294 PLANT PHYSIOLOGY 



However in many plants, such as wheat and other cereals, 

 the leaves are not moistened by water owing to their wax cover- 

 ing. As has been proved in experiments with spray irrigation, 

 such plants may be watered at any time of the day and inde- 

 pendent of the weather conditions. But even with plants 

 sensitive to wetting of the leaves, cautious watering without 

 sprinkhng of the leaves does not produce any harmful conse- 

 quences, even when done during the hot noon hours. 



68. The Process of Transpiration, or Evaporation, of Water 

 by the Plant. Its Role in the Life of the Plant. Methods of 

 Studying Transpiration. — Land plants lose considerable amounts 

 of water through evaporation from the surface of their leaves. 

 Evaporation is essentially a physical process in which water 

 changes from a hquid to a gaseous state in an unsaturated atmos- 

 phere and then diffuses as vapor into the surrounding space. 

 The mechanism, however, is considerably complicated by a 

 number of anatomical and physiological structures of the evapo- 

 rating surface. Hence, it must be really considered a physio- 

 logical process. It leaves a marked effect on many phases of 

 the Hfe of land plants, and it is responsible for a whole series 

 of other related physiological processes. The evaporation of 

 water by the plant is usually designated by the special term 

 "transpiration." 



Transpiration is a physical necessity for all terrestrial plants. 

 In instances of great dryness of the air and lack of water in the 

 soil, a continual loss of water may lead to harmful and even fatal 

 consequences. Emphasizing this, some authors (Schwendener, 

 Timiriazev) believe that transpiration is an unavoidable evil. 

 This, however, is not entirely true. Under normal conditions 

 of growth, the loss of water by the leaves is readily replaced 

 from the soil. Moreover, the process of transpiration is very 

 probably the major cause of translocation of water toward the 

 evaporating leaves. Thus ''injury" caused by transpiration is 

 really not so great. Also, the absorption and upward translo- 

 cation of mineral salts are facilitated by the water current that 

 continually passes through the plant. Another favorable conse- 

 quence of transpiration is the reduction in temperature of the 

 leaves, which enables them to function even in the brightest 

 sunhght without injury. Experiments have shown that the 

 temperature of wilting leaves, whose transpiration is reduced, 



