CIE IGCSE Biology: 复习笔记:8.1.1 Xylem & Phloem

CIE IGCSE Biology: 复习笔记:8.1.1 Xylem & Phloem

The Xylem & Phloem

  • Plants contain two types of transport vessel:
    • Xylem vessels – transport water and minerals (pronounced: zi-lem) from the roots to the stem and leaves
    • Phloem vessels – transport food materials (mainly sucrose and amino acids) made by the plant from photosynthesising leaves to non-photosynthesising regions in the roots and stem (pronounced: flow-em)
  • These vessels are arranged throughout the root, stem and leaves in groups called vascular bundles

Vascular tissue in a dicotyledonous plant

Exam Tip

If you are asked to identify the xylem or phloem in a diagram showing a cross-section of a root, stem or leaf just remember that xylem is always on the inside and phloem is always on the outside.

Adaptations of Xylem Vessels: Extended

Xylem cells lose their top and bottom walls to form a continuous tube through which water moves through from the roots to the leaves

  • Function: transport tissue for water and dissolved mineral ions
  • Adaptations:
    • Cells joined end to end with no cross walls to form a long continuous tube
    • Cells are essentially dead, without cell contents, to allow free passage of water
    • Outer walls are thickened with a substance called lignin, strengthening the tubes, which helps support the plant
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