Plants have specialized vascular tissues for transportation of substances. There are two types of vascular tissues in plants, viz. xylem and phloem.
Xylem: Xylem is responsible for transportation of water and minerals. It is composed of trachieds, xylem vessels, xylem parenchyma and xylem fibre. Trachieds and xylem vessels are the conducting elements, i.e. they are the channels through which water and minerals are transported. The xylem makes a continuous tube in plants which runs from roots to stem and right up to the veins of leaves.
Phloem: Phloem is responsible for transportation of food. Phloem is composed of sieve tubes, companion cells, phloem parenchyma and bast fibres. Sieve tubes are the conducting elements in phloem. Rest of the components play the role of suppportive cells.
The upward movement of water and minerals from roots to different plant parts is called ascent of sap. Many factors are at play in ascent of sap and it takes place in many steps. They are explained as follows:
Root Pressure: The walls of cells of root hairs are very thin. Water, from soil, enters the root hairs because of osmosis. Root pressure is responsible for movement of water up to the base of the stem. A large number of root hairs, at the root tip, increases the surface area so that a greater amount of water can be absorbed. But root pressure can help in rise of water only up to a few centimeters. So, root pressure alone cannot explain the rise of water in tall plants.
Capillary Action: A very fine tube is called capillary. Water, or any liquid rises in the capillary because of physical forces and this phenomenon is called capillary action. In stem, water rises up to some height because of capillary action. Combined effect of root pressure and capillary action may be enough for water need of small herbs but not for trees.
Adhesion-cohesion of Water Molecules: Water molecules make a continuous column in the xylem because of forces of adhesion and cohesion among the molecules.
Cohesion: The force of attraction between molecules of same substance is called cohesion.
Adhesion: The force of attraction between molecules of different substances is called adhesion.
Water molecules stick together because of force of cohesion. Water molecules stick to a surface because of force of adhesion. Due to combined effect of adhesion and cohesion, a continuous column of water is formed inside xylem vessels. This is necessary to provide read supply of water to the topmost branches whenever there is a pull effect because of transpiration.
Transpiration Pull: Loss of water vapour through stomata and lenticels (in plants) is called transpiration. Transpiration through stomata creates vacuum which creates a suction, called transpiration pull. The transpiration pull sucks the water column from the xylem tubes and thus water is able to rise to great heights in even the tallest plants.
Ascent of water can be summed up in a few sentences. Water from soil enters the root cells due to osmosis. When water enters the root cells, there is built up of pressure inside root. Water rises up to a few centimeters due to root pressure. Beyond that, capillary action inside xylem vessels results in rise of water up to a few centimeter higher. The effect of adhesion-cohesion of water molecules helps in building a continuous column of water inside xylem tubes. Trasnpiration pulls sucks this column of water and thus water rises to the topmost bracnhes of a tree.
Transport of food in plants happens because of utilization of energy. Thus, unlike the transport through xylem, it is a form of active transport. It is important to keep in mind that transport through xylem is passive transport. Moreover, the flow of substances through phloem takes place in both directions, i.e. it is a two-way traffic in phloem. This phenomenon is called source to sink. When food is prepared in leaves, the leaves work as srtore and remainnig parts of a plant work as sink. After the leaves fall in autumn, no leaves are left on a tree. In such a situation, storage organs work as store and leaf buds become the sink which need raw materials to grow again.
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