5. Transpiration stream

Water enters the root hair cell by osmosis. This is because minerals actively transport in first, which lowers the water potential forcing the water to follow by osmosis.

Water enters the xylem vessels in the same way. Once the water has gotten into the root hair, it moves across the cortex by osmosis. It travels through the casparian *****, in the endodermis. The minerals actively transport into the xylem, then the water moves by osmosis through.

Once the water is in the xylem it moves up the stem by:

• root pressure
• transpiration pull
• capillary action.

• as the various minerals move into the medulla and xylem by active transport, this drives water in my somosis.
• As water is forced into the xylem, it pushes water up the stem.
• root pressure only accounts for getting water up the start of the stem.

cohesion = attraction of water molecules to each other. As water molecules evaporate from leaves, it pulls up attracted water molecules.

Cohesion tension theory results in the water molecules being pulled up the xylem like beads on a string.

adhesion = attraction of water molecules to the xylem walls.

The same forces taht hold water molecules together, also attract the water molecules to the sides of the xylem (lignin).

Because xylem are very narrow, the forces of attraction pull the water up via the sides of the xylem.

transpiration = the loss of water vapour from the stomata due to evaporation.

By diffusion mainly through the stomata. A small amount of water can also diffuse through the epidermis.

• changes in the diameter of trees between day and night.
• air drawn into xylem when vessel is broken rather than water leaking out.
• when air is pulled in, the plant can no longer move water up the stem.