Anatomy 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 6.

Can you read a plant from one slice?

Cut a thin transverse section of a stem, look at it under a microscope and a botanist can tell you at once whether it came from a sunflower or from maize, and whether it is a root, stem or leaf. No label needed. Plants write their identity into the arrangement of their cells. This chapter teaches you to read that arrangement, which is exactly what CUET asks when it shows you a description of a section and wants the organ.

From cells to tissues to tissue systems

Anatomy is the study of internal structure. In a plant, cells form tissues and tissues form organs (root, stem, leaf). Tissues are of two broad kinds. Meristematic tissue keeps dividing: apical meristems at root and shoot tips lengthen the plant, intercalary meristems (at the base of leaves or between nodes, as in grasses) add length at a joint, and lateral meristems such as the cambium add girth. Permanent tissues have stopped dividing and do a fixed job. They are simple if made of one cell type and complex if made of several.

Looking at a section, you do not hunt for individual tissues. You look at three tissue systems, grouped by where they sit and what they do. The epidermal system is the skin. The ground system is the bulk. The vascular system is the plumbing.

The epidermal tissue system: the skin

The epidermis is the outermost layer of the primary plant body. It is usually one cell thick, made of elongated, tightly packed parenchyma cells with a thin lining of cytoplasm and a large vacuole. On the outside, shoots carry a waxy cuticle that cuts water loss. Roots have no cuticle, and the reason is practical: roots exist to absorb water, and a waterproof coat would defeat the purpose. Instead, many root epidermal cells grow out as root hairs, which are unicellular. On stems the equivalent outgrowths are trichomes, which are usually multicellular, branched or unbranched, sometimes secretory, and help cut water loss.

Stomata are the pores in the leaf epidermis that control gas exchange and transpiration. Each pore is guarded by two guard cells, which are bean shaped in most plants and dumb-bell shaped in grasses. Guard cells have chloroplasts, and their walls are uneven: the outer wall (away from the pore) is thin and the inner wall (next to the pore) is highly thickened. The neighbouring epidermal cells that differ in shape and size are subsidiary cells. Pore, guard cells and subsidiary cells together make the stomatal apparatus.

Why the uneven wall matters. When water enters a guard cell it swells, and the thin outer wall stretches more than the thick inner wall. The cell bows outwards, pulling the inner walls apart, and the pore opens. When the guard cells lose water they straighten and the pore closes.

Pause and predict: a student sees a cuticle on a root section drawing. Why is that drawing wrong?

Check your answer: roots absorb water and minerals through the epidermis and root hairs. The cuticle is a waxy waterproof layer, so roots do not have one.

The vascular system and its vocabulary

Vascular bundles are where students lose marks, because three separate pairs of words describe a bundle and students blend them together. Take each pair on its own.

Why cambium decides everything. Cambium is a layer of cells that can still divide. A bundle with cambium can keep adding new xylem and new phloem, which is how a woody stem gets thicker year after year. A closed bundle has lost that ability, so the organ can only grow by stretching. That one fact explains why open bundles go with dicots and secondary growth, and closed bundles go with monocots.

Beyond the textbook. For the exam: dicots have open bundles and show secondary growth, and monocots have closed bundles and do not. In reality: a few monocots do thicken, but by a different route. Dracaena, for example, builds a cambium in its trunk, and palm trunks get wide through extra growth of their own tissues. Also, modern classification no longer treats "dicots" as one natural group, since the dicot label covers several separate lineages. Why it matters: NTA follows the textbook rule, so use the textbook rule for marks and remember the nuance only if a question explicitly invites it.

The dicot root

Read a root section from the outside in. The outermost layer is the epiblema, many of whose cells grow into root hairs. Under it lies the cortex, several layers of thin-walled parenchyma with intercellular spaces. The innermost layer of the cortex is the endodermis, a single ring of barrel-shaped cells with no gaps between them. In the tangential and radial walls of these cells lies a deposit of the waxy, water-impermeable material suberin in bands called casparian strips.

Why the strip exists. Water moving through a root can travel through the spaces between cells and the cell walls, and nothing is checking what it carries. The casparian strip blocks that route at the endodermis, so water must pass through the living membrane of an endodermal cell to reach the xylem. The endodermis therefore works as a gatekeeper that selects what enters the vascular tissue.

Beyond the textbook. For the exam: write that suberin is deposited on the tangential and radial walls of the endodermis as casparian strips. In reality: the strip is a band running around the radial and transverse walls, the walls that sit side by side with neighbouring cells, and it contains lignin as well as suberin. Why it matters: knowing where the strip actually sits is what makes the gatekeeper idea work, but write the standard exam wording in an answer.

Next comes the pericycle, a few layers of thick-walled parenchyma. It matters for two reasons: lateral roots begin here, and part of the vascular cambium forms from it during secondary growth. Everything inside the endodermis (pericycle, vascular bundles and pith) is the stele. Inside the stele there are usually two to four xylem and phloem patches, with xylem and phloem on alternate radii (radial). The parenchyma between the xylem and the phloem is conjunctive tissue, and the pith is small or inconspicuous. A cambium ring develops later between the xylem and the phloem, so dicot roots undergo secondary growth.

The monocot root is built on the same plan (epidermis, cortex, endodermis, pericycle, vascular bundles, pith), with three differences you must hold on to.

Pause and predict: a root section has eight xylem bundles and a broad central pith. Monocot or dicot?

Check your answer: monocot. Eight is more than six (polyarch) and the pith is large, and a dicot root rarely exceeds four patches.

The dicot stem

Move from the outside to the centre of a young dicot stem. The epidermis has a thin cuticle and may carry trichomes and a few stomata. Below it is the cortex, in three zones: a hypodermis of collenchyma that gives the young stem strength, then rounded thin-walled parenchyma with conspicuous intercellular spaces, and finally the endodermis, whose cells are full of starch grains, which is why it is also called the starch sheath. Inside that, the pericycle sits above the phloem as semi-lunar patches of sclerenchyma. Then come the vascular bundles, arranged in a ring, which is the signature of a dicot stem. Each bundle is conjoint, open and endarch. Between bundles run radial rows of parenchyma, the medullary rays, and the centre is occupied by a large, rounded, parenchymatous pith with big intercellular spaces.

The monocot stem

The monocot stem looks quite different. Its hypodermis is sclerenchymatous, not collenchymatous. A large mass of parenchymatous ground tissue fills the section, and through it are scattered many vascular bundles, each one wrapped in a sclerenchymatous bundle sheath. The bundles are conjoint and closed, and those at the edge are generally smaller than those at the centre. Two diagnostic details: phloem parenchyma is absent, and water-containing cavities lie within the bundles.

Beyond the textbook. For the exam: open bundles mean cambium, and cambium means secondary growth. In reality: that cambium is the reason a tree trunk can add a new layer of wood every growing season. It lays down secondary xylem inwards and secondary phloem outwards, with far more xylem than phloem, so most of a trunk is xylem. Why it matters: it turns an abstract word, "open", into something you can picture: it is the bundle that is still able to grow.

Pause and predict: a section shows a ring of vascular bundles with medullary rays between them. Which stem is it, sunflower or maize?

Check your answer: sunflower. A ring of bundles and medullary rays are dicot features; maize shows scattered bundles.

The leaf: dorsiventral and isobilateral

A leaf section has three parts: epidermis, mesophyll and vascular system. The mesophyll is the chloroplast-rich parenchyma where photosynthesis happens. A dicot leaf is dorsiventral, meaning the upper and lower surfaces differ. A monocot leaf is isobilateral, with both sides alike.

In the dorsiventral leaf the adaxial (upper) surface faces the light and the abaxial (lower) surface faces away. The mesophyll is sorted into two kinds of cell. Just under the upper epidermis lie palisade parenchyma, elongated cells packed vertically to catch light. Below them lies spongy parenchyma, rounded and loosely packed with large air spaces, which let gases move freely to the stomata, since the abaxial epidermis generally bears more stomata than the adaxial one, which may have none at all. Vascular bundles run in the veins and midrib and are surrounded by a layer of thick-walled bundle sheath cells. In a net-veined leaf, the bundles vary in size with the thickness of the vein.

The isobilateral leaf is similar, with two differences: stomata are on both surfaces, and the mesophyll is not differentiated into palisade and spongy. Because the veins are parallel, the vascular bundles are nearly the same size apart from the main veins.

In grasses there is one more feature. Certain adaxial epidermal cells along the veins are large, empty and colourless: the bulliform cells. When they are turgid, the leaf blade lies flat and open. When water is short, they go flaccid and the leaf curls inwards, which reduces the surface exposed to the air and cuts water loss.

Pause and predict: a wheat leaf has stomata on both sides. How does it still conserve water on a hot afternoon?

Check your answer: the bulliform cells lose turgor, the blade rolls inwards and less leaf surface is exposed to hot, dry air.

Exam traps

60-second revision

Where this chapter connects

• Previous: Morphology of Flowering Plants shows the roots, stems and leaves whose insides you have just read.
• Next: Structural Organisation in Animals moves from plant tissues to animal organ systems, using the frog.
• Related: Plant Growth and Development picks up meristems and the cambium that drive secondary growth, and Photosynthesis in Higher Plants uses the bundle sheath and mesophyll you met in the leaf.