Question

1. Compare and contrast the distribution of the following plant tissues as they relate to mesomorphic, xeromorphic and hydromorphic leaves:

(a)     palisade and spongy mesophyll cells

(b)     vascular bundles

(c)     stomata

Answer 1

Mesomorphic leaves adapt to the conditions were abundant water and humid conditions. Xeromorphic leaves adapted to conditions were the humidity is relatively low. Hydromorphic leaves adapted to the aquatic conditions, either submerged in water or standing in water.

1.) Mesomorphic leaves: (eg. Tomato, soyabean)

(i) The ground tissue system(mesophyll) is divided into two types palisade parenchyma and spongy parenchyma. Palisade parenchyma - these are composed of columnar cells which are perpendicular to the leaf surface. These are located beneath the upper epidermis. Spongy parenchyma - These composed of irregular shaped cells and located at the lower part of the leaves.

(ii) vascular bundles: The primary Xylem and phloem are surrounded by a layer of parenchyma called as bundle sheath, the secondary growth is seen only in the mid vein and some large lateral veins.

2.) Xeromorphic leaves:(eg. Snake plant)

(i) mesophyll modifications allows these plant to conserve or store more water. By transpiration - multilayer epidermis covered by thick mucilage or wax secreted by stomates. By trichomes - at lower surface of leaves, when stomata shukens are associated with trichomes, these trap the moisture and prevent water loss.

(ii) in monocot leafs - there is no mid vein, the veins are longitudinally parallel. The stomata are in rows between the veins. The mesophylls are poorly developed, mostly the parenchymatous with scattered bundles of fibers.

3.) Hydromorphic leaves: (e.g water lilies, rice crops etc)

(i) In these, mesophyll contains Arenchyma (a kind of adaptation which promotes water loss). Collenchyma are little or absent, similarly sclerenchyma. The cuticle is thin or absent, the guard cells are present only on the upper surface of the floating leaves.

(ii) Vascular bundles: The vascular system (xylem) is weakly developed because the water provides the mechanical support to the plant. More amount of water present, so there is no need to prevent water loss in these.

(iii) Through the central stomata, the guard cells present on the surface of floating leaves, monitor the rate of water loss. The mesophyll permits the rapid loss of water vapour via transpiration in these type of plant leaves.

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