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ILLINOIS BIOLOGICAL MONOGRAPHS 



[86 



tube containing the sample to be examined and a vertical slit made possible 

 a clear reading under constant illumination. This precaution regarding 

 the light was made necessary as it was found that very deceiving results 

 were obtained as the light intensity diminished, the weakening light 

 giving higher readings than normal light, thus leading to conclusions wholly 

 erroneous and misleading. The results obtained over a period of a month 

 are given in table 19. 



Table 19 



CHANGES IN HYDROGEN ION CONCENTRATION IN THE EVENING 

 OVER SHALLOW AND DEEP WATER: OCONOMOWOC LAKE, 1923 



From this table certain facts become clear: 1) that the H-ion con- 

 centration over the shallow water is lower (pH higher) during the day than 

 over the deep water. This is undoubtedly due to the great activity of 

 the shallow water vegetation, which during the day is giving off oxygen 

 and taking on carbon dioxide. 2) It is to be noted that with the coming of 

 darkeness the H-ion concentration increases, as indicated by the de- 

 creasing amplitude of the figures. 3) The change in H-ion concentration 

 is greater along the shore than over the deep water. This again is due to 

 the plant activity and the reversal of the photosynthetic process; oxygen 

 is now being taken up and carbon dioxide given off, thus increasing the 

 acidity and increasing the H-ion concentration. 4) Both the shallow and 

 deep water reach a stage of more or less stable equilibrium with the pH 

 of the shallow water higher than that over the deep. 5) Finally, Labides- 

 thes sicculus moves toward the higher pH value as the change begins to 

 occur over the deep water. That the species is sensitive to changes in the 

 H-ion concentration of the water is further made clear by a series of 

 laboratory experiments to be discussed. 



