CONDITIONS OF EXISTENCE 47 



of the organism in cold water. Dissolved nitrogen is important 

 for tho work of nitrogen fixing bacteria. Oxygen is necessary for 

 the production of CO2. Carbon dioxide is necessary for the starch 

 building of chlorophyll-containing plants and animals. These green 

 organisms form the chief food basis of all other organisms. Pro- 

 teids or other complex foodstuffs are necessary for all animals. It is 

 only animals which contain chlorophyll in the form of algae living 

 symbiotically in their bodies, that can survive without taking in 

 complex foodstuffs. Proteids are made only when starch, nitrates, 

 and several other inorganic foods are present. Because of their 

 proteid and starch demands light is indirectly necessary to animals 

 which can live in darkness. 



According to Piitter and Raben, who confirmed his determina- 

 tions using better methods, sea water, and probably fresh water as 

 well, contains amino-acids, oils, and carbohydrates. Putter has 

 shown that many aquatic animals absorb nutrition from solution 

 which renders them only in part dependent upon plankton. 



Plants are commonly covered with a coating of small organisms, 

 so that animals such as snails may rasp the surface and secure food 

 without eating the plant tissues themselves. One could probably 

 remove all the larger plants and substitute glass structures of the 

 same form and surface texture without greatly affecting the immedi- 

 ate food relations. Aquatic plants are of particular use to animals 

 as cUnging, hiding, and nesting-places. 



The quantity of plankton has been much studied. Quantity 

 is usually expressed as 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. Ward found an 

 average of 11.5 cc. per cubic meter in water from the surface 2 m.; 

 from 2 to 25 m., 3.9 cc; 25 m. to bottom, 0.4 to 1.5 cc, in Lake 

 Michigan (August). Pine Lake, a small lake adjoining, contained 

 relatively less plankton than Lake Michigan, the surface stratum 

 containing more and the deeper strata much less. Lake Michigan 

 contains twice as much plankton as Lake St. Clair. A small 

 European lake (Dobersdorfer See) contains about ten times as 

 much plankton as Lake Michigan. Kofoid found the average for 

 the year to be 2.71 cc. per cubic meter for the Illinois River and 



