Phloem transports organic materials and ions throughout a plant. Phloem (from the Greek word phloios, meaning “bark”) transports dissolved organic materials (especially sucrose, stored energy as a sugar) throughout the plant. Unlike xylem, which primarily transports water upward, phloem transports solutes (dissolved molecules) upward and downward.
The conducting cells of phloem are called sieve elements. Sieve elements lack a nucleus (it disintegrates after division) but have otherwise intact protoplasts at maturity. They also have thin areas along their cell walls that are perforated by sieve pores. Solutes move from sieve element to sieve element through these pores. Sieve plates produce thin, primary cell walls and are the most delicate cells in the plant, usually living less than a year.