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MIN'NKSOTA BOTANICAL STUDIES 



higher than from 1 to 10 standard units per cubic centimeter until 

 late in August. The third treatment, August 27, was not responded 

 to by Eiidorina and Pandorina as quickly as the second treatment 

 had been. Instead of decreasing at once as they had done before, 

 they continued to increase slowly in numbers for about two weeks 

 after the treatment. They were finally forced to succumb, however. 



l"ig. 3. Effect of copper sulphate upon certain green algae. 



and after about two weeks of stubborn resistance, Endorina, with 

 1,200, and Pandorina, with nearly 700 standard units, dropped rather 

 suddenly to less than 100 standard units per cubic centimeter each. 

 They did not rise materially again during the season. 



Most of the blue-green algae are very sensitive to copi)cr sul- 

 phate. At the time of the first treatment they were running from 

 1,500 to 2,000 standard units per cubic centimeter (figure 4), 

 Anabaena oscillarioides being responsible for the greater part of this. 



