The Xylem & Phloem
- Plants contain vascular tissue for transport of substances, including:
- Xylem vessels (pronounced: zy-lem) transport water and minerals from the roots to the stem and leaves
- Phloem vessels transport sucrose and amino acids from sources to sinks
- These vessels are arranged throughout the root, stem and leaves in groups called vascular bundles
- In stems vascular bundles are arranged around the outer edge, with phloem vessels closer to the outside of the stem, and xylem vessels closer to the inside
- In roots there is a single vascular bundle in the centre of the root, inside which the xylem forms an 'x' shape and phloem are arranged around the outside of the xylem
- Stems and roots also contain a region of tissue known as the cortex; this is a layer of tissue that lies between the vascular tissue ands the outer surface of the plant
- In stems the cortex is a narrow band of tissue, as the vascular tissue is close to the surface of the stem
- In roots the cortex is wide and takes up most of the root
- Functions of the cortex include:
- Transport, e.g. of water towards the vascular tissue
- Storage, e.g. of starch
Distribution of xylem and phloem diagram
Vascular tissue is distributed differently in different parts of a plant
Examiner Tip
Remember that xylem is always on the inside of the vascular bundle in stems/roots, while phloem is always on the outside.
