1910.] NATURAL SCIENCES OF PHILADELPHIA. 547 



mesophytic forest the change, sometimes abrupt, is usually gradual. 

 Of the trees Quercus stellata, Q. marylandica and Pin us rigida do not 

 pass beyond the limits of the Serpentine, and over any extended view 

 may be taken to indicate its position. But Sassafras sassafras, Acer 

 rubrum and Nyssa sylvatica pass beyond, Quercus alba, Castanea 

 dentata and Quercus velutina come successively into prominence, 

 though not till the barrens are left does Liriodendron tulipifera become 

 a predominant forest tree. 



The flora of one other plant-association was noted, though only 

 partially. In the rich swamps at the base of the hills, derived mainly 

 from the washing of the barren soil, there is a varied and abundant 

 vegetation. Mostly this is the normal swamp-flora of the surrounding 

 district, but a few species seem constantly present here which are 

 much less frequently seen elsewhere. Such are Osmunda spectabilis, 

 Spiraea latifolia, Sanguisorba canadensis and Heliopsis helianthoides. 



The local variations of this flora are considerable. The isolation 

 of the different areas, the meagre size of some, the nearness of some 

 to one or another barren of a different type, all tend to modify the 

 flora. Yet the above characteristic species remain remarkably 

 constant. 



Because of its large continuous area the State-line Barrens probably 

 better illustrate the complete flora of the type than any other. Pin us 

 rigida, Sporobolus heterolepis, Ascyrum hypericoides, Pieris mariana, 

 Galium boreale, and Eupatorium pubescens were found here only, 

 while Quercus marylandica , Cassia chamcecrista , and Acerates riridiflora 

 present here are very local in the Chester group. 



The distance separating the two groups of barrens makes the 

 distinction between them the most important variation in the flora. 

 It is impossible from the meagre information as to the Nottingham 

 flora to speak with certainty of what plants present in the Chester 

 Group are absent in the State-line group. The following species of 

 the former have not been noted in the latter, Atheropogon curtijx n- 

 dulus, Deschampsia ccespitosa, Cyperus aristatus, Carex bicknellii and 

 Scutellaria parvula ambigua. 



In the Chester Group the variation is mostly in accessory non- 

 characteristic species. The only general division which can be made 

 is between the relatively deeper-soiled barrens of Delaware County 

 and the bare rocky ridges frequent in southeastern Chester County. 

 On the latter only occur Atheropogon curtipendulus, Talinum tereti- 

 folium and Arenaria stricta, on the former Quercus marylandica and 

 likely Lilium philadelphicum. 



