(a) : Xylem is the principal water conducting tissue of the plant. It consists of four types of cellstracheids, vessels, xylem fibres and xylem parenchyma. Xylem vessels are hollow, elongated cells with open ends and pitted walls. Cells walls are lignified. At maturity nucleus is absent in vessels. The constituent cells of the phloem are sieve elements (sieve cells, sieve tubes), companion cells, phloem fibres and phloem parenchyma cells. Sieve tube members are long, slender, tube-like cells joined end to end, to form long tubular channels the sieve tubes. Sieve tube members possess specialized sieve areas on the end walls called sieve plate. Young sieve tube members have abundant cytoplasm but their is no nucleus. The nucleus disintegrates during their development.