464 Part IV. Chapter 2. 
it does not occur, (Nuphar advenum) replaces it and forms a pure growth 
(Nuphar Association). In July, two colors of reed-grass are noticeable: a 
dark green mature form in deep water and a light green immature growth of 
reed-grass inside of the dark green area. In the shallower water behind the 
reed-grass and spatterdock, several associations of plants are seen. One area 
is characterized by the commingling of Sagittaria variabılis, Impatiens fulva, 
Ambrosia trifida, Nuphar advenum, with an occasional association of Aabiseus 
moscheutos. In other areas /mpatiens fulva, Rudbeckia laciniata and Sagittaria 
variabilis occur, and on drier soil Thalictrum polygamum and Heracleum lana- 
tum. Typha latifolia forms a pure association (Typha Association) sur- 
rounded by Ambrosia and Sagittaria; in other areas this forms pure associa- 
tions and in still other situation it is mixed with Cicufa maculata (Sagittaria- 
Cicuta Association). Cephalanthus occidentalis always occurs in the drier 
soil of the tidal marshes and in a number of places with it grow Cornus 
sericea. Alluvial islands occur in the channels of the streams, These are at 
first covered by the spatterdock, and as the soil becomes drier through the 
building action of these plants, the spatterdock is replaced by other plants 
until the island becomes dry enough to support tree vegetation. The marshes 
have been drained in many cases sufficiently to raise marsh-grass for hay. The 
ditches of such converted marshes afford many interesting plants of the Lemna- 
Association (vide p. 463), Pontederia, Hibiscus, Asclepias incarnata, Typha — 
Sagittaria — Zizania — Association. Outside of these associations and 
growing in the deeper water along the banks of the Delaware River is an a5- 
sociation of Scirpus lacustris (Scirpus Association). The between-tide, mud- 
inhabiting plants along the Delaware River shore are Micranthemum Nuttallü 
(= M. micranthemoides), Isoetes riparia (on gravelly beaches), Tillaea simplex 
— T. aquatica), Limosella aquatica, Eriocaulon (Dupatya) flavidulum, Sagittaria 
subulata and Elatine americana. Iso@ätes-Limosella Association. 
Swamp-plant Formation. 
The swamps of the region under consideration are of three kinds, as de- 
termined by their origin: (1) Swamps that owe their origin to a stream flowing 
over a low, flat area of country; (2) swamps due to underground springs; 
(3) swamps due to the collection of the drainage water of an area into a na- 
tural basin-shaped depression. True sphagnum bogs are not represented in 
southeastern Pennsylvania. This does not exclude sphagnum mosses from the 
category of swamp plants, but bogs which owe their character to them are 
absent, and peaty deposits such as one finds associated with the sphagnum 
are entirely wanting. The swamp plants characteristic of the region fall na- 
turally into several ecologic associations. Thus, we have the Symplocarpus“ 
Association, consisting of Symplocarpus (Spathyema) foetida, Osmunda re- 
galis, O. Claytoniana, O. cinnamomea, Cardamine rhomboidea and Onoclea 
sensibilis. The Iris-, Typha- and Acorus-Associations are clearly de- 
