Absorption and Movement of water in Plants
Root system is the absorbing system of the plants. Root system of plants consists of main root and lateral roots. Lateral roots bear large number of fine outgrowths called root hairs which absorb water from the soil. Cell wall of root hair is made up of an outer pectic layer and inner cellulose layer. Root hairs absorb soil water as a result of diffusion pressure deficit gradient. Active absorption and passive absorption are two mechanism of water absorption by plants. Active absorption occurs in the plant due to activities of root hairs. According to Osmotic theory of active water absorption, absorption of water takes place due to osmotic differences between soil solution and root cell sap. According to non osmotic theory of active water absorption, absorption of water occurs against the concentration gradient through the expenditure of energy. Transpiration pull causes passive absorption. Out of the total uptake of water by the plants, 98% takes place through passive absorption. After absorption by the root hair, water moves along two pathways; symplast pathway and apoplast pathway. According to the theory of transpirational pull proposed by Dixon and Jolly in 1894, in the process of transpiration, water is pulled up through the xylem and then evaporates through stomata Availability of soil water, soil temperature, concentration of soil solutes, aeration of soil and transpiration affect water absorption. Vascular tissues are responsible for long distance transport in plants are xylem (for water) and phloem (for food). Root pressure is the positive pressure developed within the cells of a root system which causes sap to rise up in the xylem ducts. Root pressure is responsible for guttation which is exudation of drops of xylem sap through specialized pores present at the tip of veins of leaves called hydathodes. Exudation of sap from the injured parts of the plant is called bleeding.
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