Why are xylem and phloem called complex tissues?
Xylem and phloem are composed of several types of cells and they work as a unit. Hence they are called complex tissues. Xylem: Xylem is composed of four different kinds of elements, namely, tracheids, vessels, xylem fibres and xylem parenchyma. • Tracheids are elongated or tube like cells with thick and lignified walls and tapering ends. These are dead and are without protoplasm. The inner layers of the cell walls have thickenings which vary in form. In flowering plants, tracheids and vessels are the main water transporting elements. • Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity. The vessel cells are also devoid of protoplasm. Vessel members are interconnected through perforations in their common walls. The presence of vessels is a characteristic feature of angiosperms. • Xylem fibres have highly thickened walls and obliterated central lumens. These may either be septate or aseptate.