PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 119 



suit; namely, the JDroduction of a flora less mesophytic in com- 

 position than that found in the environments afforded by the 

 activities of the earlier stages of the stream. As suggested 

 before, the evergreen stage may persist until the development 

 of the valley. This is not so commonly true of the glaciated 

 area as of the unglaciated region farther east, and seems to be 

 associated with more favorable moisture conditions. For in- 

 stance, it is frequently retained upon the lower portions of the 

 slope, but gives way upon the higher portions to a deciduous 

 forest as the process of widening continues. The evergreens 

 here retain a structure closely comparable to that described for 

 the ravine stage, but during the transition to the upper zone it 

 becomes more open in formation and the undergrowth as well 

 as the trees indicate a lower mesophytism. 



Since the point at which the water table is lowest is near the 

 crest of the valley slopes, the specific forms encountered there 

 have a more pronounced xerophytism. This upper region may 

 be represented by Quercus alba, Quercus coccinea, Hamamelis 

 virginiana, Prunus virginiana, Castanea dentata, Epigea repens, 

 Pyriis coronaria, Polytrichum commune, Fragaria virginica, 

 Prenanihes alba, Pteris aquilina, Hieraciiim venosum, Potentilla 

 canadensis. Aster umbellatus. Aster cordifolius, Solidago ccesia, 

 Smilax hispida, Smilax herbacea, Vaccinium vacillans, Corylus 

 americana, Rubus allegheniensis, Verbascum Thapsus, Rubus 

 villosus, Gaultheria procumbens, Epipactus repens, Aspidium 

 noveboracense, Phegopteris hexagonoptera, etc. 



The upper zone is more xerophytic when the lower one is 

 deciduous. The dominant species is regularly Quercus alba; 

 and with it may appear Quercus coccinea, Castanea, Prunus 

 serotina, and Quercus rubra, or they may be absent. The lower 

 slopes when deciduous are represented by such forms as Tilia, 

 Fagus, Castanea, Acer, Quercus rubra, Betula lenta, Prunus 

 serotina, and an undergrowth of Carpinus, Hamamelis, Ostrya, 

 and numerous herbaceous forms, the composition varying with 

 the factors of the habitat. Even the smaller of the valleys have 

 developed flood plains upon which occur a few species parti- 







