Meaning Of Transportation In Plants: The methods of transportation of substances are different in plants and animals. In plants, xylem and phloem tissues are present which help in the transport of the materials throughout the plant body. But in animals, a well developed 'circulatory system is present, which helps in transport of substances within the body.
Transportation In Plants
Transportation in plants is not as effective as in animals. However, higher plants have well developed tissues, called vascular bundles, which help in transportation of substances. Vascular bundles contain two types of tissues : (1) Xylem and (2) Phloem.
Xylem tissues carry out the transport of water and minerals. On the other hand, transport of food and other substances is carried out by phloem.
1. Transportation of water and Minerals
Water (and also minerals) are absorbed from the soil by roots and transported to various parts of the plant. like stem, leaves and flowers. In the leaves, water and carbon dioxide are used for photosynthesis. And transportions of materials is necessary, because every part of the plant requires food for obtaining energy and for building and maintaining its structure. But how do water and minerals move up from roots to leaves?
For conducting water and minerals to every part of the body, plants have xylem tissues. Xylem tissues contain two important types of elements: 1. tracheids and 2. vessels. Both these are dead and highly thick walled.
1. Tracheids
· Tracheids are long, thin, spindle-shaped cells with pits (small apertures), in their thick, lignified cell walls.
· In non-flowering plants, tracheids are the only conducting cells because in these plants, vessels are absent.
2. Vessels
· Vessels are long cylindrical cells with almost even diameter throughout the length. They also have liquified walls.
· These also bear pits. Vessels are found only in flowering plants.
Besides these, xylem also contains fibres and parenchyma. In older trees, xylem tissues form wood.
Full Mechanism Of Transportation In Plants
· Plants take up minerals in inorganic forms, such as nitrates, phosphates etc. These inorganic forms are absorbed by the roots, along with water.
· Minerals and water both are transported to different parts of the plant through vessels and tracheids (xylem elements).
· Water and minerals are absorbed by the roots, with the help of root hairs. Root hairs are the outgrowths of epidermal (outer) cells of the roots.
· The absorption occurs under the effect of diffusion.
· When water is absorbed by the root hairs, it goes deep into the roots upto the xylem tissues under the effects of diffusion pressure.
· When the water reaches the xylem tissues (vessels and tracheids), it moves upwardly towards stem. This upward movement of water and minerals is called ascent of sap.
· Various theories have been proposed by different scientists. But the best accepted theory is the 'transpiration pull' theory.
Transpiration In Plants
Total amount of water, absorbed by the plants, is not utilised by them. Plants use only 5 to 10% of this absorbed water. Rest 90 to 95% of the water is lost from the aerial parts of the plants in the of water vapours.
This loss (evaporation) of water, in the form of water vapours, form the aerial parts of plants (mainly form leaves) is called Transpiration. On an average, a tree transpires water, equivalant to 100 times its weight in its lifetime.
Transpiration Pull Theory
· This theory suggests that the water uplifts under the tension created due to rapid loss of water from the leaves. (This water loss is called transpiration).
· The loss of water creates water deficiency in the leaves. So, leaves demand more water from stem. Stem in turn demands more water from roots. In this way, this demand for more water, creates a suction pressure in the xylem tissues. This suction is called transpiration pull.
· Under the effect of this pull (suction or tension), water moves upwardly. Therefore, transpiration helps in the upward transport of water and minerals.
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2.Transportation of Food and Other Substances (Translocation)
· The transport of food (the product of photosynthesis) and various other substances, in plants occurs through another special type of tissues, called phloem. This transport of products of photosynthesis (food) from leaves to other parts of the plant is known as translocation.
· The food is synthesised in leaves (by photosynthesis) and hormones etc., are synthesised at the shoot and root tips. These are transported to other parts of the plant through phloem.
· The cells of phloem tissues are living and, therefore, the transport through phloem is carried out by the activity of living cells.
· These include sieve tubes (or sieve cells). (This is opposite to the transport of water, which is carried out by dead xylem cells, i.e., tracheids and vessels). Sieve tubes are supported by the companion cells.
Sieve Tubes
· These are elongated tube like living cells.
· They have thin cellulosic cell walls and are placed end to end.
· They are non-nucleated at maturity but protoplasm has large vacuoles. They do not have thickening of lignin.
· Between the two sieve tubes, there is present a perforated sieve plate. This plate contains many small pores, like a sieve, through which, the food is filtered
Transport Mechanism
· Translocation of food is a vital life process. It is necessary, because every part of the plant requires food for obtaining energy and for building and maintaining its structure.
· Similar to xylem, the transport of food material through phloem, can not be explained simply on the basis of physical forces. The transport of food material requires metabolic energy (from ATP).
· Translocation of food from leaves to other parts occurs due to difference in the concentration of solute (food particles).
· Leaves, which produce the food (sucrose), have higher concentration of solute. But other parts of the plants have lower concentration of solute. The high solute concentration in leaves, increases the osmotic pressure due to which water enters the leaf. The food along with water (in dilute form), enters the phloem tissue (which have low osmotic pressure).
· Therefore, food particles move from a region of higher concentration (i.e., leaves) to a region of lower concentration and reach to various parts of the plant), under concentration effect and use metabolic energy (ATP).
· In spring season, sugars get stored in roots and stems. These sugars can be transported to the buds (which require energy) according to the same phenomenon.
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