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POPULAR SCIENCE MONTHLY 



lection of data. The higher orders of animal life are, of course, de- 

 pendent upon the lower. The low free-swimming forms, called collect- 

 ively zooplankton, are preyed upon by larger animals. The latter in 

 turn are devoured by still larger forms and so on up to the fish. It is 

 evident that all the animals above the zooplankton are dependent upon 

 it, and whatever increases or decreases the quantity of zooplankton 

 causes a fluctuation in the food supply of the fish. Thus it is that a 

 quantitative study of the plankton forms so conspicuous a feature of 

 the extended investigations which have been made both here and abroad 

 of fresh-water biology. 



Aquatic animals, just as terrestrial, are dependent upon plants for 

 the organization of the elements of food into food. As there is an 

 animal or zooplankton, just so there is a vegetable or phytoplankton. 

 The latter is the living basis of the food supply of the aquatic fauna. 



Fig. 2. Vullixntria spiralis alter 7 weeks' growth rooted in gravel. Plants in figures 1 and 



2 originally the same size. 



This phytoplankton lives on substances which it makes for itself out 

 of carbon dioxid and water and the mineral matter in solution in the 

 water. The supply of water and carbon dioxid is, of course, unlimited. 

 The supply of mineral food varies considerably in different bodies of 

 water and in the same lake several factors may operate to cause a 

 fluctuation. The rooting aquatic plants have long been suspected of 

 being one of these factors, but whether they increase or decrease the 

 mineral food has not until recently been known. 1 



The rooting aquatic plants may be considered in two groups accord- 

 ing as they are submerged or emergent. The latter vegetation must, 



1 Pond, Raymond H., ' The Biological Relation of Aquatic Plants to the 

 Substratum,' U. S. Fish Commission Report for 1903, 483-520, published 1905. 



