In the given figure of phloem tissue, identify the marked part (A, B and C) which help in maintaining the pressure gradient
in the sieve tubes.

In the given fig. of phloem tissue A, B & C are respectively sieve tube, phloem parenchyma and campanion cells. Companion cells (c) are specialized parenchymal cells adjacent to a sieve tube in the phloem of flowering plants. It is beliveved to regulate the flow of nutrients through the tube.