Plants transfer large amounts of water from the soil to the atmosphere by a process called transpiration. An acre of corn can discharge about 3,000 to 4,000 gallons of water each day and a large oak tree can transpire about 40,000 gallons per year. The U.S. Geological Survey estimates that about 10 percent of the water vapor in the atmosphere has been introduced by plants, the remainder being from evaporation from the oceans, lakes and rivers.
Why do Plants Drink Water
Plants need water for nutrition and cooling. Water evaporation through the leaves cools plants by a process similar to perspiration in humans; like all living organisms, plants cannot survive very hot temperatures. Water exits plants by opening up pores in the leaves, and while the pores are open, they also absorb carbon dioxide which is essential for photosynthesis and plant growth. Water also transports essential nutrients from the soil to the plants.
Driving Mechanisms for Water in Plants
From the time of Aristotle onwards (about 350 BC) there has been speculation and research into how plants absorb water from the soil and transport it through their internal structures. The concept of water potential, introduced in the 1960s, is generally accepted today as the basis of the working model. A water potential gradient exists within plants and water flows down this gradient. Solar heating is the primary energy source that creates the potential gradient. During the first half of the 20th century, osmotic pressure was assumed to be the primary driving force. Osmosis causes a fluid to move through a membrane from a zone of higher pressure to one of lower pressure.
Groundwater enters plants through a network of root hairs at the tips of root caps. Water absorption depends on the magnitude of the water potential difference between the soil water and the root tips, and also on the resistance to water flow offered by the internal structure of the roots. Water absorption is also affected by the amount of air in the soil, its temperature and the concentration of solutes in the soil.
After its passage through the roots, water enters the xylem, the woody part of a tree, which includes a complex system of water columns for carrying the water. Vascular bundles in the xylem are interconnected and spiral upward in complex patterns. In most plants only a few outer rings of the sapwood usually conduct water.
Finally, the water reaches the pores on the undersides of leaves, called the stomata, and exits into the atmosphere. Stomata are pores in the skin of the leaves whose aperture is controlled by pairs of special cells called guard cells. Carbon dioxide from the air is vitally essential for the plants for creating tissue by photosynthesis, and the only way the air can enter a plant is through the stomata and only when they have opened to let the water out.