150 The Department of Plant Physwology [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 1s 
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 tate 
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- 
