1904] FINK—A LICHEN SOCIETY 279 
the southward exposure, Buellia myriocarpa has in part replaced 
the Biatora. The better adaptation of the Buellia appears in the 
greater tendency toward disappearance of the thallus and the better 
development of such protective structures as exciple, hypothecium, and 
paraphyses. The Biatora is further replaced by the Acarosporas. 
This is due to two causes. Primarily, the disintegration being 
slower on the drier southward-exposed riprap, the more highly 
developed thalli of the Acarosporas have time for development and 
the production of apothecia on the more permanent rock surfaces, 
and are consequently frequent and often fruited. Secondarily, with 
their well-developed upper cortices the Acarosporas are even more 
able to endure extreme xerophytic conditions than is the Biatora. 
Also Lecanora muralis, once seen on the north side, was noted several 
"mes on the south side, where thalli with well developed cortices 
are better adapted to the conditions. Response to conditions is 
beautifully shown in that while Cladonias are common enough on 
the moister and more shaded north side, only a single specimen could 
be found on the south side. Also the total absence of the Stereo- 
caulon from the southward exposure is quite significant, especially 
sca of recall that it occurs commonly in quite exposed places a 
a. away on the north side. Finally, the response of lichens 
tons of environment, as shown here, is quite remarkable 
and fully justifies the detailed attention to a limited area. 
B. The Society of the sandstone ledges near Boone, Towa. 
— of lichen structure found in the societies of other sand- 
Megs studied in Iowa and Minnesota have been quite 
have bie the € places studied have been ledges along streams, which 
the rocks a. part either carried away the loose sand as fast as 
Which few ie sintegrated, or frequently subjected it to inundation, 
ose or fruticose lichens will endure. In such societies, 
faces rk a Cladonias found have been those growing on the 
of erosion “Se Bes, while in the present society some of the products 
row. Th, “"* Temained to make a soil in which these lichens could 
* Study of a ledge along a stream was recently carried out 
8 tely at the “Ledges” (fig.5) in Boone county, Iowa, 
More com 
th wri 
ter has done elsewhere; and while the study of the col- 
an the 
