Transpiration

Transpiration is the evaporation of water from plants, especially leaves. It is a type of translocation and part of the water cycle. The amount of water lost by a plant depends on its size, the light intensity, temperature, humidity, wind speed, and soil water supply. The process of Transpiration helps in regulating temperature in the plant.

Transpiration was first worked out by Stephen Hales (17 September 1677 – 4 January 1761), an English clergyman.^[1]^[2] He proved what is still believed, that the evaporation of water molecules from leaves is the main force pulling the water column up from its origin in the roots.

How transpiration works

Leaf surfaces are dotted with openings called stomata, which act rather like pores. In most plants there are more on the undersides of the leaves than on the top. The stomata are bordered by guard cells that open and close the pore. Transpiration happens when the guard cells open the stomata. This lets oxygen and water vapour flow out, and carbon dioxide flow in. The carbon dioxide is used in photosynthesis, and the oxygen is produced by photosynthesis.

Transpiration also pulls water through the plant. This brings with it the mineral nutrients from roots to shoots. Water moves out of the leaves into the atmosphere. This exerts a pull on the water column, which brings the water up against gravity. Water gets into the plant at the roots by osmosis, and it transports dissolved mineral nutrients to the upper parts of the plant through the xylem. A fully grown tree may lose hundreds of gallons (thousands of liters) of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for this process; most of the rest is used in photosynthesis.

Desert plants and conifers have adaptations which reduce water loss. Examples are: thick cuticles, reduced leaf areas, sunken stomata and hairs. All these reduce transpiration and conserve water. Many cacti do photosynthesis in succulent stems, rather than leaves. The surface area of even a fat stem is far less that the total surface of leaves in a tree. Also, stomata of desert plants are usually closed during the day and open at night, when transpiration is lower.

Transpiration Media

Overview of transpiration:
1. Water is passively transported into the roots and then into the xylem.*
2. The forces of cohesion and adhesion cause the water molecules to form a column in the xylem.*
3. Water moves from the xylem into the mesophyll cells, evaporates from their surfaces and leaves the plant by diffusion through the stomata*
Transpiration of water in xylem
Stoma in a tomato leaf shown via colorized scanning electron microscope
The clouds in this image of the Amazon Rainforest are a result of evapotranspiration.
Some xerophytes will reduce the surface of their leaves during water deficiencies (left). If temperatures are cool enough and water levels are adequate the leaves expand again (right).
The effect of temperature on the transpiration rate of plants.
The effect of wind velocity on the transpiration rate of plants.
The effect of humidity on the transpiration rate of plants.