Companion cell is a thin walled elongated cell which is associated with sieve tube. The sieve tube elements lack nucleus, they remain living, being dependents upon the adjacent companion cell nucleus.
Phloem is the food conducting tissue of plants, it consists of four type of cell.
(i) Phloem parencyma
(ii) Phloem fibre
(iii) Sieve tube elements
(iv) Companion cell.