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It has been comparatively easy to recognize the larger plant societies in the 
Pittsburgh district by relating the vegetation to the physiographic development 
of the habitat. However, it has not been so easy to definitely designate eco- 
logically the habitats and societies of the mosses of the district. This is evidently 
largely due to the dependence of the mosses more or less directly upon larger 
plants; as in bark mosses and mosses growing on decaying logs. Not only will 
mosses be influenced by the general ecological features of the habitat, such as 
heat, light, moisture, exposure, nature of the soil, etc. — such factors as are all- 
important in determining the major plant societies — but the very existence of 
the mosses may be determined by the absence or presence of certain other plants 
upon whose live trunks they may grow, or upon whose decaying wood they are 
restricted, or upon whose shade-giving moisture-conserving habits they are 
dependent. It would thus appear that the sum total of ecological factors which 
may determine the absence or presence of a moss may be even greater than is 
the case with most of the plants of larger size in the same habitat. On the other 
hand, many mosses are not so restricted as are many of the larger seed plants 
and ferns because of their independence of direct connection with the soil; Stere- 
odon imponens being a good example of such a moss. 
In the following classification of the mosses of the Pittsburgh district the 
attempt has been made to group them under the ecological societies determined 
for the larger seed-plants and ferns by the physiographic development of the 
habitat . 1 Those species of mosses particularly characteristic of the habitat or 
particularly abundant in the habitat have been indicated by an asterisk. It is 
fully realized that the classification as proposed is not yet perfect, and further 
investigations may lead to radical changes. 
I. Quercus alba Association. White Oak Forest. 
This forest covers the rounded tops and upper slopes of the hills everywhere 
in the district, the soil being usually rather thin, infertile, and often quite acid, 
so that the area has not been so largely cleared for farming purposes as have 
some of the lower slopes and valleys. In this forest occur several of the huckle- 
berries ( Vaccinium ), the trailing arbutus ( Epigaea ), etc., while the following 
mosses are common there: 
A. On the ground. 
Ceratodon purpurascens (Hedw.) Jenn. 
Dicranella heteromalla (Dill.) Schimp. 
Leptobryum pyriforme (L.) Wils. 
Leucobryum glaucum (L.) Schimp.* 
Poly trichum Ohioens - R. & C. 
Webera nutans (Schreb.) Hedw. ( Pohlia nutans Lindb.) 
B. On base of White Oak (and Chestnut Oak). 
Anomodon attenuatus (Schreb.) Hueben. 
Anomodon minor (Hedw.) Fuernr. (A. obtusifolius Bryol. Eur.) 
Jennings, O. E. “A Note on the Ecological Formations of Pittsburgh and Vicinity.'' 
Science, N. S. 21 : 828-830. May 22, 1908. 
