276 BOTANICAL GAZETTE {ocroBER 
habitat, and the same may be stated regarding the rarer crustose 
members of the society, such as the Lecanoras, the Placodiums, the 
Acarosporas, the Verrucarias, the Rinodina, and the Lecidea. Veta 
few of these crustose forms may have come from the trees some 1 50™ 
away, some of the trees no doubt being old enough to bear these lichens 
at the time when the riprap was built or shortly after. These species 
are the first and the last Placodium of the list, the Rinodina, and the 
Lecidea. As to the C adonias, there are not any conspicuous 
Cladonia-bearing substrata within a mile, except the loess and Kan- 
san drift of railroad cuts, on which the first Cladonia of the list is 
very common. However, all of the Cladonias except C. /urcal 
have been found within a few miles of the society, and in all probability 
arrived from various places not faraway. The northern Stereocaulon, 
not known elsewhere in Iowa, doubtless originated in the society 
through fragments of thalli brought to the spot on railroad cars, and 
very probably on ties or telegraph poles. It is not so common 4 
member of the society as a number of the similarly constructed 
Cladonias, and is usually found on the more solid rocks, the more 
disintegrated spots having been previously occupied by other lichens 
or mosses, and the rocks on which it occurs not having had time 1 
disintegrate conspicuously since its advent into the society. The 
fact that the Stereocaulon (fig. 4) is almost uniformly sterile = 
indicate either that it has only recently gained access to the society; 
or that it is poorly adapted to the climatic conditions of the AGB 
Also the position of this species on the northward exposure is ve y 
of note. The Ramalina, the Physcia, and the first Parmelia doubtless 
came as fragments from trees from one to several miles away, pir 
these rare members of the society may be thought to have come sy 
fragments blown from the trees some 1 50™ away, since the trees 
became large enough to bear these species. + te by 10 
Before leaving this subject it must be pointed out that Its : ie 
means a matter of chance what species will reach such ” ae is 
lichen society and survive. But the matter is determined . sub- 
instance largely by adaptability and early access before oe 
Stratum is occupied. As has been noted elsewhere, 
where rock-lichen societies are adjacent to tree-lichen ag 
lichens of the trees, though scarcely so well adapted 1 me 
ties, the 
