TRANSPIRATION
The loss of water in the form of vapours from the living tissue of the aerial part of plants is called transpiration. Less than 3% of total water absorbed by plant is utilized by plants for the development and metabolic processes. Remaining 97% of water gets transpired.
TYPES OF TRANSPIRATION
Loss of water can take place from any exposed part of the plant as it is a physical process. Depending on the surface through which water is lost, transpiration of three types.
- Stomatal transpiration: Stomata are minute pores on the epidermis of leaves. In herbaceous plants, stomata are also found on the epidermis of green stem. They form a connecting link between internal tissue and surrounding atmosphere. Therefore, they are the main passage through which water vapour escapes. Stomatal transpiration accounts for 80%-90% total water loss of the plant.
- Cuticular transpiration: Cuticle is a wax-like layer of cutin that covers the epidermis of leaves and stems. Cuticles present on the surface of the epidermis reduces the direct evaporation of water. If the cuticle is thin, up to 20% of total transpiration may take place through it. But as its thickness increases, the extent of water vapour loss is significantly reduced. Cuticular transpiration accounts for about 10% of the total amount of transpiration from leaves under ordinary condition.
- Lenticular transpiration: Lenticels are small openings in the cork of woody stem, twigs and fruits. Water vapours are lost through these openings. Loss of water through lenticels present on woody stem and fruit is called lenticular transpiration. It accounts for 1% of total water loss of a plant. However, in deciduous trees, which shed their leaves during autumn, lenticular transpiration can be significant.
STOMATAL TRANSPIRATION
Water absorbed by root is translocated to the mesophyll cells of the leaves through the xylem elements. These cells have large intercellular spaces. Water evaporated from the surface of mesophyll cells accumulates in the intercellular spaces and then escape to the outer atmosphere through stomata. This is the process of stomatal transpiration.
Structure of stomata: Stomata are tiny pores or openings present in the epidermal surface of leaves and young stems. A typical stomata usually consists of two kidney-shaped or bean-shaped guard cells, which differs from other epidermal cells, mainly in two respects.
- Contains chloroplast and perform the phenomena of photosynthesis.
- Their walls next to stomatal pores are thick as compared to inner walls.
The guard cells are also characterized by accumulation of starch during night and decrease during day. Guard cells are usually more smaller in size as compared to other epidermal cells. Therefore, they are rapidly affected by small change in turgor. The opening and closing of stomata depends upon the turgidity of guard cells. Guard cells are bordered by one or more modified epidermal cells, called subsidiary cells or accessory cells.
TYPES OF STOMATA (ACCORDING TO THEIR DISTRIBUTION)
- Apple type or mulberry type: These types of stomata are present on abaxial or lower surface of the leaves. For example, apple, peach, walnut etc. Such leaves in which stomata are restricted to lower surface only are called hypostomatic.
- Potato type: This type of stomata is present on abaxial or adaxial side but more on the abaxial surface of the leaves. For example, potato, tomato, brinjal and other members of the family Solanaceae. Such leaves with stomata on both the surfaces are called amphistomatic.
- Oat type: This type of stomata is present on both the leaf surfaces, being almost equally distributed, for example, wheat, rice, grasses etc. These leaves are also called amphistomatic.
- Water lily type: This type of stomata is found only on the upper surface of the leaves, for example, water lily. The leaves of such plants are found floating on water surface. These leaves could be termed as epistomatic.
- Potamogeton type: In submerged aquatic plants, the stomata are generally absent or if present, they are non -functional , for example, Potamogeton, Hydrilla,Vallisneria.
TYPES OF STOMATA (ACCORDING TO THE DAILY MOVEMENTS)
Loftfield classified stomata into following four types on the basis of their daily movement:
- Alfalfa type: The stomata remain open throughout the day and closed throughout the night. These are found in all the thin leaved mesophytes, for example, alfalfa, pea, bean, raddish mustard, turnip, grapes etc.
- Potato type: The stomata remain open throughout the day and night except for a few hours following sunset. For example, onion, potato, cabbage, bananas etc.
- Barley type: The stomata open only for a few hours during the day and remains closed for the rest of the period. For example, barley, maize, wheat and other cereals.
- Equisetum type: The stomata remain open throughout the day and night. For example, Equisetum.
TYPE OF STOMATA (DEPENDING ON THEIR ASSOCIATION WITH SUBSIDIARY CELLS)
- Atropa type: Such stomata are not surrounded by any subsidiary cells.
- Sedum type: In this type, stomata remain surrounded by three subsidiary cells of unequal type.
- Maize type: Guard cells are dumb-bell shaped and are surrounded by paired subsidiary cells.
- Allium type: Guard cells remain associated with two subsidiary cells.
DIFFERENCE BETWEEN EVAPORATION AND TRANSPIRATION
| Evaporation | Transpiration |
| 1. Water vapours are formed at the free surface of water. | 1. Water vapours are formed mostly in the internal tissues of the plant. |
| 2. It is a physical process which is controlled by the environmental condition like relative humidity and air current. | 2. It is both a physical and physiological process besides being controlled by the environmental conditions, it is influenced by the rate of water absorption, osmotic pressure of cell, thickness of cuticle and number, possession and opening of stomata. |
| 3. Evaporation takes place at the surface of nonliving objects. | 3. It occurs at the exposed surface of the plants. |
| 4. Evaporation stops when the air is fully saturated. | 4. Formation of vapours continues for some time even after the saturation of outside air. This is due to the fact that temperature of leaf is slightly higher than the outside air and therefore, the saturation point of the internal air is slightly higher. |
| 5. Light does not directly influence the rate of evaporation . | 5. Light influences transpiration to a great extent. Stomata usually open in the presence of sunlight increasing the rate of transpiration. Stomata close during darkness, thereby lowering the rate of transpiration. |
| 6. It varies directly according to the velocity of wind. | 6. The rate of transpiration is slightly lower than evaporation under influence of wind velocity as it lowers the leaf temperature. |
| 7. It is not much influenced by the structures of the evaporating surface. | 7. Transpiration is controlled by the anatomy of the transpiring organ. |
| 8. Carbon dioxide, pH and hormones have no effect on evaporation. | 8. Transpiration is influenced by pH, hormones, and carbon dioxide. |
| 9. It continues as long as water is available along the surface. | 9. It is largely dependent upon the absorption of water from the soil. |
| 10. It is comparatively faster. | 10. It is comparatively a slower process. |