Anatomy of Flowering Plants: NEET notes

1. Three tissue systems

NCERT §6 introduction, §6.1

• Anatomy is the study of the internal structure of an organism. In a plant, cells make up tissues and tissues make up organs, and each organ has its own internal plan.
• Monocots and dicots differ inside as well as outside, and internal structure also changes as plants adapt to different environments.
• Grouping tissues by where they sit and what they look like gives three tissue systems: the epidermal system, the ground (fundamental) system and the vascular (conducting) system.
• Epidermal system: the outer covering, made of epidermal cells, stomata and appendages such as hairs and trichomes.
• Ground system: everything except the epidermis and the vascular bundles. It forms most of the plant body and is divided into cortex, pericycle and pith.
• Vascular system: xylem and phloem, which together form vascular bundles and carry water, minerals and food.
• To compare roots, stems and leaves, their mature zones are cut across (transverse sections) and viewed under the microscope.

2. Epidermis and stomata

NCERT §6.1.1, Figure 6.1

• In the primary body of a plant, the epidermis is the layer on the very outside. Its cells are elongated and packed tightly into one unbroken layer, which is usually a single cell thick.
• Epidermal cells are parenchyma: a large vacuole with only a thin film of cytoplasm pressed against the wall.
• A waxy, thick cuticle often covers the outer surface and cuts down water loss. Roots have no cuticle.
• Stomata are found in the epidermis of leaves and control transpiration and the exchange of gases.
• Each stoma has two guard cells around a pore. Guard cells are bean-shaped in most plants but dumb-bell shaped in grasses.
• The wall of a guard cell facing the pore (inner wall) is heavily thickened; the wall away from the pore (outer wall) is thin. Guard cells contain chloroplasts and control opening and closing.
• Epidermal cells next to the guard cells may change shape and size; these are subsidiary cells.
• Stomatal apparatus = stomatal pore + guard cells + the subsidiary cells around them.

3. Hairs and ground tissue

NCERT §6.1.1, §6.1.2

• Root hairs are single-celled outgrowths of epidermal cells. They take up water and minerals from the soil.
• Hairs on the stem are called trichomes. In the shoot they are usually multicellular, branched or unbranched, soft or stiff, and some secrete substances.
• Trichomes help reduce the water lost in transpiration.
• Ground tissue is every tissue other than the epidermis and vascular bundles. It is built of simple tissues: parenchyma, collenchyma and sclerenchyma.
• In primary stems and roots, parenchyma is found in the cortex, pericycle, pith and medullary rays.
• In leaves the ground tissue is the mesophyll: thin-walled cells that contain chloroplasts.

4. Vascular bundles

NCERT §6.1.3, Figure 6.2

• Xylem and phloem are complex tissues; together they make up a vascular bundle.
• Open bundle: cambium lies between phloem and xylem, so the bundle can later add secondary xylem and phloem. Found in dicot stems.
• Closed bundle: no cambium, so no secondary tissues form. Found in monocots.
• Radial arrangement: xylem and phloem lie in separate patches on different radii, alternating round the centre. This is the root pattern.
• Conjoint arrangement: xylem and phloem lie together on the same radius. This is common in stems and leaves.
• In a conjoint bundle the phloem is usually only on the outer side of the xylem.
• Combine the two words for a full description: dicot stem bundles are conjoint and open; monocot stem bundles are conjoint and closed; root bundles are radial.

5. Dicot and monocot roots

NCERT §6.2.1, §6.2.2, Figure 6.3

• The dicot root studied is sunflower. From outside in: epiblema, cortex, endodermis, pericycle, then the vascular strands and a small pith.
• Epiblema is the outer layer; many of its cells grow out as single-celled root hairs.
• Cortex: several layers of thin-walled parenchyma with spaces between the cells.
• Endodermis: the innermost cortex layer, one row of barrel-shaped cells with no intercellular spaces. Their radial and tangential walls carry casparian strips of suberin, a waxy material that water cannot pass through.
• Pericycle: a few layers of thick-walled parenchyma just inside the endodermis. Lateral roots start here, and so does the vascular cambium when secondary growth begins.
• Dicot root: usually two to four xylem patches and as many phloem patches, with parenchyma between them called conjunctive tissue. Pith is small or inconspicuous. A cambium ring forms between xylem and phloem later.
• Stele = everything inside the endodermis: pericycle, vascular bundles and pith.
• Monocot root: the same layers, but usually more than six xylem bundles (polyarch), a large, well-developed pith, and no secondary growth.
• In both roots the protoxylem is drawn towards the outside, next to the pericycle, and the metaxylem towards the centre.

6. Dicot stem

NCERT §6.2.3, Figure 6.4

• Epidermis: the outer protective layer, with a thin cuticle; it may carry trichomes and a few stomata.
• Cortex lies between epidermis and pericycle and has three sub-zones: hypodermis, general cortex and endodermis.
• Hypodermis: a few layers of collenchyma just under the epidermis, giving the young stem mechanical strength.
• Below it, rounded thin-walled parenchyma cells with clear intercellular spaces.
• Endodermis: the innermost cortex layer. Its cells are full of starch grains, so it is also called the starch sheath.
• Pericycle: semi-lunar patches of sclerenchyma sitting just above the phloem, inside the endodermis.
• Vascular bundles are many and arranged in a ring, the mark of a dicot stem. Each is conjoint and open, with endarch protoxylem (protoxylem towards the centre).
• Medullary rays: a few layers of radially placed parenchyma between neighbouring bundles.
• Pith: the central region of many rounded parenchyma cells with large intercellular spaces.

7. Monocot stem

NCERT §6.2.4, Figure 6.4

• Hypodermis is made of sclerenchyma, not collenchyma as in the dicot stem.
• Vascular bundles are many and scattered through the ground tissue instead of forming a ring.
• Each bundle is wrapped in a sclerenchymatous bundle sheath.
• Bundles are conjoint and closed: there is no cambium.
• Bundles near the edge are generally smaller than those near the centre.
• Phloem parenchyma is absent, and the bundles contain water-containing cavities.
• The bundles sit in a large, conspicuous ground tissue of parenchyma that fills the stem.
• Identification key: scattered, conjoint, closed bundles with sclerenchyma sheaths and no phloem parenchyma mean a monocot stem.

8. Dorsiventral leaf

NCERT §6.2.5, Figure 6.5

• A dicot leaf is dorsiventral. A vertical section through the lamina shows three parts: epidermis, mesophyll and vascular system.
• Upper surface = adaxial epidermis; lower surface = abaxial epidermis. Both carry a conspicuous cuticle.
• The abaxial (lower) epidermis usually has more stomata than the adaxial one, which may have none.
• Mesophyll is the parenchyma between the two epidermal layers. It contains chloroplasts and carries out photosynthesis.
• Palisade parenchyma lies on the adaxial side: long cells standing upright, side by side.
• Spongy parenchyma lies below the palisade and reaches the lower epidermis: oval or round cells loosely packed with many large spaces and air cavities.
• Vascular bundles run in the veins and midrib, and their size matches the size of the vein. Dicot veins vary in thickness because the venation is reticulate.
• A sheath of thick-walled cells rings each bundle. Xylem lies towards the upper (adaxial) side of the bundle and phloem towards the lower side.

9. Isobilateral leaf

NCERT §6.2.6, Figure 6.5

• A monocot leaf is isobilateral. It is built much like the dorsiventral leaf, with two key differences.
• Stomata occur on both the upper and the lower epidermis.
• The mesophyll is not divided into palisade and spongy parenchyma.
• In grasses, some adaxial epidermal cells along the veins become large, empty, colourless bulliform cells.
• When bulliform cells are full of water (turgid), the leaf lies open and its surface is exposed.
• When water is short and they go flaccid, the leaf rolls inwards, which reduces water loss.
• Because venation is parallel, the vascular bundles in a vertical section are of nearly the same size, except in the main veins.

Must-know facts

1. Three tissue systems: epidermal, ground (fundamental), vascular (conducting).
2. Cuticle is absent in roots.
3. Guard cells: bean-shaped in most plants, dumb-bell shaped in grasses; inner wall thick, outer wall thin; they contain chloroplasts.
4. Stomatal apparatus = pore + guard cells + subsidiary cells.
5. Root hairs are unicellular; shoot trichomes are usually multicellular.
6. Ground tissue = all tissue except epidermis and vascular bundles; in leaves it is the mesophyll.
7. Open bundle has cambium (dicot stem); closed bundle has none (monocot).
8. Radial bundles: roots. Conjoint bundles: stems and leaves, phloem usually outside the xylem.
9. Dicot root example in NCERT: sunflower.
10. Endodermis of root: barrel-shaped cells, no intercellular spaces, casparian strips of suberin on radial and tangential walls.
11. Lateral roots and the vascular cambium arise from the pericycle.
12. Dicot root: two to four xylem patches, small pith. Monocot root: usually more than six (polyarch), large pith, no secondary growth.
13. Stele = pericycle + vascular bundles + pith (everything inside the endodermis).
14. Dicot stem hypodermis: collenchyma. Monocot stem hypodermis: sclerenchyma.
15. Starch sheath = endodermis of the dicot stem.
16. Dicot stem pericycle: semi-lunar sclerenchyma patches above the phloem.
17. Dicot stem bundles: ring, conjoint, open, endarch protoxylem.
18. Monocot stem bundles: scattered, conjoint, closed, sclerenchymatous bundle sheath, no phloem parenchyma, water-containing cavities; peripheral ones smaller.
19. Dorsiventral leaf: more stomata on the abaxial side; palisade adaxial, spongy abaxial.
20. Isobilateral leaf: stomata on both sides; mesophyll undifferentiated; bulliform cells in grasses roll the leaf when flaccid.

Common traps

Key terms

Anatomy

The study of the internal structure of an organism.

Cuticle

Waxy layer on the outside of the epidermis that limits water loss; missing in roots.

Guard cells

The pair of cells that enclose a stomatal pore and open or close it.

Subsidiary cells

Epidermal cells near the guard cells that have become different in shape and size.

Stomatal apparatus

The pore, its guard cells and the subsidiary cells taken together.

Trichome

A hair on the shoot epidermis, usually multicellular.

Mesophyll

The chloroplast-bearing ground tissue of a leaf.

Open vascular bundle

A bundle with cambium between xylem and phloem, able to make secondary tissue.

Closed vascular bundle

A bundle without cambium, so no secondary tissue forms.

Radial bundle

Xylem and phloem in separate patches on different radii, as in roots.

Conjoint bundle

Xylem and phloem together on the same radius, as in stems and leaves.

Epiblema

The outermost layer of the root, many of whose cells form root hairs.

Casparian strips

Bands of suberin on the radial and tangential walls of root endodermis cells.

Pericycle

Layer just inside the endodermis; origin of lateral roots and the vascular cambium.

Conjunctive tissue

Parenchyma lying between the xylem and phloem patches of a root.

Stele

All tissues inside the endodermis: pericycle, vascular bundles and pith.

Polyarch

Having many xylem bundles, usually more than six, as in the monocot root.

Starch sheath

The starch-rich endodermis of the dicot stem.

Endarch

Protoxylem towards the centre and metaxylem towards the outside of the bundle.

Bulliform cells

Large, empty, colourless adaxial epidermal cells of grass leaves that roll the leaf when flaccid.