Morphology of Flowering Plants

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 5.

Why is a carrot a root but a potato a stem?

Both grow underground, both are swollen with stored food, and both end up in the same curry. Yet a carrot is a root and a potato is a stem. The potato has little dents called "eyes", each of which is a bud sitting where a leaf once was, and a bud-bearing spot is a node. Roots never have nodes, buds or leaves.

That is the whole spirit of this chapter. Plant organs are defined by what they carry and where they come from, not by where they sit or what they look like. Once you can read those clues, every organ you meet in this chapter becomes easy to place.

A flowering plant has two systems. The root system is underground, and the shoot system is everything above it: stem, leaves, flowers and fruits.

The root: anchor, pipe and pantry

Roots absorb water and minerals, anchor the plant, store reserve food and make plant growth regulators.

The four zones of a root tip. Walking up from the tip, you meet four regions in order.

Why it works this way. Division, elongation and maturation happen in sequence along the same line of cells, like an assembly line. New cells appear just behind the cap, stretch in the next zone and finally take on their jobs in the last. The root hairs are in the oldest zone, because absorption can begin only after a cell has matured.

The stem: the plant's highway

The stem is the ascending axis that bears branches, leaves, flowers and fruits. It grows from the plumule of the embryo. The point where a leaf arises is a node, and the stretch between two nodes is an internode. The stem bears buds, which may be terminal or in the axil of a leaf. It spreads branches, carries water, minerals and the products of photosynthesis, and in some plants stores food, gives support, protects and helps vegetative propagation. Potato is a stem because its "eyes" are buds at nodes.

The leaf: the food factory

A leaf is a lateral, usually flat structure borne at a node, with a bud in its axil. Leaves arise from the shoot apical meristem in an acropetal order (youngest at the top) and are the main organs of photosynthesis.

A typical leaf has a leaf base, a petiole and a lamina. The leaf base may carry two small lateral stipules. In monocots it expands into a sheath that clasps the stem, and in some legumes it swells into a pulvinus. The lamina has a central midrib with veins and veinlets that give rigidity and carry water, minerals and food.

Venation is the pattern of veins in the lamina. Reticulate venation forms a network, and is typical of dicots. Parallel venation has veins running side by side, and is typical of monocots.

A bud is present in the axil of a petiole, in both simple and compound leaves, but never in the axil of a leaflet.

Pause and predict: a leaf is deeply lobed, and the cuts go almost to the midrib but stop short. Simple or compound?

Check your answer: Simple. Only when the incisions actually reach the midrib and break it into separate leaflets is the leaf compound.

Phyllotaxy is the arrangement of leaves on the stem.

Inflorescence: how flowers are arranged

An inflorescence is the arrangement of flowers on the floral axis. When a plant is ready to flower, the shoot apical meristem turns into a floral meristem, the internodes stop elongating and the axis becomes shorter. If the shoot tip itself becomes a single flower, the flower is solitary.

The flower: the reproductive unit

A flower sits on the swollen end of the pedicel, the thalamus (receptacle), and has four whorls. The calyx (sepals) and corolla (petals) are accessory whorls. The androecium (stamens) and gynoecium (carpels) are the reproductive whorls. In lily, the calyx and corolla look the same and are together called the perianth. A flower with both stamens and carpels is bisexual. A flower with only one is unisexual.

Symmetry and counts. An actinomorphic flower has radial symmetry and can be cut into equal halves by any vertical plane through the centre, as in mustard, datura and chilli. A zygomorphic flower can be divided into equal halves in only one plane, as in pea, gulmohur, bean and Cassia. An asymmetric flower, such as canna, cannot be divided into two equal halves by any plane. Flowers whose parts come in multiples of 3, 4 or 5 are trimerous, tetramerous or pentamerous. A flower with a bract at the base of the pedicel is bracteate, and one without is ebracteate.

Where the ovary sits. This depends on how the other whorls are attached to the thalamus.

Pause and predict: in a guava fruit, where do you find the dried remains of the sepals?

Check your answer: At the top of the fruit. Guava is epigynous, so the thalamus wraps around the ovary and the sepals, petals and stamens arise above it.

Calyx and corolla. Sepals are green and protective. They may be united (gamosepalous) or free (polysepalous). Petals are often brightly coloured to attract pollinators, and may be united (gamopetalous) or free (polypetalous). The corolla can be tubular, bell shaped, funnel shaped or wheel shaped.

Aestivation is the arrangement of sepals or petals in the bud.

Androecium. Each stamen has a filament and an anther. The anther is usually bilobed, with two pollen sacs in each lobe packed with pollen grains. A sterile stamen is a staminode. Stamens can be joined to other parts or to each other.

Gynoecium. It is made of one or more carpels, each with a stigma, style and ovary. Inside the ovary, ovules sit on a cushion like placenta. Many carpels may be free (apocarpous, as in lotus and rose) or fused (syncarpous, as in mustard and tomato). After fertilisation the ovules become seeds and the ovary becomes the fruit.

Placentation is how the ovules are arranged inside the ovary.

Fruit and seed

A fruit is the ripened ovary. Normally it develops after fertilisation. A fruit formed without fertilisation is parthenocarpic, as in the seedless banana. The wall of the fruit is the pericarp, which may be dry or fleshy. If fleshy, it forms three layers: the outer epicarp, the middle mesocarp and the inner endocarp.

A drupe is a fruit that develops from a monocarpellary, superior ovary and is one seeded. Mango has a thin epicarp, a fleshy and edible mesocarp and a stony endocarp. In coconut, which is also a drupe, the mesocarp is fibrous.

Beyond the textbook. For the exam: a fruit is a ripened ovary, and mango and coconut are drupes. In reality: in many familiar "fruits" the fleshy part is not the ovary at all. The edible part of an apple or a strawberry is largely the swollen thalamus. Tomato and brinjal, called vegetables in the kitchen, are botanically berries, since a berry is a fleshy fruit with a soft pericarp and many seeds. Why it matters: it shows that the botanical definition of a fruit does not match everyday use, which is exactly what exam questions on "true" fruits play with.

The seed has a seed coat and an embryo. The embryo is made of an embryonal axis and one or two cotyledons.

Remember the pairings by their names: coleoptile is the shoot sheath (plumule), coleorhiza is the root sheath (radicle).

Beyond the textbook. For the exam: the maize seed has a seed coat fused with the fruit wall, a scutellum and an aleurone layer. In reality: because the seed coat and fruit wall are fused, a maize grain is technically a whole fruit containing one seed. The same is true of wheat and rice grains. Why it matters: it explains why the covering of a maize grain is described as membranous and fused with the fruit wall.

Dicots and monocots in one table

Beyond the textbook. For the exam: learn the table above as a set of rules. In reality: these are tendencies, not laws. A few monocots, such as Smilax and yam (Dioscorea), have netted leaf veins. Modern classification also shows that "dicots" are not a single evolutionary group, because some of the oldest flowering plants share features with both. Why it matters: if a question asks about the typical character, use the table, but treat "always" with suspicion.

Describing a plant like a botanist

A botanist describes a plant in a fixed order: habit, then the vegetative characters (root, stem, leaves), then the floral characters (inflorescence and flower parts), and finally the floral diagram and floral formula.

A floral diagram shows the number and arrangement of the parts and how they are related. The mother axis is shown as a dot at the top, and the whorls are drawn from the calyx on the outside to the gynoecium at the centre.

A floral formula is a shorthand with these symbols.

Mustard (Brassicaceae): ⊕ ⚥ K2+2 C4 A2+4 G(2), with a superior ovary.

Solanaceae: the potato family

Solanaceae is widely spread in the tropics, subtropics and even temperate zones. Most members are herbs or shrubs, a few are small trees. The leaves are alternate, simple and exstipulate, with reticulate venation. The potato (Solanum tuberosum) has an underground stem.

Floral formula: ⊕ ⚥ K(5) C(5) A5 G(2), with a superior ovary. The "persistent" calyx explains the green cap you pull off the top of a brinjal or tomato, since the sepals stay on the fruit.

Solanaceae is important to people as food (tomato, brinjal, potato), spice (chilli), medicine (belladonna, ashwagandha), fumigatory (tobacco) and ornamentals (petunia).

Exam traps

60-second revision

Where this chapter connects

• Previous: Animal Kingdom closed the diversity unit. From here the book turns to the structure of the organism itself.
• Earlier link: Plant Kingdom introduced angiosperms as flowering plants with seeds in fruits, and the dicot and monocot division used here.
• Next: Anatomy of Flowering Plants looks inside the root, stem and leaf you have just described from outside.
• Class XII link: Sexual Reproduction in Flowering Plants follows the stamens, carpels and ovules from this chapter through pollination to seed and fruit.