88 



BOTANICAL GAZETTE 



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leaving toward the latter 

 part of the month, particu- 

 larly when this exposure is 

 forested. On March i6 on 

 a northwest exposure of a 

 prairie knoll the frost was 

 recorded at the following 

 depths: at base of slope 

 13 •07''™, at middle of slope 

 30.48'^'", at crest consid- 

 erably below 35'^'". Thus 

 the base of the exposure 

 recovers much more slowly 

 than the crest, a condition 

 holding equally true for all 

 exposures; hence vegetation 

 starts earlier upon the upper 

 slopes. However, the chre- 

 sard increases toward the 

 base of the slope. On 

 March 16 the slope in 

 question showed a chresard 

 at the crest of 7 . i per cent., 

 at the middle of 14. i per 

 cent., and at the base of 18. 2 

 per cent., the differences 

 being more pronounced and 

 the chresard greater than 

 in any subsequent aspect. 

 The physical conditions 

 of the period are shown in 

 the adjacent table. 



Prevernal floral aspect 



Floral activity begins 

 only in the first week of 

 April, and is characterized 



