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

Xylem vessels are adapted to allow them to carry out the following functions:

• Transport of water and minerals from roots to leaves

• Structural support of plant stems

The features of xylem vessels that enable them to carry out these functions include:

• The outer walls of xylem vessels are thickened with a substance called lignin

  • Lignin strengthens the tubes, reducing the chance of breakages which might impede the flow of water

  • Lignin provides structural support to the plant stem

• Xylem cells are dead and contain no cell contents

  • The xylem tubes are hollow, allowing free passage of water

• Xylem cells are joined end-to-end with no cross walls

  • This means that a continuous tube is formed, through which water and dissolved minerals can flow