360 BOTANICAL GAZETTE [NOVEMBER 
of the vegetation of the general area. Several or numerous species of certain 
genera, again, may be of similar habitat distribution, so that a species of one 
locality may be replaced in the same habitat in another locality by a second 
species of that genus. Care has been taken to select species for the floristic 
lists which are as representative over a considerable area as may be. In the 
southern foothills, as in the plains of southern Colorado, many more plants of 
southwestern derivation enter into the flora, particularly in the primitive 
grassland and mixed grassland assemblages. Many are true desert plants; a 
few cacti and chenopods and many composites fall in this group. That this 
geographic variation from north to south is ecological as well as floristic should 
be apparent. It is paralleled by a similar altitudinal variation in composition, 
from the montane zone to the plains. 
LICHEN ASSOCIATIONS 
TUuCKERMAN (23); W1xt1AMs (27), lichens in the Black Hills; HERRE (7), 
lichens of Mount Rose, Nevada; SHANTz (22, p. 187).—Fig. 2. 
A considerable proportion of the area of the foothills is exposed 
rock, and its vegetation, except in crevices, is mostly composed of 
lichens. The study of lichen vegetation has usually been left to 
the specialists in that group of plants, since they are so poorly © 
known to most other botanists. For this reason brief notes on 
external appearance, as to color and vegetative form, are given 
with the speciesnames. The writer is indebted to R. HEBER Howe, 
Jr.,2 who has made the species determinations and examined this 
part of the manuscript (fig. 2). 
_ Dry surfaces, much exposed to sun and wind, occupy most of the 
area of bare rock in the foothills. They slope considerably, so that 
run-off is rapid and absorption minimal. The first lichens to 
invade dry rock, forming primitive xerophytic stages of lichen 
growth, are fine-grained crustose forms, notably the black-gray 
Rinodina radiata Tuck. (?=R. thysanota Tuck.) and an indetermi- 
nable lead-gray species which is especially characteristic. Theseare 
soon followed, but not displaced, by lichens of an intermediate 
stage, mostly coarser crustose forms; the gray-green Rinodina 
oreina (Ach.) Wain. is a character species both locally and geogr: aph- 
ically abundant. The established stage on dry rock is marked by 
the gray-green small-foliose Lecanora rubina (Vill.) Wain. and the 
2 Some of the crustose species were determined for Dr. Howe by H. E. Hasse; 
the Stereocaulon by L. W. Ruwwpte. 
€ 
