Brent Cornell

    
The flow of water through the xylem from the roots to the leaf, against gravity, is called the transpiration stream

• Water rises through xylem vessels due to two key properties of water – cohesion and adhesion

Cohesion:

• Cohesion is the force of attraction between two particles of the same substance (e.g. between two water molecules)
• Water molecules are polar and can form a type of intermolecular association called a hydrogen bond
• This cohesive property causes water molecules to be dragged up the xylem towards the leaves in a continuous stream

Adhesion:

• Adhesion is the force of attraction between two particles of different substances (e.g. water molecule and xylem wall)
• The xylem wall is also polar and hence can form intermolecular associations with water molecules
• As water molecules move up the xylem via capillary action, they pull inward on the xylem walls to generate further tension

Water Transport via the Xylem

Structure of the Xylem

The xylem is a specialised structure that functions to facilitate the movement of water throughout the plant

• It is a tube composed of dead cells that are hollow (no protoplasm) to allow for the free movement of water
• Because the cells are dead, the movement of water is an entirely passive process and occurs in one direction only
• The cell wall contains numerous pores (called pits), which enables water to be transferred between cells
• Walls have thickened cellulose and are reinforced by lignin, so as to provide strength as water is transported under tension

Basic Xylem Structure

Xylems can be composed of tracheids (all vascular plants) and vessel elements (certain vascular plants only)

• Tracheids are tapered cells that exchange water solely via pits, leading to a slower rate of water transfer
• In vessel elements, the end walls have become fused to form a continuous tube, resulting in a faster rate of water transfer

All xylem vessels are reinforced by lignin, which may be deposited in different ways:

• In annular vessels, the lignin forms a pattern of circular rings at equal distances from each other
• In spiral vessels, the lignin is present in the form of a helix or coil 

Additional Xylem Features