A type of cell found within the phloem of flowering plants. Each companion cell is usually closely associated with a sieve element, with which it communicates via numerous fine cytoplasmic strands (plasmodesmata) penetrating the adjoining cell walls. The companion cell shares its nucleus and RNA with its adjacent sieve element and regulates the latter’s activity. In some cases it may also take part in loading and unloading sugar into the sieve element (see phloem loading; phloem unloading). In gymnosperms a similar function is attributed to albuminous cells, which are found closely associated with gymnosperm sieve elements.