Why Xylem is Complex Permanent Tissue?
Complex tissues: As the name suggests more than one type of cell work together as a unit. The complex tissues present in plants are the xylem and phloem.
Xylem: The main function of the xylem is to conduct water and minerals from roots to stems and leaves. Additionally, it also provides mechanical strength to parts of the plants. The xylem is composed of four elements namely, tracheids, vessels, xylem fibers, and xylem parenchyma.
Tracheids: Elongated, tube-like cells with thick and lignified walls and tapering ends. These cells lack protoplasm and hence are usually dead with thickened cell walls, varying in form. In flowering plants, tracheids and vessels are the main water-transporting elements.
Vessels: It is a long, cylindrical tube-like structure composed of several cells called vessel members that lack protoplasm and are made up of lignified thickened walls and a large central cavity (lumen). Vessels are the characteristic feature of angiosperms whereas; gymnosperms lack vessels in their xylem. The vessels function by forming a network of vessel members through perforations in their common walls.
Xylem fibers: These fibers either septate or aseptate have highly thickened walls and obliterated central lumens.
Xylem parenchyma: These cells are composed of cellulose, living and thin-walled. The radial conduction of water is carried out through ray parenchymatous cells. Their function is to store food materials in the form of starch or fat and other substances like tannins.
Primary xylem: Primary xylem is of two types namely, protoxylem and metaxylem. In stems, the protoxylem lies towards the center and the metaxylem lies towards the periphery of the organ. This type of arrangement is called Endarch. In roots, the protoxylem lies towards the periphery and the metaxylem lies towards the center. This type of arrangement is called Exarch.