Ch. Ill, 6] TRANSPIRATION FROM PLANTS 43 



of their very different habits, centering especially in their 

 different ways of acquiring their food. 



6. The Water Loss, or Transpiration, from Plants 



A special feature of the physiology of leaves, and other 

 green tissues, is the constant loss of water therefrom to the 

 air, — a matter which profoundly influences the forms and 

 distribution of plants. It is called scientifically transpira- 

 tion, and the student should not permit the resemblance 

 between this word and respiration to confuse in his mind the 

 two processes, which are wholly unrelated. 



The general fact that much water evaporates from plants is 

 well known to all who grow them. The rapid wilting of shoots 

 when cut but not placed in water, is visible evidence thereof. 

 The water which gathers in drops on the glass covers of ferner- 

 ies, or on windows in which house plants are kept, has mainly 

 this origin, though of course it comes partly from wet soil. The 

 reality of the transpiration from the green parts, as distinct 

 from evaporation from the soil, can fje shown very perfectly 

 by the arrangement pictured herewith (Fig. 18) ; for only the 

 leaves and stem are inside the closed chamber, the pot and soil 

 being excluded by a special glass plate. Within a few minutes 

 some water appears on the glass, at first as a faint vaporous 

 cloud, and later in large drops which run down the sides. 

 Thus we have a perfect demonstration of transpiration, or 

 the removal of water as vapor from leaves and young stems. 



The precise amount of transpiration can be determined 

 in several ways, but most accurately by weighing, which 

 requires potted plants. To secure transpiration without 

 evaporation from soil and pot, we use the arrangement shown 

 in our picture (Fig. 19). When a plant thus prepared is 

 weighed at intervals on a good balance, the transpiration is 

 determined exactly, and since the cover may be raised and 

 known quantities of water added at intervals, the experi- 

 ment may be continued as long as desired. By this method 

 it is found that living green parts in the hght never wholly 



