PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 113 



files, the common presence of water in the stream bed, and the 

 seepage of water from the walls, afford an abundance of avail- 

 able moisture not common to the adjoining regions. Additional 

 features conducive to mesophytism are found in the protection 

 afforded from the wind by the nature of the depression itself, 

 and the prevention of rapid evaporation by the development of a 

 shade along the margins. 



During the early stages of such a ravine three plant zones are 

 usually distinguishable. Lowermost is one which is character- 

 ized by the presence of the liverworts, Marchantia polymorpha, 

 Conocephalus conicus, and Pellia epiphylla. At this point 

 there is commonly found a deep shade, and plenty of moisture. 

 Immediately above the liverwort zone may occur one in which 

 the dominant forms are mosses. The difTerentiation of this 

 region sometimes does not occur, so that the mosses appear more 

 or less scattered throughout the lower zone and form a transition 

 directly to the crevice pioneers above. The crevice plants form 

 an assemblage represented by species which develop upon the 

 steep faces of the rock walls, sending their roots into the joint 

 planes and crevices where they depend upon the small amount 

 of humus and the moisture it retains, for their sustenance. 



When the rock faces are strongly exposed to the desiccating 

 influences of wind and sun, such pioneers are of necessity quite 

 xerophytic in structure, and restricted in their development. 

 In such situations occur Aquilegia canadensis, Poa compressa, 

 Verbascum Thapsus, Aster oblongifolius, Aster anomalus, Hedeoma 

 pulegioides, Melilotus alba, etc. However when this same position 

 is protected by shade there is a distinct mesophytic aspect pre- 

 sented, even by the crevice plants, and one finds Cystopteris 

 bulbifera, Arabis canadensis, Arabis laevigata, Solidago latifolia, 

 Aspidium marginatum, Aspidium spinulosum, Polypodium vid- 

 gare, Asplenium Trichomanes, and associated with them various 

 lichens including Endocarpon, Psychia, etc. 



Under the best of conditions the rock walls offer but uncertain 

 anchorage; so that small species are dominant. When, however, 

 the talus formed at the base of the rock walls has accumulated 

 in sufficient quantity, there is initiated, a condition more favor- 



THE PLANT WORLD, VOL. 20, NO. 4 



