PLANT ASSOCIATIONS OF WESTERN PENNSYLVANIA 111 



perfoliatum, Gentiana Andrewsii, Gentiana crinita (rare), Asclepias 

 incarnata, Hypericum virginicum, Lobelia syphilitica, Campanula 

 aparinoides, Typha latifolia, Acorus calamus, Gerardia paupercula, 

 and Ranuncidus ahortivus. 



The outer portion of the fen is infringed upon by a belt of 

 shrubs which is usually narrow. It is composed of Salix glau- 

 cophylla, Salix longifolia, Cornus stolonifera, Cephalanthus occi- 

 dentalis, etc. The zone of shrubs may be very extensive when 

 the area is low, but commonly forms a mere fringing belt about 

 the edge of the forest which succeeds it, if the slope from the 

 fenland is appreciable. The latter is commonly the case, so 

 that the forest is comparatively well drained and is distinctively 

 of the mesophytic type. This is, in most respects, similar to 

 the floodplain forest which will be discussed later. 



The Forest 



The forest margin may contain such outposts as Crataegus, 

 Prunus virginiana, Pyrus coronaria, Populus tremuloides, and 

 Populus grandidentata. Its inner structure in brief is as fol- 

 lows. The most important tree members are Ulmus americana, 

 Acer ruhrum Tilia americana, Juglans cinerea, Carpinus caro- 

 liniana Quercus alba, Quercus rubra, Carya ovata. Magnolia 

 acuminata, Ostrya virginiana, etc. 



The herbaceous representation is dependent upon the density 

 of the forest, but the number of species is commonly large. The 

 vernal flora developed here is very rich and includes among it's 

 members Erythronium americanum, Phlox divaricata, Viola 

 cucullata, Viola pallens, Viola pubescens, Cypripedium parvi- 

 florum. Geranium maculatum. Podophyllum peltatum, Claytonia 

 virginiana, Anemone quinquefolia, Anemone pennsylvanica, Tril- 

 lium grandifiorum , Trillium erectum, Streptopus amplexif alius, 

 Thalictrum dioicum, Smilacina racemosa, Hepatica acutiloba, etc. 



THE RAVIXE-VALLEY SERIES 



Most ravines are formed upon slopes with a comparatively 

 steep gradient, where the cutting of a channel is facilitated both 

 because of the inclination, and the great amount of sediment 



