146 HARSHBERGER: PHYTO-GEOGRAPHIC SKETCH 
present are a number of associations characterized by a single 
plant, as follows : 
April. — Hloerkea proserpinacoides Willd. 
May. — Polemonium reptans ., Valerianella chenopodifolia 
(Pursh.) D.C., Ornithogalum umbellatum ., Heracleum /anatum 
Michx. 
June. — Lysimachia nummularia L. 
July. — Impatiens biflora Walt., 7. aurea Muhl., Thalictrum 
polygamum Muhl., Adicea pumila (L.) Raf. (Pilea pumila Gray). 
August. — Leptamnium Virginianum (L.) Raf. (Epiphegus 
Virginiana Bart.), Lobelia cardinalis L., Commelina nudifiora ie 
C. Virginica L. 
September. — Eupatorium purpureum L. 
The rocky outcrops in the woods consisting either of ledges or 
of angular boulders formed by frost action are covered by several 
well-characterized associations formed by pure growths of the 
following species : 
Camptosorus rhizophyllus (L.) Link, Polypodium vulgare L. 
April. — Saxifraga Virginiensis Michx., Aquilegia Canaden- 
sts L, 
May. — Heuchera Americana L. 
The herbaceous flora of the woods found on the serpentine 
areas of southeastern Pennsylvania has been discussed in a papét 
by the writer entitled ‘The flora of the serpentine barrens of 
southeast Pennsylvania.” * As the facts have been presented 
there in some detail, it is hardly necessary to repeat the obset- 
vations so recently published. The student is referred to that 
paper for an account of the distribution of the serpentine plants. 
Besides these pure associations of 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, sep® 
rated from each other by spaces filled up with other characteristic 
woodland species, likewise isolated. These plants, therefore, form 
a mixed vegetation which covers the ground except where the 
pure associations of single species occur and give character to the 
herbaceous flora of such woodland areas. The following species : 
may be included in this category. They are arranged, because : 
* Science, II. 18: 339-343. 11S 1903. 
