Permanent Tissues-Complex Tissues-Xylem Homework Help
Define Permanent Tissues – Complex Tissues - Xylem
Complex tissues: As the name suggests more than one type of cells work together as unit. The complex tissues present in plants are xylem and phloem. Get Permanent Tissues-Complex Tissues-Xylem Homework Help by experts
Xylem:The main function of xylem is to conduct water and minerals from roots to stem and leaves. Additionally, it also provides mechanical strength to parts of the plants. The xylem is composed of four elements namely, tracheids, vessels, xylem fibres and xylem parenchyma.
Figure 1 Xylem tissue:
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 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 large central cavity (lumen). Vessels are 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 fibres: These fibres 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 centre and the metaxylem lies towards the periphery of the organ. This type of arrangement is called as endarch. In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. This type of arrangement is called as exarch.