44 



IOWA STUDIES IN NATURAL HISTORY 



TABLE 21 — Obsei'vations on the dissolved gases of Okoboji lake, July 

 31, 1919. In the column for free carbon dioxide the 

 minus sign indicates that the water gave an alkaline re- 

 action and the plus sign that it gave an acid reaction 

 with phenolphthalein. 



TABLE 22 — Observations on the dissolved gases of lake Mendota on 

 August 6, 1919. In the column for free carbon dioxide 

 the minus sign indicates that the water was alkaline and 

 the plus sign that it was acid to phenolphthalein. 



VI. THE PLANKTON 



The small floating and free swimming organisms that 

 constitute what is known as the plankton may be divided 

 into two groups, namely, (a) those which are large enough 

 to be easily obtained with the regular plankton net and (b) 

 those which are so small that they readily pass through the 



