Anatomy of Flowering Plants

You can see the outside of a plant easily. To see the inside you cut a thin section and put it under a microscope. The study of the internal structure of plants is called anatomy. Plants are built from cells; cells are organised into tissues; tissues are organised into organs. Different organs (root, stem, leaf) differ in their internal structure, and within angiosperms the monocots and dicots are also anatomically different. Internal structure also shows adaptations to different environments, which is why a grass leaf looks different from a mango leaf when sectioned.

The rationalised NCERT chapter opens directly with the tissue systems, and the older detailed treatment of meristems, simple and complex tissues and secondary growth has been removed from the CBSE syllabus. But the chapter still uses those names on every page, so you need the following as background. A tissue is a group of cells having a common origin and usually performing a common function. On the basis of the ability to divide there are two kinds: meristematic tissue (cells that keep dividing: apical meristems at root and shoot tips give primary growth; intercalary meristems between mature tissues in grasses regenerate parts eaten by cattle; lateral meristems such as vascular cambium and cork cambium give increase in girth) and permanent tissue (cells that have lost the ability to divide and have taken a fixed shape and function).

Permanent tissues are simple when made of one cell type: parenchyma (thin cellulose walls, living, storage, photosynthesis, packing), collenchyma (living, walls thickened at the corners with cellulose, hemicellulose and pectin, gives mechanical support to growing parts such as the young stem and petiole) and sclerenchyma (dead at maturity, thick lignified walls, as fibres and sclereids, gives strength). They are complex when several cell types work together: xylem (tracheids, vessels, xylem fibres and xylem parenchyma; conducts water and minerals upward and gives mechanical strength) and phloem (sieve tube elements, companion cells, phloem parenchyma and phloem fibres; conducts food from leaves to the rest of the plant). Two xylem words appear again and again: the first-formed xylem is protoxylem and the later-formed is metaxylem. When protoxylem lies towards the centre and metaxylem towards the periphery, as in stems, the arrangement is endarch; when protoxylem lies towards the periphery and metaxylem towards the centre, as in roots, it is exarch. These terms are still examined in NEET and are used freely in the sections that follow.

Tissues can be classified by the kinds of cells they contain, which is what the background above did. NCERT now asks a different question: how do tissues vary depending on their location in the plant body? Their structure and function depend on where they sit. On the basis of structure and location, there are three tissue systems: the epidermal tissue system, the ground or fundamental tissue system and the vascular or conducting tissue system.

Think of any organ in transverse section as three concentric zones. The outermost skin is the epidermal system. Everything inside the skin that is not a vascular bundle is the ground system: cortex, endodermis, pericycle, pith and medullary rays in stems and roots, and the mesophyll in leaves. The strands of xylem and phloem, wherever they are placed, are the vascular system. Every T.S. you draw in this chapter is just these three systems arranged in the pattern characteristic of that organ, so if you learn the three systems well the six sections in 6.2 become a matter of arrangement rather than new material.

The examiner's standard 3-mark or 5-mark question here is the old NCERT exercise: name the three basic tissue systems and give the tissue names under each. Answer it as a table. Epidermal: epidermis (epidermal cells), stomata (guard cells and subsidiary cells), epidermal appendages (root hairs, trichomes), cuticle. Ground: parenchyma, collenchyma, sclerenchyma, forming cortex, hypodermis, endodermis, pericycle, pith, medullary rays, mesophyll. Vascular: xylem (tracheids, vessels, xylem fibres, xylem parenchyma) and phloem (sieve tube elements, companion cells, phloem parenchyma, phloem fibres) arranged in vascular bundles, with cambium in open bundles.

The epidermal tissue system forms the outermost covering of the whole plant body. It comprises the epidermal cells, the stomata and the epidermal appendages (trichomes and hairs).

The epidermis is the outermost layer of the primary plant body and is usually single-layered. Its cells are parenchymatous, with a small amount of cytoplasm lining the cell wall and a large vacuole. The outside of the epidermis is often covered with a waxy, thick layer called the cuticle, which prevents loss of water. The cuticle is absent in roots, which makes sense because the root epidermis has to absorb water rather than hold it in.

Stomata are structures present in the epidermis of leaves. They regulate the processes of transpiration and gaseous exchange. Each stoma is composed of two bean-shaped cells called guard cells which enclose the stomatal pore. In grasses the guard cells are dumb-bell shaped. The outer walls of the guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened. This unequal thickening is what lets the guard cells bend outward when turgid and open the pore, and straighten when flaccid and close it. Guard cells possess chloroplasts and regulate the opening and closing of stomata. Sometimes a few epidermal cells in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells. The stomatal aperture, the guard cells and the surrounding subsidiary cells together are called the stomatal apparatus. That definition is a favourite 1-mark and 2-mark question.

The cells of the epidermis bear a number of hairs. Root hairs are unicellular elongations of the epidermal cells and help absorb water and minerals from the soil. On the stem the epidermal hairs are called trichomes. Trichomes in the shoot system are usually multicellular; they may be branched or unbranched, soft or stiff, and may even be secretory. Trichomes help in preventing water loss due to transpiration. So the contrast the examiner wants is: root hair = unicellular, absorption; trichome = multicellular, on the stem, reduces water loss.

NCERT's definition is short and should be quoted as it is: all tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. That single sentence is worth 2 marks whenever the examiner asks 'What is ground tissue?'

Where do you find it? Parenchymatous cells are usually present in the cortex, pericycle, pith and medullary rays in the primary stems and roots. So when you label a T.S., every one of those layers belongs to the ground system, and most of them are parenchyma. The exceptions worth remembering are the hypodermis of the dicot stem (collenchyma) and the hypodermis of the monocot stem (sclerenchyma), which are also ground tissue but not parenchyma, and the semi-lunar patches of sclerenchymatous pericycle above the phloem in a dicot stem.

In leaves the ground tissue consists of thin-walled, chloroplast-containing cells called mesophyll. In a dicot leaf the mesophyll is differentiated into palisade parenchyma and spongy parenchyma; in a monocot leaf it is not differentiated. That difference belongs to section 6.2.5 and 6.2.6, but you should already see that the mesophyll is simply the ground tissue of the leaf.

Function follows cell type. Parenchyma stores food and water, fills space and, when it has chloroplasts, photosynthesises. Collenchyma gives flexible mechanical support to growing organs. Sclerenchyma gives rigid strength. The ground system therefore does the storing, the supporting and (in the leaf) the photosynthesis, while the epidermal system protects and the vascular system conducts.

The vascular system consists of the complex tissues, the phloem and the xylem. The xylem and phloem together constitute vascular bundles. Everything the examiner asks in this section is about how the two tissues are placed relative to each other and whether cambium sits between them.

Open versus closed. In dicotyledonous stems, cambium is present between the phloem and the xylem. Such vascular bundles, because of the presence of cambium, possess the ability to form secondary xylem and phloem tissues, and hence are called open vascular bundles. In the monocotyledons the vascular bundles have no cambium present in them. Since they do not form secondary tissues they are referred to as closed. The word open therefore means 'open to further growth', not physically open.

Radial versus conjoint. When xylem and phloem within a vascular bundle are arranged in an alternate manner along different radii, the arrangement is called radial, such as in roots. In the conjoint type of vascular bundles, the xylem and phloem are jointly situated along the same radius of the vascular bundle. Such vascular bundles are common in stems and leaves. The conjoint vascular bundles usually have the phloem located only on the outer side of the xylem.

Put the two classifications together and you get the three drawings in NCERT Figure 6.4, which are asked as 'identify A, B and C': (A) radial, as in a root, with separate xylem and phloem strands on alternate radii; (B) conjoint closed, as in a monocot stem, with phloem outside xylem and no cambium; (C) conjoint open, as in a dicot stem, with a strip of cambium between phloem and xylem. Learn the pairing organ → bundle type: root → radial; dicot stem → conjoint, open, endarch; monocot stem → conjoint, closed, endarch, with a sclerenchymatous bundle sheath.

For a clear understanding of tissue organisation, NCERT looks at the transverse sections of the mature zones of the root and stem and a vertical section of the leaf, in a dicot and a monocot each. That gives six sections. The trick to learning them is to fix the order of layers from outside to inside and then note only what changes between monocot and dicot.

For a root the sequence is epiblema (with unicellular root hairs) → cortex → endodermis (casparian strips) → pericycle → vascular bundles (radial, exarch) with conjunctive tissue → pith. Dicot root: two to four xylem bundles, pith small; monocot root: more than six (polyarch), pith large.

For a stem the sequence is epidermis (cuticle, trichomes, few stomata) → hypodermis → cortex → endodermis → pericycle → vascular bundles (conjoint, endarch) → pith. Dicot stem: collenchymatous hypodermis, endodermis as starch sheath, sclerenchymatous pericycle patches, bundles in a ring, open, medullary rays between them, large pith; monocot stem: sclerenchymatous hypodermis, no distinct cortex, endodermis or pith, bundles scattered, closed, each in a sclerenchymatous bundle sheath, phloem parenchyma absent, water-containing cavities present.

For a leaf the sequence is upper (adaxial) epidermis → mesophyll → lower (abaxial) epidermis, with vascular bundles in the veins wrapped by a bundle sheath. Dorsiventral (dicot) leaf: palisade above, spongy below, more stomata on the lower surface, reticulate venation so bundles of varying size; isobilateral (monocot) leaf: undifferentiated mesophyll, stomata on both surfaces, bulliform cells, parallel venation so bundles of nearly similar size.

The exam almost always asks for a diagram plus the differences in a table. Practise drawing each section in under four minutes with eight to ten labels, and practise writing the differences as three-column tables (feature, dicot, monocot). The next six sections give you the NCERT description of each, in the words the marking scheme uses.

Look at NCERT Figure 6.5(a), the T.S. of a sunflower root, and read the layers from outside in.

The outermost layer is the epiblema. Many of the cells of the epiblema protrude in the form of unicellular root hairs. There is no cuticle. The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces. The innermost layer of the cortex is called the endodermis. It comprises a single layer of barrel-shaped cells without any intercellular spaces. The tangential as well as the radial walls of the endodermal cells have a deposition of water-impermeable, waxy material, suberin, in the form of casparian strips. These strips force water and minerals moving inward to pass through the endodermal cell membrane rather than through the wall, so the endodermis controls what enters the stele.

Next to the endodermis lies the pericycle, a few layers of thick-walled parenchymatous cells. The initiation of lateral roots and of the vascular cambium during secondary growth takes place in these cells. That is the answer to the classic question 'from which layer do lateral roots arise?' (pericycle) and its pair 'from which layer do root hairs arise?' (epiblema, the epidermis).

The pith is small or inconspicuous. The parenchymatous cells which lie between the xylem and the phloem are called conjunctive tissue. There are usually two to four xylem and phloem patches, arranged radially, with the protoxylem pointing outward (exarch). Later, a cambium ring develops between the xylem and the phloem. All tissues on the inner side of the endodermis, such as pericycle, vascular bundles and pith, together constitute the stele.

In the exam, describe the dicot root as: epiblema with unicellular root hairs, parenchymatous cortex, endodermis with casparian strips, pericycle, radial exarch bundles two to four in number, conjunctive tissue, small pith. Each phrase is a marking point.

The anatomy of the monocot root, NCERT Figure 6.5(b) of a maize root, is similar to the dicot root in many respects. It has an epidermis (epiblema) with unicellular root hairs, a parenchymatous cortex, an endodermis with casparian strips, a pericycle, radial exarch vascular bundles with conjunctive tissue between them, and a pith. So the outer half of your answer is the same as for the dicot root.

The differences are two, and both come from the centre of the section. First, the monocot root has usually more than six xylem bundles; NCERT calls this polyarch, in contrast to the dicot root with two to four. Second, the pith is large and well developed, whereas in the dicot root it is small or inconspicuous. A third point follows from the first two: monocot roots do not undergo any secondary growth, because the pericycle and conjunctive tissue never form a cambium. In the dicot root a cambium ring appears later between xylem and phloem, and the pericycle gives rise to cork cambium, so the dicot root can thicken; the monocot root stays primary for life.

How would you tell a root from a stem in an unlabelled section? Roots have radial bundles with exarch protoxylem and an endodermis with casparian strips; stems have conjoint bundles with endarch protoxylem. And how would you tell a monocot root from a dicot root? Count the xylem groups (more than six versus two to four) and look at the pith (large versus small). Those two questions between them account for most of the 2-mark and 3-mark items on roots.

NCERT Figure 6.6(a) shows the T.S. of a sunflower stem. The epidermis is the outermost protective layer of the stem. It is covered with a thin layer of cuticle and may bear trichomes and a few stomata.

The cells arranged in multiple layers between the epidermis and the pericycle constitute the cortex. It has three sub-zones. The outer hypodermis consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem. Below it the cortical layers consist of rounded, thin-walled parenchymatous cells with conspicuous intercellular spaces. The innermost layer of the cortex is the endodermis. Its cells are rich in starch grains, so the layer is also called the starch sheath. This is the stem's version of the endodermis; there are no casparian strips here in the NCERT description.

The pericycle is present on the inner side of the endodermis and above the phloem bundles in the form of semi-lunar patches of sclerenchyma. In between the vascular bundles there are a few layers of radially placed parenchymatous cells which constitute the medullary rays.

A large number of vascular bundles are arranged in a ring; the ring arrangement of vascular bundles is a characteristic of the dicot stem. Each vascular bundle is conjoint, open, and with endarch protoxylem. Phloem lies outside, xylem inside, and a strip of cambium lies between them. A large number of rounded parenchymatous cells with large intercellular spaces occupy the central portion of the stem and constitute the pith.

In the exam: epidermis with cuticle and trichomes; collenchymatous hypodermis; parenchymatous general cortex; endodermis as starch sheath; sclerenchymatous pericycle in semi-lunar patches; conjoint, open, endarch bundles in a ring; medullary rays; large pith. Ten labels, ten marking points.

NCERT Figure 6.6(b), the T.S. of a maize stem, is the one section most students can recognise on sight because of its scattered bundles. The monocot stem has a sclerenchymatous hypodermis beneath the epidermis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue. Because the ground tissue is continuous there is no clear distinction into cortex, endodermis, pericycle and pith; the scattered bundles simply sit in it.

The vascular bundles are conjoint and closed: phloem lies outside xylem on the same radius, and there is no cambium, so the monocot stem never thickens by secondary growth. The peripheral vascular bundles are generally smaller than the centrally located ones. Protoxylem is endarch, as in all stems, and the xylem in each bundle is typically seen as a Y or V shape, with the two large metaxylem vessels forming the arms and the protoxylem at the base.

Two features are unique to the monocot stem and are asked as identifiers: phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles. The water-containing cavity (protoxylem lacuna) forms below the protoxylem when its elements are stretched and torn during elongation of the stem.

The old NCERT exercise remains the model 3-mark question: a T.S. shows conjoint, scattered bundles with sclerenchymatous bundle sheaths and no phloem parenchyma; identify it. Answer: a monocotyledonous stem, because scattered bundles with a bundle sheath and absence of phloem parenchyma are characteristic only of the monocot stem. And if you are handed a fresh section from the school garden, the reasons to call it monocot are: scattered bundles in ground tissue, a bundle sheath around each, closed bundles with no cambium, sclerenchymatous hypodermis, and no differentiation into cortex and pith; to call it dicot: bundles in a ring, open with cambium, collenchymatous hypodermis, distinct cortex, endodermis, pericycle, medullary rays and pith.

A vertical section through the lamina of a dicot leaf (NCERT Figure 6.7(a)) shows three main parts: the epidermis, the mesophyll and the vascular system.

The epidermis covers both the upper surface (adaxial epidermis) and the lower surface (abaxial epidermis) of the leaf, and has a conspicuous cuticle. The abaxial epidermis generally bears more stomata than the adaxial epidermis; the adaxial epidermis may even lack stomata entirely. This unequal distribution is what makes the leaf dorsiventral, with a distinct upper and lower face.

The tissue between the upper and lower epidermis is the mesophyll. Its cells possess chloroplasts and perform photosynthesis, and it is made up of parenchyma. It has two types of cells: the palisade parenchyma and the spongy parenchyma. The adaxially placed palisade parenchyma is made of elongated cells arranged vertically and parallel to each other, packed so that the maximum number of chloroplasts face the light. The spongy parenchyma lies below the palisade cells and extends to the lower epidermis; its cells are oval or round and loosely arranged, with numerous large spaces and air cavities between them, which are continuous with the stomata and allow gas exchange.

The vascular system includes vascular bundles which can be seen in the veins and the midrib. The size of the bundles depends on the size of the veins. The veins vary in thickness in the reticulate venation of dicot leaves, so in a section you see bundles of very different sizes. Each vascular bundle is surrounded by a layer of thick-walled bundle sheath cells. Xylem faces the upper epidermis and phloem the lower, and the bundles are conjoint and closed in the leaf.

For the 5-mark question draw the V.S. with the midrib on one side, label upper epidermis with cuticle, palisade parenchyma, spongy parenchyma with air spaces, lower epidermis with stomata and guard cells, a vascular bundle with xylem above and phloem below inside a bundle sheath, and write the description above in that order.

The anatomy of the isobilateral leaf (NCERT Figure 6.7(b), a grass leaf) is similar to that of the dorsiventral leaf in many ways: epidermis on both faces with a cuticle, mesophyll between them, vascular bundles in the veins with a bundle sheath. NCERT then lists the differences, and these are the whole examinable content of this section.

First, stomata are present on both surfaces of the epidermis, and in roughly equal numbers, because a grass leaf stands upright and both faces receive similar light. Second, the mesophyll is not differentiated into palisade and spongy parenchyma; all the mesophyll cells look alike. Those two features make the two faces of the leaf equal, hence 'isobilateral'.

Third, in grasses certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells called bulliform cells. When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss. Bulliform cells are a favourite for the 2-mark 'what and why' question and for the case-based passage about a rolled grass leaf on a hot afternoon.

Fourth, the parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in the main veins) as seen in vertical sections of the leaves. In a dicot leaf, with reticulate venation, bundle size varies widely; in a grass leaf the bundles are evenly spaced and much the same size.

The standard table asked for 3 marks: stomata on lower surface mainly versus both surfaces; mesophyll differentiated versus undifferentiated; bulliform cells absent versus present; reticulate venation with unequal bundles versus parallel venation with similar bundles; dorsiventral (two faces different) versus isobilateral (two faces alike).