Chapter 6: Anatomy of Flowering Plants | Summary

The epidermal tissue system forms the outermost covering of the entire plant body. It comprises epidermal cells, stomata, and epidermal appendages such as trichomes and root hairs. Epidermal cells are parenchymatous, single-layered, compactly arranged, and covered externally by a waxy cuticle that prevents water loss. The cuticle is absent in roots.

Stomata are specialised pores present in the epidermis, mainly of leaves. Each stoma consists of two guard cells (bean-shaped in most plants, dumb-bell shaped in grasses) enclosing a stomatal pore. Guard cells possess chloroplasts and regulate stomatal opening. Subsidiary cells surround the guard cells and together form the stomatal apparatus. Trichomes on the stem are multicellular, may be branched or unbranched, and help reduce transpiration. Root hairs are unicellular extensions of epidermal cells that absorb water and minerals.

The ground tissue system comprises all tissues except the epidermis and vascular bundles. It consists of simple tissues: parenchyma, collenchyma, and sclerenchyma. Parenchymatous cells are found in the cortex, pericycle, pith, and medullary rays of stems and roots.

In leaves, the ground tissue is called mesophyll and is composed of thin-walled, chloroplast-bearing parenchyma cells. The mesophyll carries out photosynthesis and is divided into palisade parenchyma (elongated, adaxially placed) and spongy parenchyma (loosely arranged, abaxially placed) in dorsiventral leaves.

The vascular tissue system consists of complex tissues — xylem and phloem — which together constitute vascular bundles. In dicot stems, cambium is present between xylem and phloem, making vascular bundles open (capable of secondary growth). In monocot stems, cambium is absent, making the bundles closed (no secondary growth).

Vascular bundles are classified as radial (xylem and phloem alternate along different radii, as in roots) or conjoint (xylem and phloem on the same radius, as in stems and leaves). Conjoint bundles typically have phloem on the outer side of xylem. Open bundles have cambium between xylem and phloem; closed bundles lack cambium.

The transverse section of a dicot root (e.g., sunflower) shows the following layers from outside to inside: epiblema (outermost layer with unicellular root hairs), cortex (several layers of thin-walled parenchyma with intercellular spaces), endodermis (single layer of barrel-shaped cells with Casparian strips of suberin), pericycle (thick-walled parenchymatous cells that initiate lateral roots and vascular cambium), and the stele (all tissues inner to the endodermis).

The vascular bundles in dicot roots are radial, with two to four xylem and phloem patches arranged alternately. Parenchymatous conjunctive tissue lies between xylem and phloem. The pith is small or inconspicuous. A cambium ring later develops between xylem and phloem for secondary growth.

The monocot root anatomy is broadly similar to the dicot root, having epidermis, cortex, endodermis, pericycle, vascular bundles, and pith. However, key differences exist. Monocot roots have more than six xylem bundles (polyarch condition), while dicot roots have fewer (two to four).

The pith in monocot roots is large and well developed, unlike the small and inconspicuous pith in dicot roots. Importantly, monocotyledonous roots do not undergo secondary growth, which distinguishes them fundamentally from dicot roots in terms of long-term structural development.

The transverse section of a young dicot stem shows the epidermis (outermost, with cuticle, trichomes, and few stomata), followed by the cortex with three sub-zones: hypodermis (collenchymatous cells providing mechanical strength), general cortex (parenchymatous cells with intercellular spaces), and endodermis (starch sheath rich in starch grains).

Pericycle is present as semi-lunar patches of sclerenchyma above the phloem on the inner side of the endodermis. Vascular bundles are arranged in a ring — the defining feature of dicot stems. Each bundle is conjoint, open, and endarch. Medullary rays (parenchymatous cells) lie between adjacent vascular bundles. The central pith consists of rounded parenchymatous cells with large intercellular spaces.

The monocot stem is characterised by a sclerenchymatous hypodermis (providing mechanical strength), a large number of scattered vascular bundles (not arranged in a ring), and a conspicuous parenchymatous ground tissue. Each vascular bundle is surrounded by a sclerenchymatous bundle sheath.

The vascular bundles in monocot stems are conjoint and closed (no cambium, no secondary growth). Peripheral bundles are smaller than centrally located ones. Phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles — a unique feature distinguishing monocot stems from dicot stems.

The vertical section of a dorsiventral leaf shows three main parts: epidermis, mesophyll, and vascular system. Both adaxial (upper) and abaxial (lower) surfaces are covered by epidermis with a cuticle. The abaxial epidermis bears more stomata; the adaxial epidermis may lack stomata entirely.

The mesophyll is differentiated into palisade parenchyma (elongated cells arranged vertically, adaxially placed, with more chloroplasts) and spongy parenchyma (oval/round loosely arranged cells with large air cavities, situated below palisade cells). Vascular bundles are enclosed by bundle sheath cells and are found in veins. Veins vary in thickness due to reticulate venation typical of dicots.

The isobilateral leaf anatomy is similar to the dorsiventral leaf but with key differences. Stomata are present on both surfaces of the epidermis (hence the name isobilateral). The mesophyll is not differentiated into palisade and spongy parenchyma.

Grasses have specialised bulliform cells — large, empty, colourless cells present in the adaxial epidermis along veins. When turgid (water sufficient), bulliform cells cause the leaf to remain flat and exposed. When flaccid (water stress), they cause the leaf to curl inwards, minimising water loss. Parallel venation results in vascular bundles of near-similar sizes throughout the leaf.

Chapter 6 reveals that the internal organisation of flowering plants is a beautifully coordinated system of three tissue types — epidermal, ground, and vascular — each fulfilling distinct roles of protection, metabolism, and conduction. Monocots and dicots differ markedly in the arrangement, number, and type of vascular bundles in their roots, stems, and leaves, reflecting their evolutionary divergence. The presence or absence of cambium determines whether secondary growth is possible, a key anatomical distinction underpinning the structural complexity of dicotyledonous plants.

From a board examination perspective, this chapter is highly important as it integrates knowledge of tissues with organ-level organisation. Questions frequently test the ability to compare monocot and dicot anatomy, identify structures from transverse sections, and explain the functional significance of structures such as Casparian strips, bulliform cells, and open vs. closed vascular bundles. A clear understanding of the anatomical diagrams and the ability to relate structure to function will be decisive for scoring well.