Transpiration is the loss of water from parts of plants, especially leaves (for photosynthesis) but also stems, flowers and roots.
Transpiration is the evaporation of water / loss of water from those parts of plants that are outside the earth, especially leaves but also stems, flowers and fruits. The amount of water lost by a plant depends on its size, along with the surrounding light intensity, temperature, humidity, wind speed, and soil water supply.
It happens because the plant needs to open its pores in order to obtain carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables the flow of mineral nutrients from roots to shoots. This mass flow is caused by the lower (hydrostatic) water pressure in the upper parts of the plants.
The reason for this low pressure is that water gets out of the leaves into the atmosphere. 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.
Topics of Interest
Plant Transpiration is a process similar to evaporation. It is the loss of water from parts of plants, especially leaves but also stems, flowers and roots. Leaf surfaces are dotted with openings called stomata, and in most plants they are more numerous on the undersides of the foliage. The stoma are bordered by guard cells that open and close the pore. Collectively the structures are called stomata. Leaf transpiration occurs through stomata, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.
Mass flow of liquid water from the roots to the leaves is caused by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem.
The rate of transpiration is directly related to the degree of stomatal opening, and to the evaporative demand of the atmosphere surrounding the leaf. The amount of water lost by a plant depends on its size, along with the surrounding light intensity, temperature, humidity, and wind speed (all of which influence evaporative demand). Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate.
A fully grown tree may lose several hundred gallons (a few cubic meters) 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. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced).
Transpiration rate of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, and heat balance sap flow gauges.
Desert plants and conifers have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis, termed crassulacean acid metabolism or CAM photosynthesis in which the stomata are closed during the day and open at night when transpiration will be lower.
Evapotranspiration (ET) is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies. Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves. Evapotranspiration is an important part of the water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator.
Cacti have never lost their leaves completely; they have only reduced the size so that they reduce the surface area through which water can be lost by transpiration. In some species the leaves are still remarkably large and ordinary while in other species they have become microscopic but they still contain the stomata, xylem and phloem. Certain cactus species have also developed ephemeral or deciduous leaves, which are leaves that last for a short period of time when the stem is still in its early stages of development. A good example is Opuntia ficus-indica, better known as the prickly pear. Cacti have also developed spines which allow less water to evaporate through transpiration by shading the plant, and defend the cactus against water-seeking animals. The spines grow from specialized structures called areoles, homologous to the nodes on other plants.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)