85 



is held I)y plants of the bunch-grass growth which more often adjoins 

 the blowout (PI. Ill, Fig. i). In an active blowout the sand which 

 falls to the bottom of the slope is removed by the wind, and the wind- 

 ward slope thus works its way back, enlarging the blowout in the 

 direction of the wind. 



Xo annual plants are found on the windward slope, as there is 

 no chance for burial of seeds. The plants of the association are 

 always in the form of relics from the surrounding associations, usu- 

 ally bunch-grasses. In the Havana region, Sporoholus cryptandnis 

 (Torr. ) Gray is often found on the windward slope. Most grasses 

 are killed as soon as they become dislodged from the top of the 

 sand-bluff, but this species seems to grow nearly as well on the steep 

 slope as in more stable sand in competition with other species. A 

 few other grasses and a few perennials are also occasionally found 

 on the windward slope. The tufts of grass are always very few and 

 scattered, the whole face of the slope being sometimes altogether 

 bare. The grasses sometimes become a part of the basin association, 

 after reaching the bottom of the slope, but more often they are un- 

 dermined and blown away. 



The animals of the windward slope association are veiy few in- 

 deed, and the forms that are found are accidental species. Burrow- 

 ing animals are excluded, because of the instability of the sliding 

 sand. iMany animals of other associations, however, cross the wind- 

 ward-slope areas, and practically all the animals of the bare sand are 

 seen there, particularly Cicindela lepida, Stachyocnemis, mutlUids, 

 and spiders. The windward slope is very closely related to the basin 

 association ; the same process is involved in each association. The 

 removal of sand and the biotic conditions in each area approach 

 those of the desert. 



THE BLOWSAND ASSOCIATION 



This association occupies the lee slope of the blowout, which is 

 of gentle gradient. The sand movement is merely a drifting in the 

 direction of the wind, with but little removal or deposition of the 

 sand. The constant shifting of the surface layer may allow the 

 burial and germination of a few of the countless seeds which are 

 blown across the blowouts, and consequently a large number of an- 

 nual plants are regularly found in this association. The conditions 

 of burial vary, however, from year to year, so that the growth of 

 annual plants is not always present. The plant species found on the 

 lee slope of a nearly typical blow'out at the Devil's Hole are as fol- 

 lows : Cassia cJiaiiiacchrista and Anibrosia psilostachya are the two 



