The phloem vessels consist of sieve tubes and companion cells.
Sieve tubes are long tubes which are made by end to end union of a large number of cells called sieve tube cells. The end walls of these cells are called sieve plates. These sieve plates bear small holes called sieve pores. The sieve pores help in the movement of food in the phloem tube. Internally, each sieve tube cell contains only cytoplasm as nucleus is absent in it.
Companion cells are narrow, elongated and thin walled cells which lie on the sides of sieve tube cells. Each companion cell contains nucleus. It is believed that the nuclei of companion cells control the functioning of sieve tube cells.
Test Your Understanding and Answer These Questions:
- What is phloem?
- What are sieve tubes?
- What are companion cells?
- Give differences between xylem and phloem.