1912] GLEASON—PRAIRIE GROVE 43 
‘fined to the ridges, and measurement with a Locke level or alidade 
not, only confirms this idea, but shows that the lower and outer 
margin of the forest follows a definite contour line, so that the 
forest margins on all sides of a slough lie at exactly the same level. 
This contour line is approximately two feet above the maximum 
level of standing water in the slough. The forest margins on 
opposite sides of a ridge will be at the same level if the sloughs are 
connected, but otherwise they may differ slightly in elevation. 
Within the forest, the various species of trees, with one exception, 
show no relation to the elevation, but are equally abundant on 
the sides and top of the ridges. The exception is formed by Gledit- 
sia triacanthos, which regularly chooses the lower outer margin, 
nearest the sloughs and in the wettest soil. This feature has been 
observed also in other isolated groves in the county. The shrubs, 
if present at all, seem to prefer the margin of the forest, but in this 
case the controlling factor is probably light instead of water. 
Some very definite and sharply marked zones of vegetation occur 
between the forest and the center of the pastured sloughs. The 
first is a zone of blue grass which extends out beyond the forest 
margin to a distance depending on the steepness of the slope, and 
down to the former level of maximum high water in the slough. 
Because of the continual pasturage it contains few secondary 
species. The second zone is composed of a very dense and rank 
growth of Ambrosia artemisiifolia, with scattered plants of Ver- 
nonia fasciculata, Eupatorium serotinum, Bidens cernua, B. aristosa, 
Polygonum acre, and Verbena hastata. It is probable that the 
dominance of Ambrosia, and the relative infrequency of other 
species, is caused by pasturing, in which certain species are selected 
for food, and others with rank smell or taste are avoided by the 
cattle and horses. This idea is substantiated by the different com- 
position of the same zone around a slough in an unpastured field. 
In this tangle of weeds may be found a few scattered plants of 
some typical swamp hydrophytes, such as Jris versicolor, Mimulus 
ringens, Scirpus fluviatilis, and Penthorum sedoides. ‘These are 
naturally most abundant in the deepest part of the slough, but show 
no present relation to contour lines (fig. 2). 
Around those sloughs which contain permanent ponds a better 
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