IOWA ACADEMY OF SCIKNCES 
61 
aoutli slope showed no evidence of having ever been tree-covered, here 
again the direct rays of the sun having doubtless made it impossible 
for seedling trees to survive (Fig. 2). 
Some of the hills have been pastured, and the pasturing has very 
largely killed out the earth-lichen societies while the rock-inhabiting 
species have for most part persisted. In portions of the hill-areas not 
pastured, there is evidence of a struggle for possession of the dry soil 
constantly going on between the lichens and the seed plants, and sta- 
tistical studies extended over a long series of years would bring some 
exact results, provided a considerable and unmolested area could be 
studied. Along a portion of the summit of the esker, we found an 
almost pure formation of Pea compressa, with an assemblage of seed- 
plants among which Poa pratensis was conspicuous extending down the 
slope on either side, while the more xerophytic Poa compressa had 
almost complete possession of the limy soil at the summit in this par- 
ticular spot, having even largely supplanted the lichens which flourished 
on the calcareous soil a few rods away. Of course the lichens of the 
pebbles and limy boulders were persisting successfully, even where Poa 
compressa was thickest. 
The conditions at Bald Mound and at Johnson’s Mound were very 
much the same, the former, as the name indicates, being bare of trees, 
v/hile the greater portion of the latter is well wooded (Fig. 3). The 
lichen societies of all of these hills are simdlar to those which the pres- 
ent Vv^riter has described from various localities in Iowa and Minnesota, 
viz. Lecanora calcarea contorta formations of calcareous pebbles and 
Biatora decipiens formations of calcareous earth. As in the other areas 
studied in Iowa, and Minnesota, the two lichen formations were found 
occupying the same areas, those of the soil also as usual competing 
with other plants, mainly spermaphytes. The lichens of the pebbles and 
boulders show xerophytic adaptation in scantiness of thallus, and often 
in hypolithic position and endocarpic fruit conditions and certain minute 
structural conditions v/hich have been considered by the present writer 
in extended papers on the lichen flora of Minnesota and need not be 
repeated in this brief discussion. The lichens of the earth are some- 
what larger and have in general good cortices, but are still small lich- 
ens, single thalli seldom exceeding four or five millimeters in diameter. 
Their main structural adaptation lies in the small size and the posses- 
sion of a good protective cortex. At Johnson’s Mound, two specimens 
of a small GolJema were found, and in all of the regions a thin, flat, 
blue-green Nostoc colony was common, lying loose on the ground, even 
in the most xerophytic places at hill tops in pastures. This Nostoc, 
a form of N. commune Vauch.. has never been seen by the present 
writer in any of the similar habitats studied elsewhere, and seems 
quite out of harmony with surroundings, Nostocs usually growing in 
moist places, while a form having a thin, flat colony would be the last 
one to look for in a dry place. A physiological adaptation of several, 
or possibly all, of the lichens recorded below is their power of build- 
ing up fats or oils from the lime of the rocks and the soil. 
