90 BOTANICAL GAZETTE [august 



hourly velocity of ii miles, which is not exceeded during the other 

 aspects. Precipitation is slight (y.Sy*^'") and falls on only about 

 ^^ per cent, of the days. The average cloudiness of the sky (4.7) 

 is low, and the sunshine (63 per cent.) is correspondingly higher, 

 resulting in a low light intensity (0.547). The saturation deficit 

 is here not at its maximum (36.3), but, augmented by the high 

 wind velocity, evaporation, which must necessarily serve as a rough 

 comparative index of transpiration, is rising in amount. While far 

 more favorable for vegetation, it becomes progressively more so, 

 the last half of the aspect presenting conditions in soil and air notice- 

 ably more congenial to growth. The chresard decreases steadily 

 during the aspect from 14.8 per cent, on April 17 to 12.2 per cent, 

 on April 25, and a marked difference in holard was evident at crest 

 (15.8 per cent.), slope (18.4 per cent.), and base (22.6 per cent.) 

 on April 18. The average chresard is 12.8 per cent. 



Species of the prevernal aspect 



Facies. — None. 



Principal species. — Antennaria campestris,t Carex pennsylvanica,t Peu- 

 cedanum nudicaule,t Pulsatilla hirsutissima.*t 



Secondary species. — Astragalus crassicarpus,t Peucedanum foeniculaceum,* 

 Draba micrantha, Ranunculus ovalis. 



* Not occurring in area proper but in vicinity. t Forming associations. 



The earliest flowering form is Pulsatilla and occurs copiously to 

 subcopiously and characteristically upon the upper slopes of the 

 prairie hills. It appears several days earlier upon the south to south- 

 west exposure, which holds equally true for the other prevernal 

 bloomers. The early warming-up of this exposure accounts for the 

 above phenological precocity. But the greatest abundance of these 

 prevernal forms occurs on the north to northeast exposure. Matura- 

 tion follows close upon anthesis, which likewise holds for all prevernal 

 flowering species. 



Pulsatilla is followed by the blooming, during the first week of 

 April, of Peucedanum foeniculaceum and P. nudicaule. The former 

 with its umbel of yellow flowers is of rare occurrence; but the latter 

 with its umbel of white flowers appears even copiously in restricted 

 plats, and in its distribution occurs mainly upon the upper xerophytic 



