A specialized * parenchyma cell, characterized by a dense cytoplasm and conspicuous nucleus, associated with an adjacent * sieve tube element in the phloem of angiosperms. Companion cells are analogous to the * albuminous cells of gymnosperms. A sieve tube element and its associated companion cell arise from a single division of the same mother cell. The protoplast of the companion cell is connected with the enucleate protoplast of the sieve tube element by means of * plasmodesmata and is believed to regulate the rate of flow along the * sieve tube. |
