CHAPTER VI 

 THE LOSS OF WATER BY THE PLANT 



49. Transpiration. Its Role in the Life of a Plant.— Land 



plants lose considerable amounts of water through evaporation 

 from the surface of their cell walls. Evaporation is essentially a 

 physical process in which water changes from a liquid to a gaseous 

 state in an unsaturated atmosphere, and then diffuses as vapor 

 into the surrounding space. The mechanism, however, is consid- 

 erably complicated by a number of anatomical and physiological 

 peculiarities of the evaporating surface. Hence it must be really 

 considered a physiological process. It leaves a marked effect on 

 many phases of the life of land plants and 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; 

 but, 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) have come to believe that transpiration is nothing 

 more than 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 likely the major cause of translocation of water 

 towards the evaporating leaves. No great "injury" really can be 

 caused by transpiration. Also the absorption and upward translo- 

 cation of mineral salts is facilitated by the water current which con- 

 tinually passes through the plant. Another favorable consequence 

 of transpiration is the reduction in temperature of the leaves, which 

 enable them to function even in the brightest sunlight without 

 injury. Experiments have shown that the temperature of wilting 

 leaves, whose transpiration is reduced, is 4 to 6° higher than of 

 turgid ones, a difference which sometimes may prove fatal to the 



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