1909.] NATURAL SCIENCES OF PHILADELPHIA. 395 



Separated from Como Lake by a carriage road, and situated in the 

 angle which this road forms with the main or trolley road, is a small 

 triangular pond of fresh water, completely covered with a small- 

 flowered variety of the white water lily, Nymphcea odorata, with the 

 margin of the pond fringed with a circumarea of Panicum crus-galli. 

 Como Lake proper has relatively no growth of water lilies, except as 

 previously noted in a small embayment of the shore, where a small 

 stream enters from the pine woods, and yet in the pond, cut off from 

 the main lake by the construction of the carriage road, it is the domi- 

 nant and exclusive plant. 



Fresh Water Swamp Formation. 



Como Swamp. — West of the trolley road is a swamp (Fig. 6) which 

 occupies the area covered formerly by a western extension of Lake 

 Como. but cut off from it by the construction of the wagon road. As 

 this swamp is fed by a small stream about three-eighths of a mile long, 

 there is a partially open channel running through the middle of it, and 

 a small fresh water lagoon has been formed at one side by a choking of 

 the former lake basin with vegetation. This extremely wet swamp 

 illustrates the last stage in the conversion of a salt water pond with its 

 salt marsh vegetation, first into a brackish pond with the suppression 

 of most salt water plants (Sylvan Lake stage) and then into a fresh 

 water lake with its characteristic vegetation (Como Lake stage), and 

 finally the invasion of the shallow lake with vegetation peculiar to 

 fresh water swamps (Como Swamp stage, Fig. 6). 



The channel (ditch) is blocked in its lower portion by sedges and 

 grasses, in the midst of which we find an association of Potamogeton sp. 

 (f), Polygonum acre (*) and Scirpus pungcns ( + ) surrounding a 

 smaller association of Sparganium eurycarpum (= Fig. 6). On the 

 north side of the ditch we find an association of Leersia oryzoides (±), 

 Panicum crus-galli ( I ) and a few scattered clumps of Typha angusti- 

 folia (9 ). A triangular-shaped association comprises Nymphcea 

 (Castalia) odorata (O), Scirpus pungens ( + ), Panicum crus-galli (I), 

 with a few scattered specimens of Scirpus lacustris (©), and the arrange- 

 ment of the plants suggests that the water lilies represent the original 

 facies which has been invaded by other specis. The cranberry, 

 Vaccinium macrocarpon (M), forms a pure association at the edge of 

 the marsh, while nearby J uncus canadensis (cT), Polygonum acre ( • ), 

 P. sagittatum ( T ), Nymphcea odorata (©) are in association (see Fig. 

 6). Along the north edge of Como Swamp two other associations are 

 found, one consisting of the admixture of Scirpus pungens ( + ), Poly- 



