Transpiration is the process by which plants give off water into the atmosphere in the form of water vapor. It is not a simple process of evaporation since it is regulated by the physiological activity of the protoplasm. The extent of loss of water due to transpiration is different for different plants; for example, 10 to 20 liters by an apple tree, and 3 to 4 liters by a maize plant per day.
Mechanism of Transpiration
Water may transpire directly through the cuticle, a waxy layer covering the leaf surfaces (cuticular transpiration), through stomata (stomatal transpiration) or through the lenticels (lenticular transpiration). Turgor pressure inside mesophyll cells of the leaf forces water outwards through the cell wall. Water is collected in the intercellular spaces. From intercellular spaces, water diffuses out of the stomata into the atmosphere. Transpiration is greatly influenced by three factors – guard cells, water vapor in the respiratory cavities and the intercellular spaces.
Transpiration plays and important role in the translocation of water and dissolved matter. Evaporation of water from the leaf causes a drop in turgor pressure. It will pull water from the neighboring cells and a continuous chain is formed through xylem tubes. This is known as cohesion theory of transpiration pull theory to explain the ascent of water from the roots. According to this, the water molecules cohere together and form a long continuous column in the vessels extending from leaf to the root. You can compare it with taking ink from an ink bottle using a dropper. Besides translocation of substances transpiration also facilitates other physiological activities of the plant.
During photosynthesis gases like carbon dioxide and oxygen also diffuse in the same fashion. When the concentration of carbon dioxide in the cell falls, it sets up a diffusion gradient between the air and intercellular spaces in the cell. This results in diffusion of carbon dioxide into the cell from the atmosphere. The concentration of carbon dioxide increases inside the leaf. This then comes out of the leaf to the atmosphere. Since diffusion occurs in all directions, large surface area is helpful in greater diffusion.