Phloem

Phloem is the living vascular tissue in plants responsible for transporting sugars, amino acids, hormones, and other organic molecules from sites of production (sources) to sites of consumption or storage (sinks). In most plants, the primary sources are mature leaves, where photosynthesis generates sugars, and the primary sinks are growing tissues, roots, fruits, and storage organs. The phloem thus functions as the plant’s distribution network, connecting the organs that capture energy to the organs that consume it.

The phloem is composed of sieve elements — elongated cells connected end-to-end through perforated sieve plates — and companion cells that maintain the metabolic viability of the sieve elements. Unlike xylem vessels, which are dead at functional maturity, phloem cells remain alive. This is essential to their function: the transport of sugars through the phloem depends on active loading and unloading processes driven by living cells. The phloem is infrastructure that must itself be maintained as a living system.

From a relational standpoint, the phloem reveals the plant as an internally connected whole. A leaf does not photosynthesize for itself alone; it produces sugars that are distributed throughout the organism. A root does not absorb minerals for its own use alone; those minerals travel upward through the xylem while sugars flow downward through the phloem to sustain the root’s growth. The phloem and xylem together constitute a circulatory logic — not a closed loop like an animal’s bloodstream, but a bidirectional flow connecting every part of the plant to every other. This internal connectivity is what makes the plant a genuine autopoietic unity rather than a mere collection of independent modules.

Beyond bulk transport, the phloem carries signaling molecules — hormones, RNA, and small proteins — that coordinate development and stress responses across the plant body. Through these signals, a root under drought stress can communicate its condition to the leaves, triggering stomatal closure far from the site of water deficit. The phloem is therefore not only a distribution network but a communication channel, linking the plant’s dispersed meristems and organs into a coordinated, responsive whole.

Related terms

• Xylem — the complementary vascular tissue transporting water and minerals
• Stomata — pores whose regulation is coordinated via phloem-carried signals
• Meristem — growing regions sustained by phloem-delivered nutrients
• Homeostasis — the dynamic balance maintained through vascular coordination