floema

Floema, or phloem, is a vascular tissue in plants responsible for transporting organic nutrients, mainly sugars such as sucrose, from photosynthetic tissues to non-photosynthetic parts of the plant. It functions alongside xylem and forms part of the vascular system that distributes resources required for growth, storage, and metabolism.

Structure and components: In many plants, phloem is organized in vascular bundles. The main living transport elements are sieve tube elements, which form sieve tubes, and companion cells that regulate their function. Other phloem components include phloem parenchyma and fibers. In angiosperms, sieve tubes and companion cells are common, while gymnosperms often use sieve cells with associated albuminous cells. Sieve plates, with pores, connect sieve elements and facilitate flow.

Function and transport mechanism: Phloem translocates photosynthates from sources (e.g., mature leaves) to sinks (e.g., growing tissues, roots, fruits). The movement is explained by the pressure-flow hypothesis: loading of sugar into sieve elements at the source lowers water potential, water enters from the xylem, creating positive turgor pressure that drives sap toward sinks, where sugars are unloaded and used or stored. Phloem also transports amino acids, hormones, RNAs, and signaling molecules, supporting long-distance communication and coordinated responses.

Development and distribution: Primary phloem forms part of the young plant’s vascular tissue, while secondary phloem is produced by the cambium in woody plants, contributing to the inner bark (bast). The phloem’s integrity is crucial for growth and resilience, though it is sensitive to damage and disease.