410 MINNESOTA BOTANICAL STUDIES 



justment of the draw-tube of the microscope, a Whipple eyepiece 

 micrometer may be made to cover exactly one square millimeter. 

 This will give as a field for counting, exactly one cubic millimeter, 

 and makes the computation very simple. Usually ten counts were 

 made from each sample, though when organisms were greatly re- 

 duced in numbers so that many cubic millimeters of the concen- 

 trated sample did not contain a single organism, one hundred such 

 counts were made, in order to reduce as much as possible the 

 error arising from the difficulty of picking put typical areas for 

 the count. From the number of organisms found in a given volume 

 of this concentrated sample it is a very simple matter to compute 

 the number in the water as it is found in the lake. 



On account of the great variation in size and form of diflferent 

 organisms, and the consequent impossibility of making comparisons 

 between different organisms that would really mean anything, the 

 figures used in the charts or diagrams for indicating the quantity 

 of organisms present do not refer to the actual number of indi- 

 viduals or colonies of a given organism, but to standard units, 

 a system of measurement suggested by Whipple for convenience 

 in comparing the various forms. The standard unit is represented 

 by a square 20 microns on a side (one micron equals 0.001 milli- 

 meter). In computing the number of standard units in a given 

 individual or colony, an attempt has been made to approximate the 

 volume rather than the area of the upper surface. In the majority 

 of cases where individuals are quite uniform in size, the number 

 of individuals was multiplied by a number found to be the average 

 size for that species. In other cases, however, where there was 

 great variation in size, as in filamentous forms, and forms grow- 

 ing in colonies of an indefinite size, the size was computed sep- 

 arately for each occurrence of the species in the part of the sam- 

 ple counted. 



The method of distributing the copper sulphate in the lake was 

 that which has commonly been employed in such treatments. About 

 fifty pounds of the crystals were placed in" a burlap sack, this tied 

 to the stern of a boat and the boat rowed back and forth until the 

 sulphate was dissolved, care being taken to distribute systemati- 

 cally, and as uniformly as possible. In addition to the rowboats 

 which were used near the shore, a small motorboat with a bag of 



