RELATION OF ALGAE TO DISSOLVED GASES. 181 
them from other sources, and more than is made good at 
night. The process goes so far that mono-carbonates are 
precipitated to be redissolved by the free COz in the lower 
water or accumulated in the mud at the bottom. 
Davis (’01), in accounting for the origin of the marl in 
some of the small lakes in Michigan, gives the following :— 
after pointing out the inadequacy of animal life, evaporation 
and escape of COz on relief of pressure, as causes of the pre- 
cipitation of the carbonate of lime, the author discusses at 
length the only alternative hypothesis, namely, that the cal- 
cium salts are precipitated through the agency of plant life. 
In the marly lakes aquatic plants of all kinds become in- 
crusted with calcium carbonate, which is not a true secretion 
of the plants, for it is purely external, and the same species in 
other districts are not incrusted. 
If the calcium carbonate is in excess in the water, and is 
held in solution by carbon-dioxide, then the absorption of the 
latter by the plants causes precipitation of the carbonates 
upon those parts, stems and leaves abstracting the gas. But 
if the proportion of calcium carbonate in solution is so small 
that it would not be deposited even if there were no carbon- 
dioxide present, the precipitation is explained by the action of 
the oxygen set free by the living plants, in converting calcium 
bicarbonate to mono-carbonate:—CaH2(COs)2 + O = H20 
+ CaCOs + CO2 + O. 
In lakes such as these the vigorous photosynthesis, thus 
made possible by this increased supply of CO in an available 
condition, is shown by the resulting excretion of oxygen, 
which often exceeds the point of saturation. This we should 
expect to occur, since Devaux (’89) has shown that the ratio, 
o =1, holds true for submerged aquatics as well as for 
aérial plants or leaves. Even in winter Knauthe (’99) found 
a much larger excess of oxygen in some of the ponds which 
he studied, when they were covered with ice. In one pond 
he found the surprisingly large amount of 49 cc. of oxygen 
per liter of water, just under the ice. This was about 515 
per cent of the amount required for saturation. A phenom- 
enal increase in oxygen was noted in this pond, the amount 
