Page:Popular Science Monthly Volume 68.djvu/256

 252 of data. The higher orders of animal life are, of course, dependent upon the lower. The low free-swimming forms, called collectively 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.

 Vallisneria spiralis after 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.

The rooting aquatic plants may be considered in two groups according as they are submerged or emergent. The latter vegetation must,