(327) 



obliged to select for determination certain conditions which 

 would serve as indicators of others. The two most important 

 sets of conditions are moisture and temperature. As an index of 

 moisture we measured evaporation, because in turn evaporation 

 is influenced by factors which influence transpiration, or water 

 loss, by the plant, such as temperature, relative humidity, and 

 wind movement. Of course no instrument which has been or 

 probably ever could be devised would respond to evaporation in 

 the same way that the plant does. On account of the living 

 protoplasm which it contains, the plant, when exposed to unfavor- 

 able external factors, sets up internal resistances which we are 

 unable to imitate in our instruments. Evaporation therefore 

 tells us the conditions to which the plant is subjected, not the 

 rate of water loss or of other life processes of the plant. The 

 instruments selected for evaporation were Livingston porous cup 

 atmometers, the evaporating surface being a white sphere of 

 porous porcelain which draws distilled water up from a reservoir 

 bottle to which it is attached by a glass tube. A mercury seal in 

 the tube permits the upward passage of the water, but prevents 

 its downward flow, and thus keeps out rain. The instruments 

 are read by measuring the quantity of water required to refill the 

 reservoir bottle. This gives the total amount of water evapo- 

 rated, in cubic centimeters, since the last reading, whenever that 

 may have been. In this case readings were made once a week. 

 The instruments are standardized, so that by the application of a 

 correction coefficient the results are comparable with those from 

 similar instruments anywhere else. It might be added that the 

 instrument is widely used in studying plant environments 

 throughout the country, so that the records taken in this investi- 

 gation can be compared with those for other vegetation in other 

 regions. 



In addition to the white spheres, black spheres were used. 

 The black absorbs sunlight to a certain extent, so that the 

 diff'erence between the readings of the black and white spheres 

 gives a rough measure of sunlight, a very important environ- 

 mental factor. These sunlight, or solar radiation readings were, 

 however, not entirely satisfactory. Under a forest canopy a 

 fleck of sunlight may strike the white sphere while the black 

 is in the shade, and it is impossible to obtain uniform light for 

 both black and white instruments. Hence the white sometimes 

 gives higher readings than the black, an obvious contradiction. 



