62 



EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



The evaporation from the two types of atmometers, transpiration 

 from potometers and phytometers, relative humidity, absolute 

 humidity, saturation deficit of the air, vapor-pressure deficit, dew- 

 point depression, barometer pressure, wind velocity, air-temperature, 

 and sunlight are shown in figure 32. These data were taken from 

 series 33 started at 6 p. m. September 8, and ended 7 p. m. September 

 9, 1919. The evaporation from the two types of atmometers does 

 not agree, and a study of the maxima and minima of the curves shows 

 that the Livingston white cylinder is much more responsive to wind 



6.0 

 .8 

 .6 



.2 

 5.0 



.6 



.2 



4.O 



.8 



.4 



3.0 



.6 



.2 



2.0 



8 



.2 



1.0 



0.0 



r 



\ 



6 7 6 9 K> II MT. I23456789IOH NOON I 23456 



FIG. 33. Series 33, showing evaporation from white-cylinder porous cup in 

 cubic centimeters per hour (A), compared with product of vapor- 

 pressure deficit and wind velocity (B), calculated by Johnston's 

 method. 



and much less to sunlight than the blotting-paper 'type. Neither 

 type of atmometer showed any correlation whatever between its 

 water-loss and transpiration from potted plants or cut leaves. The 

 lack of any relationship in the curves representing water-loss from 

 potometers and phytometers shows the entire unreliability of cut 



