Definition of companion cell in US English:

companion cell

noun

Botany
  • (In the phloem of flowering plants) a specialized elongated cell, one or more of which is typically connected to, and regulates the movement of substances into and out of, each sieve cell; (in early use also) †a cell that has recently divided from another (obsolete).

Origin

Mid 19th century. In later use after German Geleitzelle.