Delphinidin

Phloem tissue, which conducts dissolved food materials (primarily sugars) produced by photosynthesis throughout the plant, is composed mostly of two types of cells without secondary walls. The relatively large, more or less cylindrical sieve tube members have narrower, more tapered companion cells closely associated with them.

Phloem is derived from the parent cells of the cambium, which also produce xylem cells; it often also includes fibers, parenchyma, and ray cells. Sieve tube members, like vessel elements, are laid end to end, forming sieve tubes. Unlike vessel elements, however, the end walls have no large openings; instead, the walls are full of small pores through which the cytoplasm extends from cell to cell. These porous regions of sieve tube members are called sieve plates.

Sieve tube members have no nuclei at maturity, even though their cytoplasm is very active in the conduction of food materials in solution throughout the plant. Apparently, the adjacent companion cells form a very close relationship with the sieve tubes next to them and aid in the conduction of the food.

Living sieve tube members contain a polymer called callose that stays in solution as long as the cell contents are under pressure. If an insect such as an aphid injures a cell, however, the pressure drops, and the callose precipitates. The callose and a phloem protein are then carried to the nearest sieve plate where they form a callus plug that prevents leaking of the sieve tube contents.

Sieve cells, which are found in ferns and cone-bearing trees, are similar to sieve tube members but tend to overlap at their ends rather than form continuous tubes. Like sieve tube members, they have no nuclei at maturity, but they have no adjacent companion cells. They do have adjacent albuminous cells, which are equivalent to companion cells and apparently function in the same manner.