AND TRANSPIRATION OF TREES 107 



3. self-registering apparatus made especially to determine 

 amount of transpiration. 



4. calculating the amount of water absorbed by plants and 

 considering this the equivalent of the amount transpired. 



b. Chemical methods. 



1. determination of increase of water absorbed by such 

 substances as calcium chloride and concentrated sulphuric 



acid. 



2. color change in paper impregnated with palladium chloride 

 and cobalt chloride. 



The method used was that of direct weighing, which, according to 

 Burgerstein is the most reliable. 



Water Content 



For determining water content the following methods are offered: 



a. Physical methods. Any physical property which changes with 

 the water content offers a possible basis for experimentation. 



1. electrical properties, for example, electrical resistance- 



2. thermal properties, such as thermal conductivity and 

 capacity for heat. 



3. mechanical properties, such as weight and tensile strength. 



4. optical properties, such as coefficient of absorption and 

 reflection of radiation. It is known, for instance, that 

 water very strongly absorbs infra-red radiation. The 

 amount of absorption would probably indicate the amount 

 of water present in the leaf. 



b. Chemical methods. 



1. qualitative test for water. This test sensitive to less 

 than 0.1 mg. is easily and quickly made by bringing the 

 substance to be tested into contact with calcium carbide 

 in the presence of an acetylene solvent. This is then 

 decanted or distilled into an ammoniacal solution of 

 cuprous chloride.^ 



The method selected was the one which had been satisfactorily used 

 in preliminary experiments carried on in the summer of 1914 at the 

 University of Michigan Biological Station. It was based upon the 

 mechanical property of weight. 



♦Journal Franklin Institute, March, 1916. p. 408. 



