296 



PLANT PHYSIOLOGY 



transpired. In stagnant air, however, transpiration is retarded. 

 It is expedient, therefore, to draw a current of air through the 

 vessel containing the plant and then to pass this air through 

 calcium chloride tubes or an apparatus containing some other 

 water-absorbing substance (Fig. 90). The advantages of this 

 method are that the leaf or branch under investigation remains 

 attached to the plant. This is almost the only method that can 

 be used to determine the transpiration of trees under natural 

 conditions. Its defect is that the transpiring organs are enclosed 



Fig. 90. — Apparatus for the study of transpiration {redrawn after Freeman). 



in a container, which must undoubtedly interfere with the normal 

 course of transpiration. 



Somewhat analogous to the quantitative methods, which are 

 based on the increase in weight of a water-absorbing substance, 

 is the cobalt chloride method, in which the color changes exhibited 

 by filter paper that has been impregnated with a solution of 

 cobalt chloride and dried are used to indicate the transpiration 

 rate. When dry, this paper has a blue color; when moist, it 

 becomes pale pink. If a slip of such paper is placed on the 

 transpiring surface of a leaf and then covered with glass, its 

 color will change (Fig. 91). 



