107 



edge of the bare sand and soon cover the whole area. When a col- 

 ony is once established it effectually checks any further movement 

 of the sand. Colonies in young blowouts are usually surrounded by 

 Panicuin pseudopubescens, which soon closes in and reoccupies the 

 space. A few plants of Hndsonia may persist for a time, but their 

 life is short. But few other species occur in the association. Those 

 observed are Cassia Chamaechrista, Buphorbia corollata, Polygonella 

 articulata, Cares innhellata, and Andropogon scoparius. The last 

 two are probably pioneers in the redevelopment of the Panicum 

 pseudopubescens or bunch-grass association. 



Successions from the Beowout Formation 



As the blowouts increase in size and become progressively deeper 

 the movement of the sand becomes less active, and in nearly everv' 

 case they finally become stabilized and revert to the bunch-grass 

 association. The few cases where the reversion does not take place 

 are of interest, since they illustrate a peculiar series of successions 

 and introduce some associations not found elsewhere in the sand 

 regions under discussion. 



The general vegetational and environmental conditions of one 

 type of these successions have been given by Gleason {igoy: 167-169) 

 from observations on a few blowouts in the Havana area. The prin- 

 cipal changes in the ecological conditions are in the direction of pro- 

 tection against wind, resulting in a stable substratum, and a larger 

 supply of water, depending upon the depth of the blowout. These 

 conditions, rather unusual for the sand areas, pemiit the develop- 

 ment of a more mesophytic vegetation. This is generally simply a 

 more luxuriant growth of bunch-grass composed of the usual species. 

 More rarely entirely different species appear, especially if there are 

 groves or cultivated grounds near. 



THE BLOWOUT THICKET ASSOCIATION 



A simple case, leading to what may be named the blowout thicket 

 association, is shown in a blowout near Oquawka. The blowout is 

 nearly stabilized, with the usual gTowth of Lespcdcza and Oenothera, 

 l)ut in the very bottom two locust trees (Robina Pseudo-Acacia) 

 have become established. Under them are plants of Oxybaphus 

 uycfagineus, Lactuca canadensis, and an unknown grass, of a species 

 not occurring in the surrounding bunch-grass. In another blowout 

 several young plants of Populus deltoides are growing (PI. XI, 

 Fig. 2). In a similar deep blowout in the Hanover area is a large tree 



