Transport of Water and Minerals
The water and minerals absorbed by roots are conducted upwards to the leaves, flowers and other parts of the plant. It takes place through xylem tubes. The upward movement of sap that contains war and minerals is called ascent of sap.
Take a freshly cut twig of balsam plant and place its lower end in a dilute solution of eosin. After sometime prepare cross sections of different heights and examine under the microscope. Xylem vessels and tracheids become colored and this proves that the movement of the sap takes place through xylem.
Translocation of Food
Food is manufactured in the green cells of leaves by photosynthesis. This food is Trans located to different parts of plant for its utilization. Food prepared in the leaves moves downwards or upwards in the sieve tubes of phloem.
Methods and pathway of Translocation
Water, mineral salts and sugar (food) are transported by two methods in higher plants: (1) translocation, which is the movement of dissolved substances from one part of the plant to another, and (2) transpiration, in which water evaporates from the leaves and the subsequent movement of absorbed water takes place through xylem. Water and dissolved salts travel upwards in the xylem vessel, while food passes downwards and upwards in the sieve tubes of the phloem.
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Vascular bundles (xylem and phloem) are well suited to carry out these functions. Vessels (tubes0 in the xylem are cylindrical in shape with their ends open and are placed one above the other. It constituents a continuous columns stretching from roots to leaves. Phloem has sieve tubes that are also cylindrical but the ends are not open, instead they are covered with sieve plate. A continuous column from leaves to other parts of the plant is formed due to arrangement of these tubes one above the other. Sucrose is the main form of carbohydrate which is translocated in plants.
There are four main differences between xylem and phloem: (1) xylem is dead, where as phloem cells are alive. (2) Xylem carries mainly water and minerals, while organic compounds such as sugar and amino acids are transported to the phloem. (3) The flow of liquid in xylem is upward only, whereas the flow of liquid in phloem is bidirectional. (4) The contents in phloem can move up and down the plant simultaneously.
Ringing or girdling experiment can prove the movement of organic food through phloem tissue. If the stem of a woody plant is girdled completely by removing a ring of the bark including phloem then the portion of the stem just above the ring swells up after some days. This is due to accumulation of food material in tissues. The tissues below the ring gradually lose all the stored food and dry up. If the xylem is blocked by wax and the phloem is kept intact, the translocation of food is not affected. This shows that phloem conducts food.
Although a large quantity of water is absorbed by the root yet a very small portion (about 10%) of it is used for plants vital activities. The greater part of it (90%) is lost from the surface of leaves into the atmosphere.