Studies in Prairie and Woodland 25 



were recorded for a considerably shorter period of time in the 

 sumac thicket than in the prairie. Even under these brief periods 

 of greatest stress some of the more typical mesophytes in the 

 thicket were noticeably wilted. The duration of the periods of 

 stress was so short, however, that no permanent injury resulted 

 to the vegetation of the thicket. As Weaver and Thiel have al- 

 ready pointed out it is the humidity-time relation that is critical 

 here, i.e., the minimum humidity multiplied by the time of dura- 

 tion of that minimum. This renders the true physiological cri- 

 terion for the air-drought of a plant habitat. 



The same relations also hold for the temperature conditions in 

 our series of stations. The mean weekly temperatures computed 

 upon the weekly maximum and minimum temperatures are un- 

 safe in yielding a true criterion of temperature as an ecological 

 factor. The maximum temperature for the shrub community 

 may be, and actually was, sometimes slightly above that for the 

 prairie station, but the duration of the maximum temperatures in 

 the shrub was nearly uniformly less than that in the prairie. 

 Frequently, even when there was considerable difference in the 

 maximum temperatures in the prairie and shrub communities, the 

 prairie maximum was sustained for a considerably longer period. 



A point of considerable interest comes from a comparative ex- 

 amination of all of the hygrothermograph ■ records for the three 

 stations for the entire season. This may not be, probably is not, 

 significant ecologically, and it may be due to poor technique in 

 the operation of the instruments, but we speak of it at this point. 

 The reference is to the fact that our hygrothermographs fre- 

 quently recorded the highest maximum humidity for certain days 

 at the station in the high prairie and the lowest maximum in the 

 woodland. Also during some periods of several days duration 

 the night air of the prairie was recorded as containing several 

 per cent, more moisture than was present in the shrub or in the 

 woodland; or the air in the shrub may be more moist at night 

 than that of the prairie, but very much more moist than that of 

 the woods. Rather frequently also the minimum temperature for 

 corresponding days was lower in the prairie than in the shrub com- 

 munity. These points are shown in the records for several days. 



31 



