Explain companion cells.

Explanation

The thin walled,elongated,specialized parenchyma cells,which are associated with the sieve elements,are called companion cells.These cells are living and they have cytoplasm and a prominent nucleus.They are connected to the sieve tubes through pits found in the lateral walls.Through these pits cytoplasmic connections are maintained between these elements.These cells are helpful in maintaining the pressure gradient in the sieve tubes.Usually the nuclei of the companion cells serve for the nuclei of sieve tubes as they lack them.The companion cells are present only in Angiosperms and absent in Gymnosperms and Pteridophytes.They assist the sieve tubes in the conduction of food materials.