WATER RELATIONS OF PLANTS 295 



is 4 to 6°C. higher than that of turgid ones, a difference that 

 sometimes under conditions of a high temperature may prove 

 fatal to the plant. In greenhouses and hotbeds, where the 

 moist air lessens transpiration, overheating and severe burning 

 of the leaves have been noted. Another physiological result of 

 transpiration has been pointed out by L. A. Ivanov. According 

 to this author, a certain saturation of the plant is required, 

 especially for its blooming and fruiting. This optimum degree 

 of turgor is maintained by transpiration. All this evidence 

 compels one to beheve that transpiration is not an unnecessary 

 evil, but that it is a very important physiological process. 



One must not suppose, however, that in order to function 

 normally 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 mini- 

 mum. Transpiration can often be considerably reduced not 

 only without injury, but even with advantage, to the plant. It 

 has been noted (Art. 58) that the amount of salts absorbed by a 

 plant is by no means proportional to the amount of water trans- 

 pired, being to a considerable degree an independent process, and 

 following its specific laws. Later it will be shown that the drier 

 the climate and the higher the transpiration, the more water is 

 used for the production of a certain quantity of organic matter. 

 But it is not possible to make a sharp distinction between useful 

 and excessive loss of water. Transpiration therefore represents 

 a unified physiological process. 



The methods for the study of transpiration are comparatively 

 simple. They 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. 



To determine the amount of water vapor transpired, the plant 

 under investigation is placed in a tight container, made of glass 

 in order not to exclude light, which plays an important role in 

 transpiration. The water transpired is collected by means of 

 some hygroscopic substance, such as dried calcium chloride. The 

 increase in weight of the drying agent shows the amount of water 



