves become a pitcher and a bladder respectively, and trap small animals. In Australian acacia the true leaves are small and short-lived, and the petiole broadens into a flat green phyllode that does the work of a leaf blade.

When a plant finishes its vegetative phase, the shoot apex stops making leaves and turns into a floral meristem. The internodes near the tip do not lengthen, the appendages that would have been leaves become floral parts, and the axis condenses into a short swollen structure called the thalamus or receptacle. This is why a flower is described as a modified shoot: it is a shoot with very short internodes whose lateral appendages have become sepals, petals, stamens and carpels.

The arrangement of flowers on the floral axis is called the inflorescence. There are two main types, and the difference lies in whether the main axis keeps growing. In a racemose inflorescence the main axis continues to grow and flowers are borne laterally on it. Since the tip keeps producing new flower buds, the oldest flower is at the bottom and the youngest is near the tip, an arrangement called acropetal succession. Radish and mustard show a racemose inflorescence. In a cymose inflorescence the main axis ends in a flower, so its growth is limited; further flowers develop on lateral branches, which again end in flowers. Here the oldest flower is at the tip and the younger ones are below it, which is basipetal order. Members of the potato family such as Solanum have cymose inflorescences, though many of them also bear solitary axillary flowers.

A flower is the reproductive unit of an angiosperm. A typical flower has four whorls arranged on the thalamus, counted from outside inwards: calyx, corolla, androecium and gynoecium. Calyx and corolla are accessory organs that protect and attract; androecium and gynoecium are the essential reproductive organs, male and female respectively. A flower carrying both stamens and carpels is bisexual; one carrying only stamens or only carpels is unisexual. When the calyx and the corolla are not distinguishable from one another, as in lily, the whorl is called a perianth and its members are tepals. A flower with a bract at the base of its stalk is bracteate; one without such a bract is ebracteate.

Flowers are also described by their symmetry. An actinomorphic or radially symmetrical flower can be divided into two equal halves by any vertical plane passing through the centre, as in mustard, datura and chilli. A zygomorphic or bilaterally symmetrical flower can be divided into two equal halves in only one particular vertical plane, as in pea, gulmohar, bean and Cassia. A flower that cannot be cut into equal halves in any plane, as in Canna, is asymmetric. Depending on whether the floral appendages come in multiples of three, four or five, the flower is called trimerous, tetramerous or pentamerous.

Finally, the position of the ovary with respect to the other whorls on the thalamus gives three arrangements. In a hypogynous flower the gynoecium sits at the highest position on the thalamus and the other parts are attached below it, so the ovary is said to be superior; mustard, China rose and brinjal are examples. In a perigynous flower the thalamus grows up as a cup, so the gynoecium sits in the centre at about the same level as the other parts arranged on the rim, and the ovary is called half inferior; plum, rose and peach are examples. In an epigynous flower the thalamus grows over and completely encloses the ovary and even fuses with its wall, so that the calyx, corolla and androecium arise from above the ovary; here the ovary is inferior, as in guava, cucumber and the ray florets of sunflower.

The calyx is the outermost whorl and its units are sepals. Sepals are usually green and leaf-like, and their main job is to protect the flower while it is still a bud. If the sepals are joined to one another the calyx is gamosepalous; if they are free it is polysepalous.

The corolla is the next whorl and is made of petals. Petals are usually brightly coloured and often scented, and they attract insects and other pollinators. A corolla with fused petals is gamopetalous and one with free petals is polypetalous. The shape of the corolla is described with words such as tubular, bell-shaped, funnel-shaped and wheel-shaped.

Aestivation is the way the sepals or petals of a whorl are arranged with respect to the other members of the same whorl while the flower is still in bud. There are four common patterns. In valvate aestivation the members merely touch each other at the margins without overlapping, as in Calotropis. In twisted aestivation one margin of each member overlaps the next member, and all the overlaps go the same way round, as in China rose, cotton and lady's finger. In imbricate aestivation the margins do overlap, but not in any fixed direction, as in Cassia and gulmohar. In vexillary or papilionaceous aestivation, seen in pea and bean, there are five petals of unequal size: the largest petal, called the standard or vexillum, covers two lateral petals called wings, and the two wings in turn cover the two smallest petals, which are joined together to form the keel.

The androecium is made of stamens, and each stamen is the male reproductive part. A stamen has a stalk called the filament and a head called the anther. The anther is usually bilobed, and each lobe contains two chambers, the pollen sacs, in which pollen grains are produced. A stamen that has lost its ability to produce functional pollen is called a staminode. Stamens may be attached to other whorls: when they are fused to the petals, as in brinjal, they are epipetalous, and when they are fused to the perianth, as in lily, they are epiphyllous. Stamens may also be joined among themselves. Free stamens make the flower polyandrous. If all the filaments unite into a single bundle the condition is monadelphous, as in China rose; if they form two bundles it is diadelphous, as in pea; and if there are more than two bundles it is polyadelphous, as in Citrus. The stamens in one flower need not all be of the same length.

The gynoecium is the female part and is made of one or more carpels. A carpel has three regions: the ovary at the base, a slender style above it, and a sticky stigma at the top on which pollen lands. Inside the ovary is a cushion of tissue called the placenta, and on this placenta are borne one or many ovules, each of which contains an egg. When a flower has more than one carpel and they remain free from one another, the gynoecium is apocarpous, as in lotus and rose. When the carpels are fused together, it is syncarpous, as in mustard and tomato.

Placentation is the arrangement of the ovules inside the ovary. In marginal placentation the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on it in two rows, as in pea. In axile placentation the fused margins form a central axis and the ovary is divided into chambers, with the ovules attached to that central axis, as in China rose, tomato and lemon. In parietal placentation the ovules lie on the inner wall of a one-chambered ovary, which may later look two-chambered because a false wall grows across it, as in mustard and Argemone. In free central placentation the ovules are borne on a central axis but the partition walls are absent, as in Dianthus and primrose. In basal placentation the placenta lies at the base of the ovary and bears a single ovule, as in sunflower and marigold.

A fruit is the mature or ripened ovary, and it develops after fertilisation. If the ovary alone forms the fruit, it is called a true fruit. In some plants the thalamus also grows and becomes part of the edible portion, and such a fruit is called a false fruit; apple is the usual example. In a few plants a fruit forms without fertilisation at all, and such a fruit is parthenocarpic and has no seeds; the banana we eat is the standard example.

A fruit has two parts: the pericarp, which is the wall developed from the ovary wall, and the seeds, which develop from the ovules. When the pericarp is thick and fleshy it may be separated into three layers, from outside inwards: the epicarp, the mesocarp and the endocarp. A fruit in which this three-layered arrangement is very clear and the endocarp is hard and stony is called a drupe. In mango the epicarp is the thin skin, the mesocarp is the juicy edible pulp, and the endocarp is the hard stone that protects the single seed inside. In coconut the epicarp is the outer skin, the mesocarp is the thick fibrous husk from which coir is made, and the endocarp is the hard shell. Compare this with a berry such as tomato, where the whole pericarp stays soft and the seeds lie embedded in the pulp, and with a dry fruit such as the legume of pea, which splits along both its sutures to release the seeds.

A seed is the fertilised, mature ovule. It consists of a seed coat and an embryo, and in some seeds also a food store called the endosperm. Seeds in which the endosperm is used up during seed development and the food is stored in the cotyledons are called non-endospermic; bean, gram and pea are examples. Seeds in which the endosperm remains in the mature seed and supplies food to the seedling are endospermic, and castor is the usual example.

In a typical dicotyledonous seed such as bean or gram, the seed coat has two layers: an outer tough testa and an inner thin tegmen. On the seed coat you can see a scar called the hilum, which marks the point where the developing seed was attached to the fruit, and just above the hilum a tiny pore called the micropyle, through which water first enters at germination. Inside is the embryo, made of an embryonal axis and two thick cotyledons, which are often fleshy because they are full of stored food. The end of the embryonal axis that lies towards the micropyle is the radicle, which will grow into the root, and the end lying above the level of the cotyledons is the plumule, which will grow into the shoot.

In a monocotyledonous seed such as maize, the picture is different. What we buy as a maize grain is strictly a fruit, because the seed coat is a thin membrane and is fused to the fruit wall; such a one-seeded fruit is called a caryopsis. Most of the grain is taken up by a bulky endosperm that stores the food. The outermost layer of this endosperm is a protein-rich layer called the aleurone layer, which marks the endosperm off from the parts around it. The embryo itself is small and lies in a groove at one end of the endosperm. It has a single large shield-shaped cotyledon called the scutellum, which absorbs food from the endosperm and passes it to the growing seedling, and an embryonal axis. In maize the two ends of the embryonal axis are protected by sheaths: the radicle is covered by the coleorhiza and the plumule is covered by the coleoptile, a hollow foliar sheath that pushes through the soil and shields the delicate shoot tip.

Once you know the terms, you can describe any flowering plant in a short, standard form that any botanist anywhere can read. This is called a semi-technical description. It follows a fixed order, and keeping to that order is what makes the description easy to read and easy to compare with another plant.

The description begins with the habit of the plant, that is, whether it is a herb, a shrub, a tree, a climber or a creeper, and whether it is annual or perennial. Next come the vegetative characters, described from the bottom upwards: the root system, then the stem, then the leaves with their arrangement, venation and whether they are simple or compound. After that come the floral characters, described from the outside inwards: the inflorescence, then the flower as a whole (bracteate or ebracteate, symmetry, sex, position of ovary), then the calyx, the corolla, the androecium and the gynoecium in that order. Finally the fruit and the seed are described. The whole description is then summarised in two ways: a floral diagram, which is a small plan drawing showing how many parts each whorl has and how they lie in relation to the mother axis, and a floral formula, which puts the same information into symbols.

The symbols of the floral formula are worth memorising, because a single wrong symbol changes the meaning of the whole formula. Br stands for bracteate. The symbol ⊕ means the flower is actinomorphic and the sign % means it is zygomorphic. ♂ marks a male flower, ♀ marks a female flower and ⚲ marks a bisexual flower. The whorls are written as K for calyx, C for corolla, P for perianth when calyx and corolla cannot be told apart, A for androecium and G for gynoecium. The number of members in a whorl is written just after the letter, so five free sepals are written K5. If the members of a whorl are fused to each other, the number is enclosed in a bracket, so five fused sepals are written K(5). If members of two different whorls are attached to each other, a line is drawn above the two symbols concerned. The position of the ovary is shown by a line placed against G: a line drawn below the letter G means the ovary is superior, and a line drawn above the letter G means the ovary is inferior. Since a line cannot always be printed clearly, it is safe to also write the words superior or inferior beside the formula in an answer.

Here is a worked example from the potato family, Solanaceae, which includes potato, tomato, brinjal, chilli, Datura, Petunia and tobacco. The plants are mostly herbs, shrubs or small trees. The stem is herbaceous and sometimes woody, and it may be underground and modified for storage, as in potato. The leaves are alternate, generally simple, and show reticulate venation. The inflorescence is solitary and axillary, or cymose as in Solanum. The flower is ebracteate, actinomorphic, bisexual and hypogynous, so no Br is written in its formula. The calyx has five sepals united into a tube, and it is persistent, which means it stays on even after the fruit has formed, showing valvate aestivation. The corolla has five united petals, also valvate in the bud. The androecium has five stamens that are epipetalous, that is, attached to the corolla tube, and they alternate with the petals. The gynoecium is bicarpellary and syncarpous, the ovary is superior and two-chambered, the placenta is swollen and bears many ovules, and the placentation is axile; the ovary is set at a slant in the flower. The fruit is a berry, as in tomato, or a capsule, as in Datura, and the seeds have endosperm.

The floral formula for such a flower is written as ⊕ ⚲ K(5) C(5) A5 G(2), with the line under G showing that the ovary is superior. Compare this with the mustard type of flower, whose formula is ⊕ ⚲ K2+2 C4 A2+4 G(2), again with a superior ovary. Mustard flowers too are ebracteate, which is why neither formula begins with Br. Reading the mustard formula out tells you the whole story: actinomorphic, bisexual, four sepals in two whorls of two, four free petals, six stamens of which two are shorter and four are longer, and two fused carpels with a superior ovary.