20 



GROWTH OF PLANTS 



main source of the fluctuating salt content of these two bodies of water. The 

 decline in duck feeds in these waters from 1918 to 1926 turned out to result 

 from the removal of the sea-level lock mentioned above. 



Figure 9. Submerged plants covered with colonial growths of the brackish-water 

 hydroid, Cordylophora lacustris AUman. 



Offhand, one might be inclined to assume that flow of water from the 

 canal injured the plants in Currituck Sound and Back Bay by increasing 

 the salt concentration. Apparently Bourn was early inclined to this view. 

 Culture of the main duck food plants of this region in various dilutions of 

 sea water disproved this. Potamogeton pedinatus and P. perfoliatus grew 

 better in dilute sea water than in fresh water; 20 per cent sea water proved 

 optimum and 36 per cent showed retardation but not complete inhibition. 

 P. foliosus withstood 36 per cent of sea water. Vallisneria spiralis could not 

 be grown successfully in concentration above 12 per cent sea water, but 

 did well in 8 to 12 per cent. Ruppia maritima thrived in any concentration 

 of sea water from to 80 per cent and lived and stayed healthy in 150 per 

 cent sea water. These facts eliminate salt concentration as any considerable 

 factor of injury to the main duck food plants in these waters. The only place 

 that salt concentration might act as an injurious factor to the dominant 

 plants of the region was in the northerly part of Currituck Sound. 



One of the big causes of injury to the plants was a hydroid, Cordylophora 

 lacustris Allman, carried do\xn Avith the brackish polluted waters of the canal 



