THE LOSS OF WATER BY THE PLANT 



157 



pired. In stagnant air, however, transpiration is retarded. It is 

 more expedient, therefore, to draw a current of air through the 

 vessel containing the plant. This air, however, must pass through 

 calcium chloride tubes or an apparatus containing some other 

 water-absorbing substance (Fig. 64). 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 in determining the transpiration of trees under natural 



Fig. 64. — Apparatus for the study of transpiration (redrawn after Freeman). 



conditions. Its drawback is that the transpiring organs are 

 enclosed 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 so-called cobalt-chloride method, in which the color changes 

 exhibited by filter paper impregnated with a solution of cobalt 

 chloride 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. The more 

 rapidly this change takes place, the higher the rate of transpira- 



