Phloem

Phloem is a complex tissue in vascular plants responsible for translocating organic nutrients, primarily sucrose, from photosynthetic organs to non-photosynthetic tissues. In angiosperms, phloem consists of sieve tube elements arranged end to end to form sieve tubes, with sieve plates at their end walls. Each sieve tube element is typically associated with a companion cell that controls loading and metabolic support. Sieve tubes lack most organelles as they mature, relying on companion cells and adjacent phloem parenchyma. Gymnosperms and some lower plants have sieve cells instead of sieve tubes and may lack companion cells; they have albuminous cells.

Phloem also includes phloem parenchyma and fibers, which provide storage and mechanical support. The phloem is distributed throughout vascular bundles and lies outside the xylem in many stems and roots.

Transport in phloem is source-to-sink and is often described by the pressure-flow hypothesis. Solutes such as sucrose are actively loaded into sieve elements at a source (e.g., mature leaves) by companion cells, increasing solute concentration. Water follows by osmosis, generating a higher turgor pressure in the sieve tube at the source. At sinks, solutes are unloaded and drawn into surrounding cells, reducing pressure and allowing sap to move toward tissues with lower pressure. The sap flow is bidirectional and dynamic, driven by the relative sizes of sources and sinks.

Phloem development arises from the plant’s procambium, and mature phloem is typically alive, with companion cells carrying out most metabolic functions. Phloem is essential for distribution of photosynthates and signaling molecules throughout the plant.