Transpiration
Transpiration
This section explains how various environmental factors can change the rate of transpiration, and also examines how the structure of the leaves has adapted to minimise this water loss.
Learners will need to understand the factors that affect the transpiration rate such as temperature, light intensity, wind and humidity. Simple experiments can be conducted to demonstrate these factors.
Transpiration is a process that involves loss of water vapour through the stomata of plants. Transpiration is thought to be a 'necessary cost or evil' to allow the plant to absorb water from the soil. It is an inevitable process.
Fact:
Turgidity, or turgor pressure, refers to the water content of cells and how this lends structural support to the plant. When cells absorb water, the vacuoles fill up and the cytoplasm increases, pushing against the cell membranes, which in turn push against the rigid cell walls. This makes the cells rigid, or turgid.
Transpiration is important in plants for three major reasons:
- Cooling of the plant: the loss of water vapour from the plant cools down the plant when the weather is very hot.
- The transpirational pull: when the plant loses water through transpiration from the leaves, water and mineral salts from the stem and roots moves, or is `pulled', upwards into the leaves. Water and is therefore taken up from the soil by osmosis and finally exits the plants through the stomata.
- Plant structure: young plants or plants without woody stems require water for structural support. Transpiration helps maintain the turgidity in plants.
Transpirational pull: results from the evaporation of water from the surfaces of the mesophyll layer in the leaf to the atmosphere, through the stomata. Evaporation of water from the leaves surface causes a negative pressure (suction force) in the xylem that pulls water from the roots and soil. This results in water being drawn up the xylem vessel.
Transpirational pull draws water from the roots to the leaves because of the effects of capillary action. The primary forces that create the capillary action are adhesion and cohesion. Adhesion is the attraction that occurs between water and the surface of the xylem, and cohesion is the attraction between water molecules.
We will revisit transpirational pull and capillarity later in the tutorial when we examine how water is transported in the plant.
Fact:
Capillary action occurs when the adhesion of water molecules to the walls of the vessel is stronger than the cohesive forces between the water molecules. Have you ever seen fluid in a drinking straw move higher than the level of the fluid in the glass? This happens due to capillary action. The narrower the straw, the greater the capillary action, and therefore, the higher the fluid will rise in the straw.
Fact:
Cohesion refers to the intermolecular, attractive forces that hold molecules in solids and liquids together. Imagine a drop of water on a waxy surface like wax paper. Even if the drop slides and rolls around, the water molecules will stay together due to the cohesive forces. Adhesion is the ability of a substance to stick to an unlike substance. If you were to take the same piece of wax paper and turn it upside down, some water droplets would still adhere to the paper. This indicates that there must be an attraction between the water and the wax paper. However, in this case the water-water cohesive force is stronger than the adhesive force between the molecules of the wax paper and the water.
This lesson is part of:
Plant Systems