418 



MINNESOTA BOTANICAL STUDIES 



much as before, but Eudorina and Pandorina continued to increase 

 in numbers for several days after the treatment, and this accounts 

 for the longer time required to bring down the total number to its 

 former position. For more than a month following this reduction 

 in numbers the total number of organisms did not rise above 1,000 

 standard units per cubic centimeter. The latter part of October, 

 however, with the circulation following the stagnation period of the 



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(c , ±r ji \a 20 30 g /g %9 S ig 26 B IS i8> 7 iT Zj 1.1 



t^^' ' ? 1" ' 



rJifeEg^^ip:" \Weir at \/atln<ais UaK^r ''^"g 



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Kig. 2. Effect of copper sulphate upon certa'n diatoms. 



summer months, some of the diatoms showed quite an increase. 

 Most notable among these was Stephanodiscus niagarae, which con- 

 tinued to increase until about the middle of November, when it 

 reached the very high number of 11,850 standard units per cubic 

 centimeter. When the water temperature of the lake had lowered 

 to 39.2 degrees Fahrenheit, the maximum density for water, and the 

 circulation ceased, Stephanodiscus began to show a reduction in 



