noun, Botany.
any of a number of specialized parenchymal cells adjacent to a sieve tube in the phloem of flowering plants, believed to regulate the flow of nutrients through the tube.
companion cell
A specialized parenchyma cell, located in the phloem of flowering plants and closely associated with the development and function of a sieve-tube element. Companion cells probably provide ATP, proteins, and other substances to the sieve-tube elements, whose cytoplasm lacks many structures necessary for cell maintenance.


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