A type of plant cell closely associated to a sieve-tube member by many plasmodesmata; the nucleus and ribosomes service one or more adjacent sieve-tube members which lack a nucleus and ribosomes.

in flowering plants, a cell closely associated in origin, position and function with a cell of a sieve-tube. A companion cell has a dense protoplasm and prominent nucleus. A companion cell and a sieve-tube cell are formed by longitudinal division of one parent cell. It appears to regulate the activity of a sieve-tube.

of phloem, a specialised parenchyma cell characterised by a dense cytoplasm and conspicuous nucleus, associated with an adjacent sieve-tube element.