Chapter 6: Anatomy of Flowering Plants (Structural Organisation)

Draw the two pairs of T.S. diagrams given in the concept notes (roots and stems), labelling the parts below. (a) Dicot root: epiblema with root hairs, cortex, endodermis with casparian strips, pericycle, 2–4 radial xylem and phloem bundles, conjunctive tissue, small pith. Monocot root: same layers but polyarch xylem (more than 6) and a large pith; no secondary growth. (b) Dicot stem: epidermis with cuticle, collenchymatous hypodermis, cortex, endodermis (starch sheath), sclerenchymatous pericycle, bundles in a ring, conjoint open, medullary rays, pith. Monocot stem: sclerenchymatous hypodermis, scattered conjoint closed bundles with sclerenchymatous sheaths, no phloem parenchyma, water cavities, ground tissue.

Look at the vascular bundles and the ground tissue. If the bundles are arranged in a ring, are open (cambium between xylem and phloem) and there are a distinct cortex, endodermis, pericycle, medullary rays and pith, with a collenchymatous hypodermis, it is a dicot stem. If the bundles are scattered in a large ground tissue, are closed (no cambium), each with a sclerenchymatous bundle sheath, with no phloem parenchyma, water cavities, and a sclerenchymatous hypodermis, it is a monocot stem.

A monocot stem: scattered conjoint (closed) bundles with sclerenchymatous sheaths and the absence of phloem parenchyma are its characteristic features.

The stomatal apparatus is the stomatal aperture (pore), the guard cells and the surrounding subsidiary cells together. (Diagram: the stoma in the concept notes.) Structure: each stoma is a pore enclosed by two bean-shaped guard cells (dumb-bell shaped in grasses). The outer walls of guard cells (away from the pore) are thin and the inner walls (towards the pore) are highly thickened. Guard cells contain chloroplasts and regulate the opening and closing of the pore, and so transpiration and gas exchange. Nearby epidermal cells specialised in shape and size are subsidiary cells.

(i) Epidermal tissue system: epidermal cells, stomata (guard and subsidiary cells), epidermal appendages (trichomes, root hairs), cuticle. (ii) Ground (fundamental) tissue system: simple tissues parenchyma, collenchyma and sclerenchyma, forming the cortex, pericycle, pith and medullary rays, and the mesophyll in leaves. (iii) Vascular (conducting) tissue system: complex tissues xylem and phloem (with cambium in dicots) forming vascular bundles.

(i) It helps to identify and classify plants (e.g. monocot vs. dicot) and to settle doubtful cases in taxonomy. (ii) It explains adaptations to environments (bulliform cells, sunken stomata, thick cuticle). (iii) It explains functions such as transport, support and photosynthesis. (iv) It helps to identify wood and timber and fibres for industry, and to detect adulteration in drugs and food. (v) It helps plant breeders and in studying diseases and the effects of pollution on tissues.

(Diagram: V.S. of a dorsiventral leaf in the concept notes.) A vertical section shows: (i) upper (adaxial) epidermis with a thick cuticle and few or no stomata; (ii) mesophyll of palisade parenchyma (elongated, vertical, parallel, chloroplast-rich cells below the upper epidermis) and spongy parenchyma (round, loosely arranged cells with large air spaces); (iii) vascular bundles in the veins and midrib, conjoint, with xylem towards the upper side and phloem below, each surrounded by thick-walled bundle sheath cells; (iv) lower (abaxial) epidermis with a cuticle and many stomata.