134 HARSHBERGER : PHYTO-GEOGRAPHIC SKETCH 
river by pleasure craft (Va//isneria association). In some of the 
smaller streams and in ponds formed in artificial depressions, Phi/- 
otria Canadensis (Michx.) Britton (Elodea Canadensis Michx.) 
abounds. A fine growth of this plant, the water-weed, is found 
in an abandoned quarry-hole at Leiperville, Pa., and another ina 
small stream near Horticultural Hall, Fairmount Park (4/odea as- 
sociation). The spatterdock, Nymphaea advena Soland. (Nuphar 
advena Ait. f.)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 stretches as a pure association for 
miles along the Delaware and Schuylkill rivers (Vuphar associa- 
tion). Associated with it in shallower water the pickerel-weed, 
Pontederia cordata L., abounds, forming in places pure associa- 
tions. This interesting plant with trimorphic flowers is most abund- 
ant 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 district in South Phil- 
adelphia known as the Neck. The agricultural land, kept in a high 
state of fertility by the application of city manurial waste, is inter- 
sected by numerous ditches where abound Sfirodela polyrhiza (LJ 
Schleid., Lemna minor L., Wolfia Brasiliensis Wedd., and Wolffia 
Columbiana Karst. (Lemna association). Orontium aquaticum 
L., the goldenclub, is also a plant that forms in places ecologic 
groups (Orontium association). The water-chinquapin, Nelumbo 
/utea (Willd.) Pers., twenty-five years ago existed in the region of 
the ‘‘Neck.’’ With the spread of the city southwest, the plant 
was destroyed. 
POND-PLANT FORMATION. — There are no ponds or lakes of 
any size natural to southeastern Pennsylvania; all that now 
exist are artificial. Some of them occupy depressions from which | 
clay has been taken for bricks; others occupy the bottoms of 
rock quarries, while still others have been formed by the dam- 
ming of streams. The only natural ponds are of small size and 
are rather pools formed in a depression near some perennial 
spring. The ecologist finds in such natural pools, or along their 
edges, a collection of species that seem to give character to 
them, such as Chrysosplenium Americanum Schwein. (Chrysosple- 
nium association), Veronica Americana Schwein., Typha /atifola 
