Transpiration (diffusion method) is a major physiological process in plants, by which loss of water from stomatal openings takes place. Stoma (singular stomata) are minute pores present in the foliage, stem, roots, and flowers. In most of the floral species, their number is higher in the lower side of leaves. They are crucial structures for exchanging gases and moisture, between plants and their environment.
Several functions of plants are associated with this process, which takes place when the stomata are open. When you study the detailed structure of a stomata, you will find the guard cells that regulate its opening and closing. The hydrostatic pressure of upper plant parts and soil moisture, collectively play a major role in opening of these structures.
The roots take up water molecules from soil, by means of osmosis, during which essential nutrients are also absorbed. Water and the minerals reach the upper plant parts. As the stoma receives the H2O molecules, the guard cells become flaccid, resulting in opening of the pores. Water is then lost by transpiration, and the cells revert to a normal state resulting in the closing of stoma.
Botanical studies have proved that this process is very important for the survival of plants. It is crucial for uptake of nutrients from the soil medium and controlling photosynthesis (most importantly for exchange of oxygen and carbon dioxide). In short, all floral species transpire and lose water for taking up carbon dioxide from the atmosphere.
It is beneficial for maintaining the plant temperature and by keeping them cool during hot conditions. The rate of transpiration and number of stomata are proportional to each other. The amount of water lost, depends on the atmospheric temperature, light intensity, humidity, wind, H2O content in soil, and size of the plant. It is claimed that maximum quantity of water absorbed by the roots, is lost through this process. Only 1% of it is utilized for growth and development of plants.
Cactus and other desert flora possess certain adaptive features, which enable them to thrive in prolonged dry spells. One of the adaptations is the presence of less number of stomata, sunken stomata, and small pore structures (in some species). The loss of water by transpiration due to the above characteristics is greatly reduced.
Transpiration and evaporation are similar aspects, which are important in maintaining the hydrological cycle. Both these processes are collectively called evapotranspiration, which contributes approximately 10% of the atmospheric moisture.