Anatomy of Flowering Plants: Class 11 Biology · Chapter 6

Chew a length of sugarcane and you are doing plant anatomy with your teeth. The sweet juice comes from soft, thin-walled cells that make up most of the stem; the tough strands you spit out are conducting bundles wrapped in thick-walled fibres. Cut the cane straight across and those strands appear as dots scattered over the cut face. Their number, size and position are not random: they follow a plan shared by maize, bamboo and most other monocot stems.

Anatomy is the study of what organisms are like inside. For a flowering plant it means cutting thin slices of a root, stem or leaf and reading them under a microscope. Three ideas run through this chapter. First, the plant body is built in levels: cells form tissues, and tissues form organs. Second, the root, the stem and the leaf each follow their own internal plan, and monocots and dicots follow different versions of each plan. Third, internal structure suits the organ's job and the plant's surroundings, as you will see in leaves that save water.

Chapter 5 showed you the outside of these organs. Here you learn to recognise six sections that are standard in practicals and exams: the root, the stem and the leaf of a dicot and of a monocot.

6.1 The Tissue System

A tissue (ऊतक) is a group of cells that share an origin and a job. Botanists also group tissues one level higher, by where they sit in the plant body and what they do there. Seen this way, every root, stem and leaf contains the same three tissue systems: a covering on the outside, a conducting network, and the filling in between (Fig. 6.1).

6.1.1 Epidermal Tissue System

Think of the epidermal tissue system as the plant's skin, together with everything set into it or growing out of it: the epidermal cells, the stomata and the hairs. It covers the whole primary body, meaning the parts formed directly by the growing tips.

Run a finger over a leaf and what you touch is wax. Above ground, the outer walls of the plant's skin carry a waxy cuticle that cuts down water loss. Roots have no cuticle, which makes sense: a root surface exists to let water in, not to keep it out. Under the cuticle, the epidermis itself is usually just one cell thick. Its elongated cells fit edge to edge with no gaps apart from the stomatal pores, and each is a parenchyma cell that is mostly vacuole, with only a thin film of cytoplasm pressed against the wall (Fig. 6.2).

Look at a peel of leaf epidermis under a microscope and, among the flat, jigsaw-shaped cells, you will find pairs of curved cells with a slit between them. Each pair is a set of guard cells, and they are unusual in two ways. They contain chloroplasts, which ordinary epidermal cells lack, and their walls are unevenly thick: heavily thickened on the side facing the slit, thin on the side away from it. This lets the pair change shape, widening or closing the gap between them.

The gap is a stoma (रंध्र; plural stomata), and it is the leaf's main gateway: carbon dioxide and oxygen pass through it, and so does the water vapour lost in transpiration. By opening and closing it, the guard cells regulate both. Their shape differs between groups: kidney- or bean-shaped in most plants, but like a dumb-bell in grasses, narrow in the middle with a swelling at each end.

Often the epidermal cells right beside the guard cells differ from the rest in shape and size; these are subsidiary cells. The pore, its two guard cells and the subsidiary cells together form one stomatal apparatus (Fig. 6.3).

Epidermal cells also grow hairs, and the hairs of shoot and root do opposite jobs. Shoot hairs, the trichomes, are usually built of several cells; they can be branched or unbranched, soft or stiff, and some secrete substances. A coat of them holds still, moist air against the surface and so slows the loss of water by transpiration. A root hair, by contrast, is a single epidermal cell drawn out into a long thin tube. Thousands of them enlarge the surface through which the root takes in soil water and dissolved minerals.

6.1.2 The Ground Tissue System

Most of a young stem is neither skin nor plumbing. This filling is the ground tissue, and it is defined by exclusion: every tissue that is not epidermis and not part of a vascular bundle belongs to it. Its building materials are the three simple tissues, parenchyma, collenchyma and sclerenchyma (Fig. 6.4).

In a leaf, the ground tissue is the mesophyll: thin-walled, chloroplast-filled parenchyma where most of the plant's photosynthesis happens. In primary stems and roots, parenchyma again dominates, forming the cortex, the pericycle, the pith and the medullary rays (strips of tissue between bundles), all of which you will meet in Section 6.2. Collenchyma and sclerenchyma join in wherever support is needed.

6.1.3 The Vascular Tissue System

The vascular system is the plant's plumbing. It is made of two complex tissues, xylem and phloem, which run side by side in strands called vascular bundles. Bundles are sorted by asking two questions about them (Fig. 6.5).

Where do xylem and phloem lie? Imagine spokes running out from the centre of the organ. In a root, xylem and phloem form separate patches that take turns around the centre, each on a spoke of its own: a radial arrangement. In stems and leaves, a patch of xylem and a patch of phloem share one spoke and form a single bundle: a conjoint arrangement, in which the phloem usually lies only on the outer side of the xylem.11A conjoint bundle with phloem only outside the xylem is called collateral. Stems of the pumpkin family have phloem on both sides (bicollateral). These two terms go beyond the current NCERT text.

Is there cambium? The words open and closed describe a bundle's future. An open bundle keeps a thin strip of cambium between its phloem and its xylem, so it can go on adding secondary xylem and phloem; the bundles of dicot stems are open. A closed bundle has no cambium and will never form secondary tissues; monocot bundles are closed.

6.2 Anatomy of Dicotyledonous and Monocotyledonous Plants

To compare organs fairly, botanists cut transverse sections (T.S.), thin slices at right angles to the long axis, from the mature region of each organ, where the tissues have finished forming. A leaf blade is cut across its width to give a vertical section that shows both surfaces. Each section is then read from the outside inwards. As you go through the six sections below, notice which layers stay the same between dicots and monocots and which change: the changes are what identify a section.

6.2.1 Dicotyledonous Root

Take a thin transverse section of a gram (chana) seedling root, a little way behind the tip (Fig. 6.7). Reading inwards, you cross a skin, a broad cortex and then a compact central core. The skin is the epiblema, the root's version of an epidermis, and many of its cells are drawn out into root hairs, each a single cell. The broad cortex is loose, thin-walled parenchyma with air spaces between the cells.

The cortex ends in a layer quite unlike the rest of it: the endodermis (अंतस्त्वचा). Here barrel-shaped cells are fitted together in one unbroken ring, without a single air space. Running through their walls is the Casparian strip, a band of suberin, a waxy substance that water cannot cross (Fig. 6.6).22NCERT places these strips on the tangential and radial walls. Plant anatomists locate the band more precisely on the radial and transverse walls, the walls at right angles to the root surface; in a T.S. it shows up on the radial walls. In NCERT-based exams, expect NCERT's wording.

Everything inside the endodermis makes up the stele, the root's central cylinder. Its outermost layer is the pericycle (परिरंभ), which NCERT describes as a few layers of thick-walled parenchyma. Pericycle cells can still divide, so this layer is where lateral roots are born and, once the root starts to thicken, where part of the vascular cambium arises.

Now count the xylem patches. A dicot root usually has only two to four, alternating with as many phloem patches: the radial arrangement of Section 6.1.3. In each xylem patch the narrow protoxylem lies at the outer tip and the wider metaxylem towards the centre, the exarch condition. Parenchyma called conjunctive tissue fills the gaps between the patches, and the centre has little or no pith. In older roots a cambium appears between the xylem and the phloem, and the root begins secondary growth.

6.2.2 Monocotyledonous Root

A maize root cut the same way shows the same sequence of tissues: epiblema with root hairs, cortex, endodermis, pericycle, radial vascular bundles and pith (last step of Fig. 6.7). Two differences stand out. There are many more xylem patches, usually more than six (the polyarch condition), and the central pith is wide and prominent. A third difference cannot be seen in a young section: monocot roots never undergo secondary growth.

6.2.3 Dicotyledonous Stem

Now cut across a young sunflower stem (Fig. 6.8). The skin is familiar: an epidermis with a thin cuticle, a scattering of trichomes and a few stomata. The difference from the root begins just beneath it, where the cortex, reaching from the epidermis to the pericycle, is divided into three zones.

• Hypodermis, the outer zone: a band of collenchyma a few cells deep, whose corner thickenings give the young stem strength without making it brittle.
• Cortical parenchyma, the middle zone: loosely packed, round cells with thin walls and plenty of air between them.
• Endodermis, the inner boundary: a single layer whose cells are packed with starch grains, which is why a stem's endodermis is also called the starch sheath.

Next come the vascular bundles, many of them, arranged in a single ring: the one feature that identifies a dicot stem at a glance. Over the phloem of each bundle sits a half-moon (semi-lunar) cap of sclerenchyma, which is how the pericycle appears in this stem. Inside, each bundle is conjoint and open: phloem outside, cambium in the middle, xylem inside, with the protoxylem pointing to the centre (endarch). The gaps between neighbouring bundles are bridged by medullary rays, a few layers of parenchyma cells lined up along the radius. The whole middle of the stem is a large pith (मज्जा) of rounded parenchyma cells with wide spaces between them.

6.2.4 Monocotyledonous Stem

A maize stem looks very different (Fig. 6.9). Below the epidermis is a hypodermis of sclerenchyma, not collenchyma. There is no ring: many vascular bundles lie scattered through a large mass of parenchymatous ground tissue, so there is no clear boundary between cortex and pith and no medullary rays. Bundles near the edge are smaller and crowded; those towards the centre are larger and farther apart.

Each bundle is wrapped in a bundle sheath of sclerenchyma and is conjoint and closed: there is no cambium, so the stem cannot thicken by secondary growth. Two finer details help in identification: the bundle has no phloem parenchyma, and it holds a water-containing cavity next to the protoxylem (Fig. 6.10).33The cavity forms when the earliest protoxylem elements stretch and break down; it is often called a lysigenous cavity.

6.2.5 Dorsiventral (Dicotyledonous) Leaf

A mango leaf has two different faces: a dark, glossy upper side and a paler lower side. A leaf like this is dorsiventral. To see why the faces differ, cut a thin vertical section across the blade (Fig. 6.11). Three regions show up: the epidermis, the mesophyll and the vascular system.

Start at the top. A clearly visible cuticle coats the adaxial epidermis, the upper layer, so called because it lies on the side facing the stem. Stomata in this layer are few, and in many leaves there are none.

Under it lies the mesophyll, the chloroplast-rich parenchyma that carries out the leaf's photosynthesis, in two zones. First comes the palisade parenchyma: long, narrow cells standing upright side by side, like fence posts. Then, all the way down to the lower epidermis, the spongy parenchyma: rounded or oval cells packed loosely around large air spaces and cavities.

At the bottom is the abaxial epidermis, which also has a cuticle. It usually holds most of the leaf's stomata, each opening into an air cavity in the spongy layer.

Running through the mesophyll are the veins, from the thick midrib to the finest branches, each carrying a vascular bundle. A dicot leaf's veins form a network of many thicknesses (reticulate venation), so its bundles range widely in size: a thick vein holds a big bundle, a fine vein a small one. A sheath of thick-walled cells, the bundle sheath, encloses each bundle.

6.2.6 Isobilateral (Monocotyledonous) Leaf

Hold a maize or wheat leaf up to the light: its two faces look much alike. Such a leaf is isobilateral. Cut it in section and the three regions of a dorsiventral leaf are all there, but two things change (Fig. 6.12). Stomata now dot both epidermal layers, upper and lower, and the mesophyll is one uniform tissue instead of a palisade layer over a spongy one.

The venation shows in section too. Monocot leaves have parallel venation, so apart from the bundles of the main veins, the bundles are all of nearly the same size, lined up in a row.

On a hot, dry afternoon, look along a maize field: many leaves have curled into tubes. The cause lies in the upper epidermis. Along the veins, some adaxial epidermal cells are far larger than their neighbours and look empty and colourless: these are the bulliform cells. Full of water (turgid), they keep the blade spread flat. When water runs short they lose turgor (become flaccid), the upper side shrinks, and the blade rolls inwards with its upper surface tucked inside (Fig. 6.13). A rolled leaf exposes less surface and so loses less water.