412 MINXESOTA BOTANICAL STUDJKS 



cate various organisms, vary a good deal with different observers. 

 Moore and Kellernian (U. S. Department of Agriculture, Bureau 

 of Plant Industry, Bulletin y6, 1905) give a table based in part upon 

 treatments in reservoirs under normal conditions ; Whipple ( The 

 Microscopy of Drinking Water, 255, 1914) gives a similar table, 

 based largely upon Kellerman's figures. These are perhaps as re- 

 liable as any that have been compiled. The results of the treat- 

 ment of Vadnais Lake, however, are so different from the results 

 given by either of these observers, that it seems well worth while 

 to direct our attention to a few of the organisms, and their re- 

 sponse to the copper treatment here. One of the most interesting 

 organisms in this connection is Synedra pulchella. Moore and Kel- 

 lerman suggest for the eradication of Synedra the use of one part 

 of copper sulphate in 600,000 parts of water. On June 14, the 

 time of the first treatment with copper sulphate, there were pres- 

 ent in the water at the weir as it enters the conduit leaving Vad- 

 nais Lake, 3,420 individuals of Synedra pulchella per cubic centi- 

 meter of water. The treatment given was about one part of cop- 

 per sulphate to 12,000,000 parts of water, and by June 25, the 

 number of Synedra had dropped from 3,420 to 1,116 per cubic 

 centimeter. Other organisms present were reduced in a similar 

 manner, but before a month had elapsed, practically all organisms 

 were rapidly increasing again, and it was decided to give a second 

 treatment. This time it was thought advisable to give a heavier 

 treatment, so one part of copper sulphate was used for each 10,- 

 000,000 parts of water in the lake. This treatment was given %n 

 July 12, and Synedra had reached the very high figure of 7,720 

 individuals per cubic centimeter of water. The results of the 

 heavier treatment were surprising. Each day showed a marked re- 

 duction in the number of Synedra, and in ten days' time the num- 

 ber had dropped from 7,720 to less than 100 individuals per cubic 

 centimeter. For about four weeks following this, Synedra re- 

 mained inactive, running from 12 or 15 per cubic centimeter to 

 100 per cubic centimeter, but not until about August 20 did they 

 begin to show signs of rapid increase in numbers again. This re- 

 markable reduction was secured by a treatment of one part in 10,- 

 000,000, or 6 per cent of that suggested by Moore and Kellerman 

 as effective for Synedra. It is true that this treatment did not re- 



