150 The Department of Plant Physiology [348 



The following paragraphs present the main contributions 

 thus far made by this laboratory. 



The water relation of plants. This relation involves the 

 plant responses that result from alterations in the supply and 

 in the consumption or loss of water. For temperate regions it 

 is the main conditional relation for plant growth in the open, 

 whether the plants be wild or cultivated. Most of the water 

 necessary for plant growth is given off into the air, by evap- 

 oration from the plant surfaces, almost as soon as the water is 

 taken up from the soil ; the amount of this liquid actually con- 

 sumed in forming the plant body is very small. Active plants 

 must be continuously impregnated with water, and the loss by 

 evaporation may be likened to a very considerable but unavoid- 

 able leak in a steam engine. The rate of water supply to the 

 plant must be great enough to counterbalance this loss by 

 evaporation, or the growth process will be retarded. To 

 understand the plant as a machine it is thus primarily neces- 

 sary to study the conditions that control the rates of water- 

 loss and of water-intake. 



One of the principal conditions that affect the rate of 

 water-loss by evaporation from plants is the evaporating 

 power of the air, and this condition needs to be studied quan- 

 titatively. To accomplish this the porous-cup atmometer has 

 been devised and perfected during the last decade, most of the 

 work having been done in connection with this laboratory. 

 The instrument, in various forms, is now widely employed by 

 students of plant growth. The readings obtained by its 

 means may be regarded as measures of the evaporating power 

 of the air and they may be obtained for any desirable time 

 intervals. 



Another condition that takes part in the control of the rate 

 of water loss from plants is the intensity of absorbed radiant 

 energy, received directly or indirectly from the sun. It is 

 therefore requisite to measure and integrate this intensity as 

 it affects the evaporation of water from moist surfaces such as 

 plant leaves. This is now possible by the use of the radio- 



