Lesson 4.4 Transpiration
Completion requirements
Section 4: How a Plant Grows
Lesson 4: Transpiration
We have learned that a plant uses water absorbed by the roots for the process of photosynthesis and the resulting cell growth. But here's a surprising fact: only about 10% of the absorbed water is used for that purpose. So what about the other 90% of the water absorbed by the roots? Does it have a purpose? And what happens to it?
A magnified stoma in a tomato leaf.
©iStock
Transpiration is the process whereby water vapour is lost through the stomata of
the leaves. That's what happens to about 90% of the water absorbed by plant roots.
And this is essential, since this process of transpiration actually pulls water from the roots through the plant. It happens while the leaf's stomata are open for the passage of CO2 and O2 during photosynthesis. The light of day stimulates the opening of the stomata to allow for a higher rate of transpiration.
And this is essential, since this process of transpiration actually pulls water from the roots through the plant. It happens while the leaf's stomata are open for the passage of CO2 and O2 during photosynthesis. The light of day stimulates the opening of the stomata to allow for a higher rate of transpiration.
©iStock
Transpiration has three essential functions:
Dissolved minerals move up from the root, in the xylem and sugars, produced by photosynthesis, move throughout the plant, in the phloem. Water serves as both the solvent and the means of transport.
Eighty percent of the cooling effect of a shade tree is from the evaporative cooling effects of transpiration. This benefits both plants and humans.
Water maintains the turgor pressure in cells, which keeps the non-woody
parts of a plant firm.
Desert plants and conifers have specially adapted structures, such as reduced leaf areas and sunken stomata, to reduce transpiration and conserve water.
Digging Deeper