The Transpiration Stream

From Alamy Stock Photo

In flowering plants, water is transported from the roots to the whole body of the plants by xylem vessels. The factor that has he most impact on this process is transpiration.

Water moves from:

Stomata to air

The air near the stomata usually has a greater water potential than the atmosphere, therefore water moves along the water potential gradient away from the stomata.

Mesophyll cells to stomata

The heat energy from the sun causes the water from the mesophyll cells in the leaf to evaporise through the stomata. These cells hen have a lower water potential than their neighbouring cells, there water moves towards the cells closer to the stomata by osmosis. This results in an overall movement of water from the xylem across the mesophyll cells and into the atmosphere through the stomata.

Up the stem in the xylem

The main factor for movement of water up the stem is cohesion tension, which relies on the cohesion-tension theory. The key in this theory is the property of water caused by hydrogen bonds - cohesion. Water molecules stick together and create a continuous column in the xylem. This column of water is pulled upwards due to transpiration as previously described. This creates tension in the xylem.

This theory can be proved by evidence:

1. Change in the rate of transpiration (e. g. higher at daytime) changes the diameter of xylem vessels because of higher pressure created in them, and therefore the diameter of tree trunks decrease.

2. If a xylem vessel is broken, the plant no longer draws water because the column of water has been interrupted.

3. If a xylem vessel is broken, water does not leak but air is drawn into the plant, showing the xylem is under tension.