156 TEXTBOOK OF PLANT PHYSIOLOGY 



plant. In greenhouses and hotbeds where the moist air lessens 

 transpiration, overheating and severe burning of the leaves has 

 been noted. Another physiological result of transportation has 

 been pointed out by L. Ivanov. According to this author, a 

 certain saturation deficit in the cells is necessary for the normal 

 function of a plant, especially for its flowering and fruiting. This 

 optimum degree of turgor is maintained by transpiration. All 

 these considerations compel one to believe that transpiration is 

 not an unnecessary evil, but that it is a very important physio- 

 logical process. 



One must not suppose, however, that in order to function nor- 

 mally a plant has to transpire as much water as is usually lost 

 under natural conditions. Observations and experiments have 

 shown that this amount greatly exceeds the necessary minimum. 

 Transpiration can often be considerably reduced not only without 

 injury, but even with advantage to the plant. It has been noted, 

 that the amount of salts absorbed by a plant is by no means pro- 

 portional to the amount of water transpired, and later it will be 

 shown that the drier the climate and the higher transpiration, the 

 more water is used for the production of a certain quantity of 

 organic matter. But no sharp distinction can be made between 

 useful and excessive loss of water. 



50. Methods of Studying Transpiration and Units of Measure- 

 ment. — The methods for the study of transpiration are com- 

 paratively simple. The}^ can be divided into three groups: 



1. The collection and determination of the evaporated water. 



2. The determination of changes in weight of the plant resulting 

 from loss of water during transpiration. 



3. The determination of the amount of water absorbed by the 

 plant to compensate for that lost by transpiration. 



In order to determine the amount of water vapor transpired, 

 the plant under investigation is placed in a container. The latter 

 is usually of glass, in order not to exclude light which plays an 

 important role in transpiration. The eliminated water is col- 

 lected by means of some hygroscopic substance. Under simple 

 arrangements a plant is put into an airtight container, together 

 with a cup containing some water-absorbing substance, such as cal- 

 cium chloride, phosphoric anhydride, or sulphuric acid. The 

 increase in weight of the cup shows the amount of water trans- 



