60 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



Unfortunately, none of these agree very closely with the water-loss 

 from a plant or even with each other, as Briggs and Shantz (1917) 

 have shown. This is due to the fact that they are not affected in 

 the same manner or to the same extent by each of the factors con- 

 cerned. In consequence, attempts to calibrate these with one another 

 have not met with much success. Even in the case of free water- 

 surfaces it has not been possible to correlate the evaporation of one 

 with that of another, except within certain limits (Thomas and Fer- 

 guson, 1917). Since the porous-cup atmometer is standardized and 

 in widespread use, it is useful as a measure of relative conditions in 

 various habitats, but can not be used in studies of hourly water- 

 loss from the plant. 



Until an evaporimeter is devised the water-loss of which will 

 correlate accurately with the transpiration of a plant having open 

 stomata, it is advisable to measure each factor by itself. It will then 

 be possible to determine the effect of changes in one factor upon the 

 plant under various constant combinations of other factors. It is 

 also possible to arrive at some conclusion concerning the effect of 

 any one factor when all are measured at hourly intervals during a 

 24-hour period, or oftener when necessary, and compared with the 

 plant responses. This, in short, was the manner of carrying out the 

 later and more elaborate series. The factors concerned in evapora- 

 tion measured were the temperature of the air, soil-surface, and 

 soil at a depth of 1 dm., the wind velocity, wet-bulb depression, 

 barometric pressure, evaporation from a white-cylinder porous-cup 

 atmometer, from a special blotting-paper evaporimeter standardized 

 in darkness or dim light against a free water-surface, and from a free 

 water-surface in a flat pan blackened inside. The atmometers were 

 standardized by the maker and given the factors 0.68 and 0.73. 

 The readings from these were reduced to unity. 



The water-loss from the free water-surface was determined by 

 weighing. The pan was a large Petri dish painted black on the inside, 

 and was filled to a depth of 6 mm., which brought the surface to 9mm. 

 of the edge. This level was not permitted to drop below 2 mm. from 

 its original height, when it was replenished from an inverted flask 

 with burette tips and stopcock, which permitted water to be added 

 drop by drop to the pan until the exact original weight was reached. 

 The atmometers were not weighed, but were calibrated for changes 

 of temperature, a thermometer running through the cork into the 

 reservoir bottle. The water-level of each was brought to the original 

 level marked on the open tube once an hour, the water being added 

 from burettes. This method was checked carefully by weighing and 

 found accurate to 0.02 c. c. The blotting-paper first used was green 

 in color, matching the shade of alfalfa leaves as closely as possible. 

 Later it was found that a standard blue-green paper had practically 

 the same rate of absorption of radiant energy, and this was used. 



