Also function in support. Vessel elements are wider, shorter, thinner-walled, and less tapered than tracheids. Vessel elements are aligned end to end. The end walls are perforated, permitting the free flow of water through long chains of vessel elements called xylem vessels. The cells are alive at functional maturity.
Protoplasts lack a nucleus, ribosomes and a distinct vacuole.
In angiosperms, the end walls of sieve-tube members have pores and are called sieve plates . The pores probably facilitate the movement of fluid between cells. Companion cells also help load sugar produced in the mesophyll into sieve-tubes of leaves of some plants. Each of the three systems is continuous throughout the plant, although, specific characteristics and spatial relationships vary in different plant organs.