FIELD BOOK OF PONDS AND STREAMS 



make frequent mention of its habits in rivers. In 1848, the 

 curator of the Botanic Garden at Cambridge introduced it 

 into a tributary of the River Cam and by 1852 it had spread 



Fig. 65. — Water-weed, Elodea cayiadensis. 



into the river, choked the drainage, and even stopped the 

 rowing. 



Occurrence. — It is very common in slow streams, in back- 

 waters off main channels, and in ponds (PI. X). Blooms 

 July-August. Eastern states south to Virginia and Ken- 

 tuck>', west to Wyoming. Other common names are ditch- 

 moss, choke pondweed. 



Animal associates. — ^Snails find good foraging grounds on 

 Elodea leaves and it is not unusual to find a six inch branch 

 loaded with four or five of them (Fig. 34). 



Fresh water eel-grass, wild celery, Vallisneria spiralis. — 

 Fresh water eel-grass (Fig. 66) is familiar to canoeists and 



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