Vascular Tissues

 In higher plants, mass flow of food and water occurs within vascular tissues to all parts of the plant. This movement of substances (mass flow) is known as translocation.

     Vascular tissues are grouped together in bundles with the plant body. Each bundle is known as vascular bundle. Vascular bundles are arranged in characteristic patterns in roots, stems and leaves to form a mass flow distribution system. The veins in leaves are composed of vascular bundles. Network (dicotyledonous leaf) and parallel (monocotyledonous leaf) venations indicate the two characteristics patterns of vascular bundle distribution in leaves.

      There are two main types of vascular tissues: xylem and phloem. Xylem conducts water and mineral salts from the roots to the leaves and the rest of the plant. Phloem conducts manufactured food from the leaves to the rest of the plant for assimilation and storage (in the case of excess food).

      Xylem tissue is made up of tracheids and vessels. Tracheids are long spindle shaped cells which form vertical columns that extend from root to leaf. Vessels consist of elongated cells stacked one on top another. Transverse end walls of these cell disappear, resulting in long hollow columns. Both the mature tracheids and vessels are non living. Their walls become thick and woody after secondary growth. 

    Phloem tissue is made up of sieve tubes and companion cells which are living. Sieve tubes consists of cylindrical cells, called sieve tube elements, arranged end to end in long rows. The cross walls of these cells or sieve plates are perforated by many fine pores called sieve pores. The protoplasm of each sieve tube element is connected with that of the next by means of narrow protoplasmic strands which pass through the sieve pores of the sieve plates.

      Lying alongside each sieve tube element is a smaller cell, the companion cell. It is filled with dense cytoplasm and a nucleus which is believed to be functionally associated with the accompanying sieve tube element.

      There are other types of cells in the xylem and phloem tissues. some of them are dead and woody, and provide support to the plant. Others are living and serve as storage and packing tissues.

         The trunk of most trees, like mango and rubber, is mainly xylem tissue with a thin surroundings layer of phloem. The phloem is covered on the outside with bark, a layer of dead cells.