468 Part IV. Chapter 2. 
are character-plants, although occurring elsewhere in the region, are here 
grouped together in such a manner and in such number, as to sharply delimit 
these areas from the surrounding country. The dominant trees on the serpen- 
tine barrens are Ouercus alba, Q.stellata, O. marylandıca, Q.rubra, Acer rubrum, 
Liriodendron, Nyssa sylvatica, Funiperus virginiana, Castanea, Fagus and 
Prunus serotina. 
The herbaceous plants of the forest floor are found in pure association, or 
they occur sparingly distributed along with other herbs that form together a 
mixed vegetation. The habitats of the different herbaceous associations are 
controlled by photic and edaphic conditions. Thus in the deep shade of the 
dominant trees, the botanist finds the following plants forming pure associa- 
tions and each association may be distinguished by the generic name of the 
plant ‘). 
*Solea concolor, *Podophyllum peltatum, *Triosteum perfoliatum, *Tradescantia vir- 
ginica, *Mertensia virginica, *Caulophyllum thalietroides, *Hydrophyllum ‚virginicum. 
- Gaultheria procumbens. 
The following ferns are also found in pure association in the forest: Adiantum pedatum, 
Nephrodium (Aspidium) spinulosum, marginale, acrostichoides and Phegopteris. 
Along the woodland streams, growing in the damp loamy soil of such 
situations and controlled by the amount of soil-moisture present are a number 
of associations characterized by a single plant, as follows: 
April. Floerkea proserpinacoides. 
May. *Polemonium reptans, *Valerianella chenopodifolia, *Ornithogalum umbellatum, *Herac- 
leum lanatum. 
une. *Lysimachia nummularia, 
: *Impatiens fulva, I. pallida, Thalictrum polygamum, Pilea pumila. 
August. Epiphegus virginiana, Lobelia cardinalis, Commelina nudiflora, *C, virginica, 
September. *Eupatorium purpureum, 
The rocky outcrops in the woods consisting either of ledges or of angular 
boulders by frost action are covered by several well-characterized associations 
formed by pure growths of the following species: 
*Camptosorus rhizophylius, *Polypodium vulgare, 
April. *Saxifraga virginiensis, *Aquilegia canadensis, 
May. *Heuchera americana, 
Besides these pure association ot plants found in the woods of southeast 
Pennsylvania occur a large number of species, rich in number of individuals, 
but scattered on the forest floor, separated from each other by spaces filled 
up with other characteristic woodland species, likewise isolated. These plants, 
ı) Those marked by an asterisk also occur in the vieinity of Washington, DC. and in about 
the same association, according to my observations, 
