International Excursion in America. 
33 
succession from fallowing back to short-grass takes from twenty to 
fifty years to complete. Parallel with this succession following 
ploughing a natural succession may be traced where areas of 
disintegrating rock occur. Here a lichen association takes the 
place of the weed association of fallow arable, and gradually passes 
over into the Artemisia-Gutierrezia association, which in this case is 
richer in associated species. The remainder of the succession is 
essentially the same, though in some cases the Artemisia-Gutierrezia 
association may maintain itself against the short-grass invasion 
for a long time. Locally, particularly on “ adobe” soils, the wheat- 
grass association, dominated by Agropyrum Smithii, occurs. This 
association is much more widespread further to the north, for 
instance in the Dakotas. 
On the areas of sandy soil there is, as has been said, quite a 
distinct vegetation which is much more luxuriant than the short- 
grass vegetation. This is owing to the fact that the rain-water 
percolates through sand much more quickly, and the air-dry 
surface-layer acts as a dry mulch, checking evaporation from the 
deeper layers. The plants are deeper-rooting and have a larger 
available water-supply. The typical climax vegetation is dominated 
by the tall bunch grass ( Andropogon scoparins) —a dry prairie-grass 
which has been already noticed as occurring on the Lake Michigan 
sand-dunes. Associated with the bunch-grass are such species as 
Bouteloua liirsuta , Andropogon Hallii, Calamovilfalongifolia,Panicum 
virgatum , Psoralen tenuiflora. Eriogonum annuum is scattered 
thickly through the association in places. 
The sand-hill succession is started by a “ blow-out ” which 
provides an area of bare sand. This is colonised by Muhlenbergia 
pungens , Redjieldia fiexuosa, Psoralen lanceolata, and Petalostemum 
villosum, forming an open association ; and is followed by the sand¬ 
hills mixed association (Shantz), a very variable “intermediate” 
association characterised by a large number of species, including 
Yucca glauca, Bouteloua curtipendula, Andropogon scoparius , Arte - 
misia canadensis, Ipotncea leptophylla, Thelesperma gracile, Sorghastrum 
nutans , Chrysopsis villosa, Amorpha canescens, etc.: and this in turn 
by the bunch grass association. 
If the bunch grass association is broken by ploughing and left 
alone ruderals at first colonise the ground and are followed by the 
sand-hills mixed association : eventually the bunch grass returns. 
If on the other hand it is burned over or cut, and then grazed, the 
surface-layers of the soil being hardened, short-grasses come in, 
