Water Absorption : Water is absorbed by the roots of the plant from the soil. It goes into the plant and then into and out of cells of different parts and then it goes outside the atmosphere.
The absorption of water by plant is known as water uptake.
Flow of Water
Bulk Flow : It is defined as the flow of fluid substances as a group of atoms or molecules due to external force. This is called bulk flow.
Diffusion : The flow of molecules or ions of solute or solvent from a region of higher concentration to a region of lower concentration is called diffusion.
Osmosis : The net flow of water from a less concentrated solution to a more concentrated solution when the two are separated by means of semi-permeable membrane.
Exosmosis : It is defined as outward flow of water molecules from the cell sap into the outer solution. This is called exosmosis.
Plasmolysis : Due to exosmosis, the plasma membrane leaves the wall of cytoplasm contracted. This is called plasmolysis.
Endosmosis : When a plasmolysed cell is kept in a low concentrated solution, the diffusion of water molecules into the cell takes place. This is called endosmosis.
Water Potential : This is the difference between the free energy of water molecules in pure form and the energy of water in any other form. The pure water at a constant atmospheric pressure has water potential of zero.
Ascent of Sap : The upward movement of water against gravitational force is called ascent of sap. The movement is from the roots to the shoot.
Gravitational water : The water that gradually penetrates through dry layers of the soil under the influence of gravity and reaches the ground water table. The gravitational water is not readily available to the plants and penetrates below the reach of roots within a few days.
Capillary water : The water that does not respond to the gravitational pull, is known as capillary water. It is retained by the forces of surface tension. It occurs in the form of thin films and is readily available to the plants.
Hygroscopic water : This is the water held by colloidal soil particles due to cohesive forces. Plants can absorb a small quantity of this water.
Chemically combined water : This is the water chemically combined with soil and minerals.
Runaway water : The rain water that flows on the surface of soil is called runaway water. Plants fail to absorb this type of water.
Holard, Chresard and Echard : The total amount of water present in the soil is called holard. Water available to the plants is called chresard and the water which cannot be absorbed by the plants is called echard.
Field Capacity or Water Holding Capacity : The amount of water actually retained by the soil against the gravitational pull is called water holding capacity of the soil.
Wilting : It is the shrinkage in the volume of water in plant cells due to loss of turgor pressure.
Incipient Wilting : When wilting symptoms are not visible externally, although the water content of plant cell decreases, the condition is called incipient wilting.
Temporary or Transient Wilting : Most of the herbaceous plants usually wilt during hot summer days and regain turgor during night. This is called temporary wilting.
Transpiration : The loss of water as vapours from the aerial parts of the plant into the atmosphere is called transpiration. Transpiration is of three types—
(a) Stomatal transpiration : Transpiration through pores (stomata)
(b) Cuticular transpiration : Transpiration through cuticles on the surface of epidermis.
(c) Lenticular Transpiration : Transpiration through lenticels present on the stems of plants.
Factors Affecting Transpiration
A. Environmental factors
1. Light : With the increase of intensity of light, the rate of transpiration increases.
2. Humidity of air : High vapour pressure of atmosphere decreases the rate of transpiration and low vapour pressure increases the transpiration rate.
3. Temperature : Increase in temperature generally increases the rate of transpiration.
4. Wind : A gentle breeze increases transpiration. A wind of high velocity increases transpiration.
5. Atmospheric pressure : The rate of transpiration is inversely proportional to the atmospheric pressure.
6. Availability of soil water : Transpiration decreases when sufficient water is not available to the plant.
B. Plant factors
1. Root-shoot ratio : The rate of transpiration increases with an increase in root-shoot ratio.
2. Leaf area : There is no relation between the rate of transpiration and the leaf area.
Significance of Transpiration
1. Transpiration is the method by which excess water gets rid off.
2. The greater the transpiration, the greater is the rate of water uptake.
3. Transpiration from the leaf surface causes transpiration pull which is the main force for ascent of sap.
4. Transpiration promotes uptake and transport of mineral salts in plants.
5. Transpiration plays a role in the distribution of organic food in the plant.
6. Transpiration makes the leaf temperature go down.
Osmotic Pressure : The pressure which is needed to check the process of osmosis is called osmotic pressure.
Turgor Pressure : The hydrostatic pressure developed inside the cell on the cell wall due to endosmosis, is called turgor pressure.
Wall Pressure : Plant cell wall which exerts inward pressure on cellular contents, is called wall pressure.
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