Phloems

Phloems are living vascular tissues in plants that conduct organic nutrients, primarily sucrose, from photosynthetic organs to non-photosynthetic tissues and storage sites.

In angiosperms, phloem tissue consists mainly of sieve-tube elements and companion cells, connected by plasmodesmata. Sieve-tube elements are elongated living cells that lack a nucleus at maturity; their cytoplasm is continuous through sieve plates. Companion cells retain a nucleus and metabolic machinery to load and unload sugars and to maintain the sieve-tube elements. Phloem also contains phloem parenchyma and fibers; phloem fibers provide structural support.

Location: In most vascular plants, phloem is part of the vascular bundle, typically located toward the exterior of the bundle relative to xylem. The arrangement differs between groups: in many dicots, phloem forms a distinct bundle with xylem; in gymnosperms, the conducting phloem often includes sieve cells rather than sieve tubes.

Transport mechanism: The pressure-flow hypothesis describes how solutes are loaded into phloem at sources (e.g., mature leaves), drawing in water and creating turgor pressure that pushes sap toward sinks (e.g., roots, developing tissues) where solutes are unloaded. Phloem loading can be active and may involve apoplastic or symplastic routes; unloading may be passive or active, depending on the tissue.

Development and evolution: The phloem is produced by the vascular cambium in many woody plants, forming secondary phloem toward the outside of the stem. Sieve-tube elements and companion cells are characteristic of flowering plants; gymnosperms rely on sieve cells and albuminous cells.

Importance: Phloem function is essential for growth and development; phloem transport also renders the plant vulnerable to phloem-feeding insects and certain pathogens. Disruption of phloem transport can cause wilting, stunted growth, or necrosis.