Anatomy of Flowering Plants

2.1 Core concepts

• Anatomy defined. Study of the internal structure of plants is called anatomy. Plants have cells as the basic unit, organised into tissues, and tissues organised into organs. Different organs in a plant show differences in internal structure; within angiosperms, monocots and dicots are also anatomically different, and internal structures show adaptations to diverse environments (NCERT Ch. 6 intro, p. 71).
• Three tissue systems. On the basis of structure and location, plant tissues are grouped into three tissue systems — the epidermal tissue system, the ground (fundamental) tissue system and the vascular (conducting) tissue system (NCERT §6.1, p. 71).
• Epidermal tissue system. Forms the outermost covering of the whole plant body and comprises epidermal cells, stomata and the epidermal appendages — the trichomes and hairs. The epidermis is the outermost layer of the primary plant body, usually single-layered, made of elongated, compactly arranged parenchymatous cells with a small amount of cytoplasm lining the cell wall and a large vacuole (NCERT §6.1.1, pp. 71–72).
• Cuticle, stomata, guard cells and subsidiary cells. A waxy thick layer called the cuticle covers the outside of the epidermis to prevent water loss; cuticle is absent in roots. Stomata are structures in leaf epidermis that regulate transpiration and gaseous exchange. Each stoma is composed of two bean-shaped guard cells enclosing a stomatal pore; in grasses, the guard cells are dumb-bell shaped. The outer walls of guard cells (away from the pore) are thin and the inner walls (towards the pore) are highly thickened; guard cells possess chloroplasts and regulate stomatal opening and closing. Surrounding epidermal cells specialised in shape and size are subsidiary cells. The stomatal aperture, guard cells and surrounding subsidiary cells together form the stomatal apparatus (NCERT §6.1.1, p. 72, Figure 6.1).
• Trichomes and root hairs. The cells of epidermis bear a number of hairs. Root hairs are unicellular elongations of the epidermal cells that absorb water and minerals from soil. On the stem the epidermal hairs are called trichomes — usually multicellular, branched or unbranched, soft or stiff, sometimes secretory, helping prevent water loss due to transpiration (NCERT §6.1.1, p. 72).
• Ground tissue system. All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. Parenchymatous cells are usually present in cortex, pericycle, pith and medullary rays in primary stems and roots. In leaves, the ground tissue consists of thin-walled chloroplast-containing cells and is called mesophyll (NCERT §6.1.2, p. 72).
• Vascular tissue system. Made of complex tissues — the phloem and the xylem. Together they constitute vascular bundles. In dicotyledonous stems, cambium is present between phloem and xylem, so the bundle can form secondary xylem and phloem and is called an open vascular bundle. In monocotyledons the bundles lack cambium and so cannot form secondary tissues; they are closed. When xylem and phloem are arranged in an alternate manner along different radii, the arrangement is radial (typical of roots). In conjoint bundles xylem and phloem are jointly situated along the same radius of vascular bundles; such bundles are common in stems and leaves, and the phloem is usually located only on the outer side of the xylem (NCERT §6.1.3, p. 73, Figure 6.2).
• Dicotyledonous Root (T.S.). Outermost layer is the epiblema; many of its cells protrude as unicellular root hairs. Cortex consists of several layers of thin-walled parenchyma with intercellular spaces. The innermost layer of cortex is the endodermis — a single layer of barrel-shaped cells without intercellular spaces. The tangential as well as radial walls of endodermal cells have a deposition of water-impermeable, waxy material suberin in the form of casparian strips. Next to endodermis lies the pericycle — a few layers of thick-walled parenchyma; initiation of lateral roots and vascular cambium during secondary growth takes place in these cells. The pith is small or inconspicuous. The parenchymatous cells between xylem and phloem are conjunctive tissue. There are usually two to four xylem and phloem patches. Later, a cambium ring develops between xylem and phloem. All tissues on the inner side of the endodermis (pericycle, vascular bundles and pith) together constitute the stele (NCERT §6.2.1, pp. 73–74, Figure 6.3a).
• Monocotyledonous Root (T.S.). Anatomy of the monocot root is similar to that of the dicot root in many respects — it has epidermis, cortex, endodermis, pericycle, vascular bundles and pith. Differences: compared to dicot roots, monocot roots usually have more than six (polyarch) xylem bundles, a large and well-developed pith, and they do not undergo secondary growth (NCERT §6.2.2, p. 74, Figure 6.3b).
• Dicotyledonous Stem (T.S.). The epidermis is the outermost protective layer covered with a thin layer of cuticle; it may bear trichomes and a few stomata. Cortex lies in three sub-zones — the outer hypodermis of a few layers of collenchymatous cells providing mechanical strength to the young stem; the cortical layers of rounded, thin-walled parenchymatous cells with conspicuous intercellular spaces below the hypodermis; and the innermost cortical layer, the endodermis, whose cells are rich in starch grains — this layer is also referred to as the starch sheath. The pericycle is present on the inner side of endodermis and above the phloem as semi-lunar patches of sclerenchyma. Between vascular bundles lie a few layers of radially placed parenchymatous cells — the medullary rays. The vascular bundles are arranged in a ring — a characteristic of the dicot stem. Each vascular bundle is conjoint, open, with endarch protoxylem. A large number of rounded, parenchymatous cells with large intercellular spaces occupy the centre — the pith (NCERT §6.2.3, pp. 74–76, Figure 6.4a).
• Monocotyledonous Stem (T.S.). Sclerenchymatous hypodermis; a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath; a large, conspicuous parenchymatous ground tissue. Vascular bundles are conjoint and closed. Peripheral vascular bundles are generally smaller than the centrally located ones. Phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles (NCERT §6.2.4, p. 76, Figure 6.4b).
• Dorsiventral (Dicotyledonous) Leaf. Vertical section through the lamina shows three main parts — epidermis, mesophyll and vascular system. The epidermis covering both the upper (adaxial) and lower (abaxial) surfaces has a conspicuous cuticle; the abaxial epidermis generally bears more stomata than the adaxial epidermis, which may even lack stomata. The tissue between the upper and the lower epidermis is the mesophyll — chloroplast-bearing parenchyma carrying out photosynthesis — made up of two cell types: the adaxial palisade parenchyma (elongated cells arranged vertically and parallel) and the spongy parenchyma below (oval/round cells loosely arranged with numerous large air spaces and air cavities). The vascular system includes vascular bundles in veins and the midrib — sizes vary with vein thickness in reticulate venation. Vascular bundles are surrounded by a layer of thick-walled bundle sheath cells (NCERT §6.2.5, pp. 76–77, Figure 6.5a).
• Isobilateral (Monocotyledonous) Leaf. Similar to the dorsiventral leaf in many ways but shows characteristic differences: stomata are present on both surfaces of the epidermis, and the mesophyll is not differentiated into palisade and spongy parenchyma. In grasses, certain adaxial epidermal cells along the veins modify into large, empty, colourless cells called bulliform cells — when turgid (after absorbing water) the leaf surface is exposed; when flaccid due to water stress, they make the leaves curl inwards to minimise water loss. The parallel venation in monocot leaves is reflected in near-similar sizes of vascular bundles (except in main veins) (NCERT §6.2.6, p. 77, Figure 6.5b).
• Tissue overview (summary). Anatomically, plant tissues are classified into meristematic (apical, lateral and intercalary) and permanent (simple and complex). Assimilation of food and its storage, transportation of water/minerals/photosynthates, and mechanical support are the main functions of tissues. There are three tissue systems — epidermal, ground and vascular. The ground tissue system forms the main bulk of the plant and is divided into cortex, pericycle and pith. The vascular tissue system is formed by xylem and phloem; on the basis of presence of cambium, and location of xylem and phloem, the vascular bundles are of different types. Monocot and dicot plants show marked variation in their internal structures and secondary growth occurs in most dicot roots and stems (NCERT Summary, p. 77).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 6.1 (p. 72) — Stomatal apparatus: (a) bean-shaped guard cells, (b) dumb-bell shaped guard cells (grasses). Note labels: epidermal cells, subsidiary cells, guard cells, stomatal pore, chloroplast.
• Figure 6.2 (p. 73) — Types of vascular bundles: (a) radial, (b) conjoint closed (no cambium), (c) conjoint open (cambium between xylem and phloem).
• Figure 6.3 (p. 74) — T.S. of (a) Dicot root showing 2–4 xylem patches, small pith; (b) Monocot root showing polyarch xylem (>6 bundles) and large pith.
• Figure 6.4 (p. 75) — T.S. of stems: (a) Dicot — ring of vascular bundles, collenchymatous hypodermis, medullary rays, cambium visible in each bundle; (b) Monocot — scattered vascular bundles in parenchymatous ground tissue.
• Figure 6.5 (p. 76) — T.S. of leaves: (a) Dorsiventral (dicot) — palisade above spongy mesophyll, bundle sheath, sub-stomatal cavity; (b) Isobilateral (monocot) — undifferentiated mesophyll, stomata on both surfaces.

2.4 Common confusions / NTA trap points

• Open vs closed refers to the presence/absence of cambium, not to whether the bundle is exposed; "open" = cambium present (dicot stem), "closed" = cambium absent (monocot).
• Radial vs conjoint is about the geometric arrangement (alternate radii vs same radius), not about open/closed. Roots are radial; stems and leaves are conjoint.
• Polyarch is specific to monocot roots (>6 xylem bundles). Dicot roots typically have only 2–4 xylem patches.
• Starch sheath is the dicot stem endodermis (rich in starch grains); the dicot root endodermis is characterised by casparian strips, not starch.
• Bulliform cells are restricted to grasses (monocot leaves) and aid leaf curling — students often confuse them with subsidiary cells or guard cells.
• Phloem parenchyma is absent in monocot stems — a favourite NTA distractor when the question lists features of a T.S. specimen.
• Endarch protoxylem in the dicot stem versus exarch condition in roots (NCERT mentions endarch in the dicot stem specifically; the root has alternate xylem/phloem).
• Cuticle is absent in roots — distractors often add a cuticle to root epidermis.
• Guard cells in grasses are dumb-bell shaped, NOT bean-shaped — chapter explicitly contrasts the two.
• Medullary rays vs conjunctive tissue — medullary rays are between vascular bundles in dicot stem; conjunctive tissue lies between xylem and phloem in dicot root.

2.5 Comparative anatomical features (NCERT-cited)