Transpiration and Structure of Stomata

 Although large quantities of water are absorbed by the plants from the soil, only a small quantity

alone is utilized by them. Thus, a considerable amount of water absorbed by the roots are carried

to the top of the plants and lost by aerial parts in the form of water vapour or rarely in the form of

liquid. About 80% of the absorbed water is lost leaving only 20% of the absorbed water for various

metabolic processes.

Definition

The loss of water from the living tissues of aerial parts of the plants in the form of vapour is

called transpiration and that in the form of liquid is known as guttation.

Types of Transpiration

There are three types of transpiration observed in the plant system.

They are:

1. Stomatal Transpiration

Most of the transpiration takes place through stomata. Stomata are usually confined in more

numbers on the lower sides of the leaves. In monocots, e.g., grasses, they are equally distributed on

all sides. While in aquatic plants with floating leaves, they are present on the upper surface. The loss

of water vapours through stomata amounts to about 80-90% of the total loss.

2. Cuticular Transpiration

Although the cuticle is impervious to water, still some water may be lost through it. It may contribute

to a maximum of about 10% of the total transpiration.

3.lenticular Transpiration

Some water may be lost by woody stems through lenticels which is called as Lenticular

Transpiration. (Transpiration from leaves is called foliar transpiration).

Structure of Stomata

Stomata are minute pores of elliptical shape surrounded by two specialized epidermal cells

called Guard Cells. The guard cells are bean-shaped in dicots and dumble-shaped in the members

of Gramineae (monocots). The inner wall of the guard cell is thick and inelastic (rigid) in nature due

to the deposition of a secondary layer of cellulose, while the outer wall is thin and elastic and permeable

in nature. Each guard cell has a cytoplasmic lining and central vacuole containing cell sap. Its

cytoplasm contains a nucleus and a number of chloroplasts. which are often poorly developed and not

capable of photosynthesis. The epidermal cells surrounding the guard cells are specialized, called a

Subsidiary Cells, which support the movement of guard cells and thus play important role in the

opening and closing of stomata.

The number of stomata in a given area varies from plant to plant and the xerophytes possess

a lesser number of stomata than the mesophytes. When widely open, the stomatal pore occupies about

5% of the total leaf surface area. Under normal condition, stomata nearly occupy1-2% of the total leaf

area.

The size and shape of the stomata and guard cells vary from plant to plant. For example, in

Phaseolus vulgaris, the stomatal size is about 7x 3μ; in Avena sativa -38 x 8μ and Zea mays -4 x 26μ.

Mechanism of Transpiration

The mechanism of stomatal transpiration, which takes place during the daytime, can be studied

in three steps :

i. Osmotic diffusion of water in the leaf from xylem to intercellular spaces above the

stomata through mesophyll cells.

ii. Opening and closing of stomata (stomatal movement), and

iii. Simple diffusion of water vapours from intercellular spaces to the outer atmosphere through

Stomata.