Sieve tubes and companion cells are the components of phloem. A Sieve tube is formed of many cells called sieve tube element or member. These members are arranged end to end on one another and the end wall is usually transverse. Many small pits are present and because of these pits end walls usually called a sieve plate. In gymnosperm and pteridophyte, sieve cells remain free.
Companion cells are narrow, long, and thin-walled living. They are present close to the sieve tube and also connected through plasmodesmata. Companion cells help to maintain the activity of the sieve tube. In a non-flowering plant, a specialized parenchyma cell named albuminous is present instead of a companion cell. Sieve tubes and companion cells are present in angiosperms only. The below given figure shows the position of seive tube and companion cell.
