They are commonly arranged at strands or cylinders beneath the dermal tissue of stems and stalks. The primary cell walls of collenchyma become thickened with cellulose and pectin at maturity, often at their corners. Sclerenchyma cells provide mechanical support and protection in mature plants. The secondary walls are thick and often impregnated with lignin, which strengthens and waterproofs cell walls. It is formed from two types of cells that are dead at maturity.
Tracheids are long cells with tapered, overlapping ends.
Vessel members are shorter cells joined end to end to form a vessel, with perforation plates at the ends of each member. Phloem Phloem transports sugars and other solutes throughout the plant body. Phloem contains living conducting cells called sieve tube members. The tubes bear clusters of pores in the walls through which the cytoplasm of adjacent cells is connected. Companion cells, adjacent to the sieve tube members, help to load sugars produced in leaves and unload them in storage and growth regions.