Transpiration may be defined as the loss of water as vapours from the aerial parts of living plants viz., leaves, green shoots, etc. Transpiration is mainly three types; Cuticular, stomatal and lenticular.

Cuticular: Cuticle is a layer of wax like covering on the epidermis of leaves and green stems. Upto Equation of the total transpiration may take place through it. With the increase in the thickness of cuticle, the loss of water vapour through it is reduced.

Stomatal: Stomata are minute pores in the epidermis. Their opening and closing is controlled by guard cells. The loss of water vapours through stomata amounts to about Equation of the total water loss. Stomata are mostly situated on leaves but in green stems they may be found in epidermis as well. The number of stomata is normally more on the lower side than on the upper side of the leaf. In xerophytes stomata are confirmed to the lower side. In submerged plants they are absent but in floating plants they are present on the upper side.

Lenticular: In woody stems, some transpiration also takes place through the loose mass of cells of the lenticels. About Equation of water losses take place through.

Mechanism of transpiration

Transpiration occurs in two stages; evaporation of water from the mesophyll cells into the intercellular spaces, diffusion of this water vapour of the inter cellular spaces into the outside atmosphere, when the outside atmosphere is drier.

Factors affecting transpiration

Transpiration is affected by several factors;

Temperature: Temperature affects the rate of transpiration by indirectly. An increase in temperature decreases the humidity. Transpiration also increases or decreases along with rise or fall in temperature.

Wind: The prevalence of wind greatly affects transpiration. During high wind, rate of transpiration is greatly enhanced as it removes the water vapours transpired, thereby, not allowing the transpiring surface to become saturated.

Light: Transpiration is not only accelerated by the direct sunlight but also by the diffused sunlight. Lights affects transpiration by increases the transpiration by raising the temperature of leaves and there is close relationship between the opening of the stomata and presence of light. During night, when there is no light, stomata remain close resulting in minimum transpiration.

Water supply: Deficiency of water supply in the soil decreases the rate of transpiration by decreasing the rate of absorption. The rate of transpiration also depends on the amount of water absorbed by the roots. If absorption lags behind, the rate of transpiration also diminishes. The factors affecting absorption of water by the roots also indirectly affects the rate of transpiration.

Atmospheric pressure: Reduction of atmospheric pressure reduces the density of external atmosphere thus permitting more rapid diffusion of water. Plants growing on high will show higher rate of transpiration hence they develop xerophytic characters.

Atmospheric humidity: Low and high humidity of air directly affect the transpiration. When the humidity is low, air becomes drier and receives moisture very readily. On the other hand, when humidity increases, air becomes moist or saturated and receives less water vapour, thereby decreasing the rate of transpiration.

Internal plant factors

Certain plant adaptations reduce transpiration;

  • Reduced size of the leaves, so reducing transpiring surface. Some xerophytic plants like Pinus and Opuntia have needle like or spine like leaves.

  • Thick deposition of cutin on the leaf surface.

  • Stomata found sunken in the cavities surrounded by hairs as in Nerium and Cycas.

  • The rate of transpiration directly depends upon the efficiency of the absorbing system (roots) and the evaporating system (leaves). The rate transpiration increases with an increase in root-shoot ratio.

  • The succulent stems and leaves contain mucilage or latex in their tissues. This reduces the rate of transpiration by holding water.

Role of Stomata in Transpiration

Water lost through stomata so plants regulate the degree of stomatal opening to reduce the water loss. Stomata show periodic opening and closing during the day depending upon the heat and light, humidity and water content of the cell. They are generally closing during the night, from early morning to mid-day the stomata are open and hence the transpiration increases till mid-day. During sunny afternoon, the stomata are closed so the transpiration decreases and late afternoon to evening, the stomata are again open and the transpiration increases. At night, the stomata are closed and hence the transpiration is very low.

Transpiration is a necessary evil

Transpiration is necessary because it helps in transpiration pull that indeed helps in the transportation of water throughout the plant body. It is an evil because bulk of water is lost during this process. The stomata are primarily meant for absorption of carbon dioxide but these also help in exchange of gases, but at that same time water vapors also escape through stomata. Thus transpiration is described as necessary evil because it is an inevitable process but potentially harmful. It can cause wilting and even death of the plant; it is for this reason that scientist Cutis in 1926 referred transpiration as a necessary evil.

Significance of Transpiration

Absorption of water: Transpiration influences the rate of absorption of water from the soil.

Water movement: By transpiration, water moves upwards and as it passes into the cell vacuoles it makes the cells turgid. This gives form and shape to cells and plant as a whole.

Mineral salt transport: The water stream moving upwards carries dissolved minerals with it. Transpiration also helps in distributing these minerals throughout the plant.

Cooling: The evaporation of water during transpiration cools the leaves.

Protection from heat injury: Some plants like cacti retain water by reducing transpiration. This saves the plants from high temperatures and strong sunlight.

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