The Water Relations of Leaves 75 



the stomata begin to close, and before sundown are completely 

 closed. The rate of transpiration begins to decrease about 

 2 o'clock and reaches the slow night rate before, or soon after, 

 sunset. The stomata, therefore, modify the rate of transpiration 

 greatly. 



It must not be supposed, however, that they act as safety 

 mechanisms to conserve water in the plant. They may open in 

 the hght, whether the plant has an adequate water supply or not. 

 Likewise they may close when the plant has an abundance of 

 water. Usually the stomata close when the leaves wilt, but there 

 are exceptions even to this rule. The opening of the stomata in 

 hght not only allows the outward diffusion of water vapor, but 

 also the inward passage of carbon dioxide used in photosynthesis 

 and the escape of oxygen liberated in this process. 



The amount of water transpired by plants. The amount of 

 water lost in transpiration is surprisingly large. During its hfe- 

 time, a well-watered corn plant may lose 40 gallons of water. 

 The water lost by a field of wheat during its entire period of 

 development would cover the field to a depth of 4 or 5 inches. 

 A medium-sized date palm growing in the Sahara Desert under 

 irrigation is estimated to require from 100 to 190 gallons of water 

 per day during at least four months of the year. For the best 

 growth of plants, therefore, there must be available in the soil 

 enough water to replace all that is lost by transpiration and 

 the smaller amount used in the growth of new parts. 



When we consider that the quantity of water transpired by 

 wheat in cultivation is one fifth to one eighth of the rainfall of 

 the central United States, we begin to realize how large a fraction 

 of all the water that falls on the soil is actually used by the plants. 

 In all rainfall, some water runs off the soil without penetrating 

 the surface, some evaporates from the soil surface itself, and some 

 sinks below the level of the plant roots. Consequently, it is 

 only when there are abundant rains, distributed throughout the 

 growing season, that the amount of water needed by the plants 



