AQUATIC PLANTS IN POND CULTURE. 25 



The introduction of the water-lily {C ast alia odor ata) into the ponds 

 at Wytheville, with the result of apparently crowding out two other 

 aquatic plants, and the somewhat similar experiences at San Marcos, 

 Tex., and at Mill Creek, Mich., suggest that the partial elimination 

 of one species by the introduction of another may at times be advan- 

 tageously attempted, and that with a full knowledge of the effects of 

 given combinations of species a desirable balance of vegetation could 

 be maintained by this means. This question also, however, enters the 

 broad field of plant physiology. 



METHODS OF CONTROLLING AQUATIC VEGETATION. 



ELIMINATION OF UNDESIRABLE PLANTS. 



Plants which are in themselves objectionable it is of course desired 

 to eliminate for all time. There is, however, no known method of 

 eradicating the higher forms of vegetation from ponds without de- 

 stroying the fish, unless it be possible first to draw off the water. 

 When this is done certain forms of plants die from exposure and the 

 roots of others can be grubbed out. Some of the lower forms of 

 vegetation, algal growths frequently described as " frog spittle," 

 " water moss," and " slime," enter into pond culture only as an element 

 of the water supply, and the more obnoxious forms may be destroyed 

 by means of copper sulphate according to the method of Moore and 

 Kellerman for the disinfection of municipal water supplies.* This 

 method has been successfully adapted, not only to pond culture but 

 also to waters containing trout, as is set forth in a report of experi- 

 ments at the White Sulphur Springs station of the Bureau of Fish- 

 eries soon to be published.^ The latter application of the method is of 

 especial interest, for the reason that trout are more than ordinarily 

 susceptible to the toxic properties of copper. 



CHECKING SUPERABUNDANT OR UNDESIRED GROWTH. 



To prevent superabundance of some vegetation, or to make less ob- 

 jectionable the presence of troublesome species that can not be eradi- 

 cated, it is sometimes desired to check the growth of the plants. Mr. 

 Kellerman states, in a letter, that in water not unusually hard the 

 waterweed {Philotria canadensis), Chara, and several species of 

 Potamogeton may be considerably checked in growth by treating the 

 water with copper sulphate in the proportion of 8 pounds to 1,000,000 

 gallons of water. In limestone regions, however, or where the water 

 contains a large amount of organic matter, the proportion of copper 



■^ Moore and Kellerman, Copper as an algicide and disinfectant in water supplies. 

 Bulletin 76, Bureau of Plant Industry, Department of Agriculture. (See p. 12.) 



* Marsh and Robinson. The treatment of fish-cultural waters with soluble remedial 

 agents, especially copper sulphate as an algicide. Paper presented before the Fourth 

 International Fishery Congress, Washington, September 22, 1908. 



