What is the function of phloem?

Phloem serves as the plant's food distribution system, transporting photosynthetically produced sugars and other organic nutrients from source organs (primarily leaves) to sink organs (roots, fruits, developing shoots, and storage tissues) throughout the plant body. This bi-directional transport system moves nutrient-rich sap through specialized living cells called sieve tube elements, which form continuous channels stacked end-to-end with perforated sieve plates facilitating flow. Unlike xylem's unidirectional upward movement driven by transpiration, phloem transport follows metabolic demand, redirecting resources to wherever growth, storage, or energy consumption is occurring.

The phloem's translocation function is essential for plant survival and productivity, enabling efficient resource allocation based on developmental priorities and environmental conditions. During spring, stored sugars move from roots to support new leaf growth; during fruiting season, massive amounts of photosynthate are redirected to developing seeds and fruits. Companion cells actively load sugars into sieve tubes using energy-dependent mechanisms, creating pressure gradients that drive mass flow of phloem sap at speeds up to one meter per hour. This system also transports amino acids, hormones, signaling molecules, and defense compounds, coordinating plant-wide responses to stress and facilitating communication between distant organs. Agricultural practices like girdling (removing bark containing phloem) can temporarily increase fruit size by preventing sugar export from branches, demonstrating phloem's critical role in crop yield and quality—though prolonged girdling eventually starves roots and kills the plant.