I908J 



HARVEY— PRAIRIE-GRASS FORMATION 



101 



less than appeared in the vernal aspect. It 

 should also be noted that six new ruderals 

 are added. Panicum capillare and Hordeum 

 jubatum not infrequently attain no small 

 significance. The latter frequently estab- 

 lishes in waste situations exclusive associa- 

 tions. Its permobile awned spikelets being 

 readily carried by wind or animal, its migra- 

 tion is rapid. In the autumnal aspect P. 

 capillare becomes detached at the surface, 

 and the large and profusely branched panicles 

 go tumbling over the prairie until lodgment 

 stops the distribution. 



In addition to the floral delineation of 

 the estival from the vernal aspect, it is 

 strongly set off by a change in the climato- 

 logical conditions. With the introduction of 

 hot dry days and prevailing southern winds 

 the estival aspect begins. The precipitation 

 is slightly higher than in the vernal aspect, 

 but its effect is strongly offset by an average 

 mean temperature some 5 higher than in 

 the vernal. The average hourly wind velocity 

 is 0.9 less and the relative humidity 3.4 

 lower, the lowest of any aspect. Coupled 

 with these conditions is the fact that the 

 relative evaporation is 1 . 6 higher, the greatest 

 evaporation of any aspect occurring in the 

 estival. 



The holard has a slight rise in the aspect 

 due to the heaviest mean daily rainfall of the 

 season. The chresard thus rises to n per 

 cent, on June 6 to fall to 8 . 9 per cent, on June 

 22 and 6.5 per cent, on July 7. On June 22 

 distinction of position seemed nearly elimi- 

 nated, the base, slope, and crest registering 

 a holard of 15 .9, 16. 1, and 16.6 per cent. 



