You might have noted very small pores on the back side of leaf surface, if not do this: hold a leaf (of any plant from your garden) against sunlight, now you would spot these pores; these are stomata!
What actually are stomata? The stomata are respiratory organs (like our nose) of the plant. Each stoma is guarded by a set of cells which control its opening and the closing. During the process of photosynthesis, stomata allow carbon dioxide to enter the leaf and oxygen to escape and vice versa when photosynthesis is not occurring. The one more function is assigned to stomata and that is transpiration. During the transpiration, the water is lost through stomata and ofcourse this water has been absorbed by roots. You might have also seen dew drops early in the morning, if you live in tropical country; it is the proof for transpiration that plant stomata of nearby trees did at night! Stomata are different from plant to plant. It means that stomata of terrestrial plants differ from stomata of aquatic plants. Moreover, the stomata of desert plants are also different from those of Tundra region, although both types of plants are terrestrial. This stomatal difference is based on variable environmental conditions, habitats and plant's specific adaptations. Desert plants bear sunken or closed stomata and they are active in the conservation of water. So to conserve water, stomata remain close during night and open at night to lower the rate of transpiration. In aquatic plants, the stomata are present on upper leaf surface and additionally have more number of stomata to increase the rate of transpiration. The shape of leaf, color, texture, arrangement of leaves and orientation to sunlight also helps the stomata to control the transpiration rate.
What is water cycle? The water cycle is unique natural process on the Earth and is responsible for raining. The oceans, seas, rivers and all types of water bodies get water only from operation of the water cycle. It involves cycle of reactions like evaporation, condensation, precipitation and collection. Water from water bodies is heated to evaporation by Sun's heat. Vapor rises up and condensed on dust particles to form clouds. Water from the clouds reaches Earth as rain, hail or snow. The rain water goes into oceans, rivers or seeps underground and the cycle continues.
Relationship between stomata and water cycle: Stomatal function, transpiration is the integral part of the water cycle. Transpiration is equivalent process to evaporation in water cycle. In other words, stoma is as a minimodel of water cycle. In stomata, water in leaf received from roots is evaporated by sunlight. Water is lost as vapor when stomata are opened and we refer this as transpiration instead of evaporation. The plant water vapor released by stomata also rises up and condensed on dust particles present in the atmosphere to form millions tiny droplets of water. The cloud is formed from gathering of droplets, gets heavier it subsequently comes down as rain or snow. Humid, wet or dry weather conditions control the transpiration rate as sunlight intensity differ in rainy, summer or winter season. These conditions also affect root's water absorption capacity or osmosis reaction and its supply to plant via xylem. Stomata respond to hydraulic perturbations in plant created by water evaporation and subsequent loss. In rainy season, stomata remain open even at night but in summer they either remain close or partially open to conserve water. Although transpiration rate is low, it helps to keep plant cool. Absorption of water by roots and transpiration by stomata are continuous and proportional to each other and to weather conditions. The process of transpiration significantly contributes to every reaction of water cycle or we must say that trees help in operating water cycle. We therefore insist that afforestation and not deforestation will save the Earth.
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