CONTRIBUTIONS TO THE BIOLOGY OF THE GREAT LAKES. 

THE PLANKTON ALGA? OF LAKE ERIE, WITH SPECIAL REFERENCE TO 
. TE sCnLONORnY CRA: 
By JULIA W. SNOW, 
Instructor in Botany, Smith College, Northampton, Mass. 
INTRODUCTION. 
The unicellular algee, which in themselves show most interesting characteristics 
in their structure and life history, come to have a double significance when consid- 
ered in connection with their environment. Investigation shows the presence of an 
intimate connection and interdependence between them and their surroundings. 
While they depend upon substances in the water for nutrition, they in turn prob- 
ably perform a valuable function, the same that has been proved by Bokorny 794 
and Strohmeyer ’97, in the case of some higher alge, that of purifying the water, 
reducing the amount of bacterial growth accompanying decay, and rendering the 
medium fit for higher life. Their value also as a food supply to the aquatic fauna 
is well known. In any biological study of a body of water the alge must therefore 
receive attention, and should be considered with reference to their environment 
rather than as independent unrelated entities. 
A study of this kind should be continued for a number of years, for aside from 
the desirability of repeated observations, it is necessary on account of variations in 
the flora from year to year. Certain species may be abundant each year, but others 
are periodic in their appearance, being found only at intervals of three or four years; 
and forms, more or less polymorphic, have been known to appear almost exclusively 
in one condition one year and in another condition the next, so that their identity 
has not been known until their life history has been traced. Such variations must 
be due to variations in environment, so that before these phenomena can be under- 
stood the environment must be known and its influence determined. 
In the natural state, the elements in environment are so numerous and so 
connected that to know definitely which of these produce a certain effect on an 
organism is impossible. This must be ascertained under artificial conditions and 
experimentation must be resorted to for this purpose. Under these circumstances 
the environment may be altered, certain of its elements may be eliminated and the 
effect of others studied, so that after repeated trials we may arrive at more definite 
knowledge of the life principles of these organisms than would be possible in the 
native state. When the relation to environment is definitely known, then we may 
go still further and, by changing this environment, exert a certain control over these 
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