QUANTITY 67 
scavengers. The rooted vegetation is eaten only to a small extent. 
Small floating or swimming plants and animals, called plankton (Figs. 
12-18, pp. 75, 76) are the basis of the food supply of larger animals. 
We could probably remove all the larger rooted plants and substitute 
something else of the same form and texture without greatly affecting 
the conditions of life in the water, that is, so far as the life habits of the 
animals are concerned. The aquatic plants are commonly covered with 
a coating of green algae, protozoa, and other small organisms, so that 
animals such as small snails may rasp the surface of the plants and secure 
food without eating the plant tissues themselves. Plants in water are 
of particular use to animals as clinging- and nesting-places. 
V. QuANTITY (47) OF LIFE IN WATER 
The quantity of living matter in water, so far as it is plankton or 
floating organisms, has been much studied. The quantity is usually 
expressed in one of two ways: number of organisms per liter or cubic 
meter of water, determined by counting a part of a collection; or in 
cubic centimeters per cubic meter of water. In Lake Michigan (August) 
Ward (75) found an average of 11.5 c.c. per cubic meter in water from 
the surface to 2 m.; from 2-25 m., 3.9 ¢.c.; 25 m. to bottom, 0.4-1.5 C.c. 
He found that Pine Lake (a small lake) contained relatively less plankton 
than Lake Michigan, the surface stratum of Pine Lake containing more 
and the deeper strata much less than the larger lake. Lake St. Clair 
contains only one-half as much plankton as Lake Michigan. Lake 
Michigan contains only about one-tenth as much plankton as some of the 
small European lakes (Dobersdorfer See). Kofoid (77) found 71.36 c.c. 
per cubic meter the maximum record for the Illinois River. The 
average for the year is 2.71 c.c. per cubic meter. The largest amount 
recorded by Kofoid is 684.0 c.c. per cubic meter (Turkey Lake, Ind.) . 
Small streams and lakes with large inflow and outflow have but little 
plankton. Large amount of plankton is commonly associated with 
high CO, content, low oxygen content, and a large amount of carbonate 
in solution. 
The amount fluctuates from season to season. Kofoid (77) found 
the maximum for the Illinois River in April to June. The amount 
gradually decreases until December and January, when the minimum 
is reached. He also found evidence that the light of the moon increases 
photosynthesis and the amount of plankton. The maximum of Crustacea 
was found by Marsh (78) to fall in July, August, and September, differing 
in different years. The maximum in Lake Michigan probably is usually 
