497 



merged aquatic flora for the possession of a body of water 

 capable of supporting either, the greater duration and perma- 

 nence of the larger plants which constitute the submerged 

 flora must m the long run inure to the advantage of the latter, 

 hence they predominate over the phytoplankton wherever 

 other conditions favor their appearance. This coarse sub- 

 merged vegetation cannot in its living condition be utilized by 

 the minute organisms of the zooplankton, and only such as 

 feed upon it in decay can find sustenance in the vegetation-rich 

 lake. The absence of an abundant phytoplankton and of the 

 greater part of the zooplankton may thus be accounted for in 

 waters rich in suboierged and non-rooted vegetation. The 

 total production of such a body of water consists mainly of a 

 large amount of coarse aquatic vegetation, which but few ani- 

 mals can utilize in its living condition as food, and a much re- 

 duced plankton, largely of animal constituents, together with 

 such larger and often attached species as find food in these 

 elements. 



Some light on the relation of vegetation and plankton to 

 certain of the chemical constituents of the food of the aquatic 

 flora can be gained from a comparison of Plates XLV., XLIX., 

 and L., and Tables X., XIL, and XIII., which show the results 

 of analyses in 1898. The appended table also gives the average 



amounts of free ammonia and nitrates in Illinois River and in 

 Quiver and Thompson's lakes in the period from June 1 to Oc- 

 tober 1 and in the remainder of the year— two periods which 

 approximately represent the times of maximum and minimum 

 of chlorophyll-bearing organisms. 



