Northern Piedmont Distriet. 463 
district in South Philadelphia 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 Spirodela polyrhisa, Lemna minor, 
Wolffia brasiliensis and W. columbiana (Lemna Association), Orontium 
aquaticum, the goldenclub, is also a plant that forms in places ecologic groups 
(Orontium Association). The water-chinquapin. Nelumbo Iutea, twenty 
five years ago, existed in the region of the “Neck”. With the spread of the 
city southward, the plant was destroyed. 
Pond Formation. There are no ponds or lakes of any size natural to southeastern Penn- 
sylvania; all that now exist are artificial. Some of them occupy depressions from which clay has 
been taken for bricks; others occupy the bottom of rock quarries, while still others have been 
formed by the damming 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 (Chrysosplenium Association), Veronica americana, Elodea ca- 
nadensis, Typha latifolia, Acorus Calamus, Iris versicolor, Myriophyllum verticillatum, Spirodela 
lyrhiza, Cardamine rhomboidea (= C. bulbosa), C. rotundifolia, Caltha palustris, Myosotis palustris, 
Nasturtium palustre, Nasturtium offieinale (Nasturtium Association), Ludwigia (Isnardia) palustris 
and various algae, viz., Spirogyra, Hydrodietyon, Conferva, Nitella, Oedogonium, Volvox, Euglena 
and Mesocarpus. 
Tidal-marsh Formation. The tidal marshes have been formed by a variety 
of contributing forces. Material brought down by the rivers has been sorted 
and distributed over the tidal areas by the tides and currents of the streams 
that meet here. The location of these deposits and their superficial extent 
has depended upon the character, force and direction of the currents. 
The banks of the streams influenced by tidal action are lined by thickets 
composed of Platanus occidentalis, Gleditschia /riacanthos, Alnus serrulata, 
Acer rubrum, Salix alba, Liguidambar, Fraxinus, Sambucus, Cephalanthus, 
Rosa lucida, Ampelopsis, and Rubus nigrobaccus. 
The herbaceous plants are in this rich alluvial soil Thalietrum polygamum, Heracleum lana- 
tum, Agrimonia parviflora, Allium vineale, Ambrosia trifida, Impatiens fulva. In mud near the bank 
associations of rose mallow, Hibiscus moscheutos, occur, with occasional patches of the sensitive 
fern, Onoclea sensibilis. Along the edges of the stream banks in the shallow water grow Jus- 
siaea repens, Calamintha clinopodium, Sagittaria variabilis, Cicuta maculata, Ludwigia alternifolia, 
Stachys palustris and Asclepias incarnata. In the deeper water, fringing the shore of the creeks, 
strips of wild rice, Zizania aquatica (Zizania Ass ociation) and of Nuphar (Nymphaea) advenum 
(Nuphar Association) are found. 
The tidal marshes of the rivers Delaware and Schuylkill are extensive and 
may be in places one half a mile wide. Several well marked zonal areas of 
marsh vegetation are noticeable in making an ecologice survey. Some of the 
marshes have been diked. Others at high tide are flooded with water, and 
yet the ecologist can distinguish relative depths of water by the character of 
the vegetation alone. If one begins with the open channels of the creeks and 
ditches that intersect the marshes, the following zonal areas can be distin- 
guished. In the deeper water along the channel, the reed-grass, Zisania agua- 
fica (Zizania Association) forms the outer fringe of vegetation and where 
