462 Part IV. Chapter 2. 
The flora of this section of country may be considered to be typic of the northern extension of 
the Piedmont Plateau. 
a) Northern Area. 
Southeastern Pennsylvania is a region of hills, of valleys, of meadows and 
of rocky ravines sloping down in general from the Laurentide hills to the 
Delaware River. It represents an original table-land whose general elevation 
was about 500 feet above tide-level. Originally the surface, hills, as well, as 
river-plain above the fresh water marshes, which line the Delaware River, was 
covered by a dense forest of trees. Since the settlement of the country, the 
most desirable land has been under cultivation, with many flourishing manu- 
facturing towns. 
otanically considered, the following ecologic plant formations and associa- 
tions may be distinguished, and these are determined approximately by the 
character of the areas above mentioned '). 
ı. Formations of the Estuaries, Ponds and Marshes. 
Aquatic Formation. The river and creeks of the region, especially in their 
lower courses, have smooth stretches of water in which grow a number of 
aquatic species. These species in the tidal estuaries are usually of the larger 
sort and are well adapted to grow in water where there is a change of level 
between high and low tide of about three feet. In many of the streams, the 
tidal flow is of considerable strength and the current established, therefore, in- 
fluences the distribution of the vegetation to a marked extent. Where the 
flow is less strong and swift, there the material in the form of mud and silt 
is deposited and upon this alluvial material aquatic plants take root and gra- 
dually raise the level of these areas by catching and holding fresh deposits of 
silt. The result is a tidal marsh intersected by numerous meandering channels 
- through which the tidal water ebbs and flows. 
Vallısneria spiralis forms a pure association of such extent, as seriously 
to interfere with the navigation of some of the streams (Vallisneria Associa- 
tion). In some of the smaller creeks and in ponds formed in artificial de- 
pressions, Zlodea canadensis abounds. The spatterdock, Nuphar advenum is 
probably the commonest aquatic plant that grows in the tidal portions of the 
streams of southeast Pennsylvania. It covers acres of mud ground and stret- 
ches as a pure association for miles along the Delaware and Schuylkill rivers 
(Nuphar Association). Associated with it in shallow water the pickerel- 
weed, Pontederia cordata abounds, forming in places pure associations. This 
interesting plant with trimorphic flowers is most abundant on the New Jersey 
side of the Delaware River (Pontederia Association). The duckweeds are 
found in the ditches and ponds of the region. This is true especially of the 
— 
1) HARSHBERGER, JOHN W.: A phyto-geographie Sketch of extreme southeastern . Pennsyl- 
vania. Bulletin Torrey Botanical Club XXXI: 125—159. March 1904. 
