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THE STUDY OF PLANT COMMUNITIES • Chapter V 



this automatic device is very convenient, particularly since it nor- 

 mally needs to be serviced but once a week. If, however, several 

 stations are to be maintained, the necessary instruments may not 

 be available, and the psychrometer is then the only solution. 



With readings of the psychrometer and the hygrothermograph, 

 the air temperature is also obtained, providing the means of calcu- 

 lating vapor pressure deficits with no extra determinations. A 



FlG. 32. A hygrothermograph, which automatically gives a continuous 

 record of relative humidity and temperature of the air.— Courtesy Friez In- 

 strument Division, Bendix Aviation Corporation. 



simple nomogram (Fig. 33) permits direct conversion from wet 

 and dry bulb temperatures to vapor pressure deficit. 



Evaporation and transpiration.— Measurement of transpiration 

 under natural conditions is often practically out of the question. 

 Although small plants may be potted or grown in cans and these 

 may be weighed at regular intervals to determine water loss, only 

 a limited number of plants can be used, and the labor involved can 



