204 BOTANICAL GAZETTE ° [SEPTEMBER 
santhus umbellatus and Chenopodium Fremontii disappear. When 
the shrub ultimately dies and breaks up, there is a very small ruderal 
place in which the succession is identical with that already described 
for denuded places. Grazing changes the thicket formation to a 
grass formation, and the more rapid the change the more nearly it 
approaches the ordinary ruderal successions. 
Due to drainage. 
Drainage is not an important factor, but it seems best to consider 
here those changes which result when an irrigated area is left without 
irrigation, a ditch abandoned, or the water of a stream turned aside 
causing the bed to become dry. 
Examples of the first case mentioned are not found excepting 
where, because of lack of attention, the water does not flow evenly 
over a meadow. The succession here is gradual, forms like Erigeron 
flagellaris, Vicia americana, and Astragalus hypoglottis taking the 
place of the more hydrophytic grasses and rushes. If the change of 
habitat is abrupt, most of the species die and a ruderal. succession 
follows. When the change is very gradual, the species mentioned 
above, together with Agropyron occidentale, take the place of the 
rushes. Bouteloua oligostachya often appears in this stage of the 
succession, but Muhlenbergia gracillima is one of the last to appeal 
In abandoned ditches a mixed formation is generally found. » 
this habitat the following species of the mesophytic bank formation 
may continue for a long time: Clematis ligusticijolia, Xanthium 
commune, Symphoricarpos occidentale, Stachys palustris, Erigeron 
flagellaris, and Vicia americana. The last two species are elements 
of the Bouteloua formation and are able to adapt themselves to Les 
change of habitat. Of the grasses of the Bouteloua formation, 
Bouteloua oligostachya and A gropyron occidentale are the first to enter. 
They appear much earlier than Muhlenbergia gracillima, and thrive 
even better in these transformed ditches than in the Bouteloua for- 
mation proper. 
There is only one example of the effect of a change in 
water course. This is not complete, but enough water 
taken from Camp Creek to cause it to be dry for a greater P 
year. The first and most noticeable effect has been the 
the natural 
has beet 
art of the 
death of 
