companion cells Modified parenchyma, linked to sieve cells by plasmodesmata. Unlike sieve cells, companion cells are nucleated and have many mitochondria. Companion cells may regulate translocation.

companion cell A type of cell found within the phloem of flowering plants. Each companion cell is usually closely associated with a sieve element. Its function is uncertain, though it appears to regulate the activity of the adjacent sieve element and to take part in loading and unloading sugar into the sieve element. In gymnosperms a similar function is attributed to albuminous cells, which are found closely associated with gymnosperm sieve elements.