Bastgewebes

Bastgewebe, or phloem tissue, is a vascular tissue in plants that transports organic nutrients, primarily sucrose, from photosynthetically active organs (sources) to other parts of the plant (sinks). It is one of the two main conducting tissues, the other being xylem, which transports water and minerals.

Composition: Bastgewebe comprises sieve elements (sieve tubes in angiosperms or sieve cells in gymnosperms), companion cells (present in most angiosperms), phloem parenchyma, and phloem fibers. Mature sieve elements are living but lack a nucleus and other organelles; they are connected end to end by sieve plates. Companion cells sustain and regulate the function of sieve elements through plasmodesmata and metabolic exchange. Phloem fibers provide mechanical strength, and phloem parenchyma stores and transports nutrients.

Location and development: In stems and roots, Bastgewebe lies exterior to the xylem as part of the bark in many plants. In woody plants, secondary phloem is produced by the vascular cambium and, as growth continues, older phloem is displaced outward and eventually shed in some species. The activity of the phloem ends at the periderm as bark thickens.

Transport mechanism: The pressure-flow (mass flow) model explains transport: sugars are loaded into phloem at sources, creating an osmotic gradient that draws water into the phloem, generating hydrostatic pressure that drives sap toward sinks where sugars are unloaded. Callose deposition can seal injured sieve plates to prevent leakage.

Economic relevance: Bast fibers, derived from phloem fibers, include flax, hemp, and jute, used to make textiles and cordage.