THE PERIODICITY OF FRESHWATER ALGAE I3I 
the ponds are drying up, I made periodic determinations of the freezing 
points during 1913, 1914 and the spring of 1915. These determi- 
nations number somewhat more than two hundred, and it has been 
possible to see the effects of torrential rains, showers, and the most 
prolonged droughts known in central Illinois. In general the results 
indicate that the highest concentrations coincide with the periods of 
greatest rainfall and higher water levels, and the periods of low con- 
centration are coincident with low water levels and drought. This 
result may be readily explained since the rains bring in the soluble 
salts from the upper layers of the soil. But the rains also bring in 
silt, clay and suspensoids. These require days and weeks to settle 
but meanwhile they have exposed an enormous surface for adsorption 
to all parts of the pond solution, and when they finally settle to the 
bottom they take nearly all the soluble salts with them. Indeed in 
the autumn of 191 3 the Beckmann thermometer scarcely distinguished 
between distilled water and certain pond solutions. Since the rains 
in Illinois are mostly in the spring and autumn there are two annual 
maxima of concentration, one coinciding with the beginning of the 
spring rains and a lesser one coinciding with the beginning of the 
autumn rains. In late summer and late winter the concentrations 
reach their minima. I wish to be clearly understood to apply this 
statement only to pools, ponds, and streams fed by surface run-ofif. 
I have a series of determinations for the underground water of a well 
and in this case the water reaches its greatest concentration in late 
summer. This harmonizes with the numerous analyses of our large 
streams fed by springs and underground water, in which it has been 
clearly shown that the water concentrates in late summer. 
The amount of concentration is also of interest to those who have 
relied on the osmotic pressure as a factor in producing the repro- 
ductive phase. The waters of the ponds and streams which I studied 
had an osmotic pressure of from one-tenth to one four-hundredth of 
the osmotic pressure of the cell sap. In the case of the waters, the 
depression of the freezing point varied from o.0O2°-o.O43°, in the 
case of the algae it varied between 0.4° and 0.9°. So that at most 
the osmotic pressure outside the cell is but a small fraction of the 
pressure on the inside of the cell. 
To return then to my observations on the fruiting of algae as 
coincident with the periods of high water, these are also the periods 
of high concentration. And if these results need be in harmony with 
