Plant Anatomy: Phloem Fibers and Vascular Bundles

Phloem Fibers

• Made up of sclerenchymatous cells, generally absent in primary phloem but found in secondary phloem
• Cell wall is thick, and at maturity, they lose their protoplasm and become dead
• Function as strengthening and supporting cells

Vascular Bundles

• Consist of xylem and phloem, which together constitute vascular bundles
• Three types of vascular bundles:
  • Collateral vascular bundles: have xylem and phloem arranged adjacent to each other, found in dicotyledonous stems and monocotyledonous leaves and stems
  • Radial vascular bundles: have xylem and phloem arranged in an alternate manner along different radii, found in roots
  • Concentric vascular bundles: have xylem and phloem jointly situated along the same radius, found in stems and leaves

Xylem Fibers

• Elongated with tapering ends, dead cells with lignified walls and narrow lumen
• Found in both primary and secondary xylem

Xylem Parenchyma Cells

• Living cells, only living component found in xylem tissue
• Thin-walled, made up of cellulose, and store food materials such as starch or fat

Phloem

• Complex tissue composed of several cell types, including sieve tubes, sieve cells, companion cells, phloem fibers, and phloem parenchyma
• Functions to conduct dissolved food materials (primarily sugars) produced by photosynthesis throughout the plant

Sieve Elements

• Can be segregated into sieve tubes or sieve cells
• Sieve tubes are more specialized, consisting of long, tube-like structures arranged longitudinally and laid end to end, forming sieve tubes
• Sieve cells are less specialized

Xylem

• Important component of the "plumbing" and storage systems of a plant
• Chief conducting tissue throughout all organs for water and minerals absorbed by the roots
• Two types of xylem tissue: primary xylem and secondary xylem
• Primary xylem develops earlier and is first formed from procambium of apical meristem, while secondary xylem develops from vascular cambium
• Primary xylem consists of protoxylem and metaxylem, with protoxylem containing few tracheary elements (tracheids or vessel)