ee SEE eS see 
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182 MISSOURI BOTANICAL GARDEN. 
at the surface rising from 29.8 cc. on January 16, noon, to 
42 ec. on January 17, 12:30 p.m. This period of very high 
oxygen content lasted three days and was correlated with a 
very large growth of chlorophyll-bearing organisms. During 
the daytime, when the organisms were actively excreting 
oxygen, the water became alkaline as a result of the consump- 
tion of carbon-dioxide, presumably taken from the bicar- 
bonates. It would be interesting to know the kind of 
organisms that were so active at so low a temperature, but this 
fact is not stated. The mantle of ice prevented the escape 
of oxygen, which often occurs during a strong wind in 
summer. 
In some of the hard-water lakes studied by Birge and 
Juday (’11) the alkalinity reached a point where some of the 
normal calcium carbonate was precipitated out, thus forming 
a layer in which the combined COz showed a pronounced 
decrease. Alkaline strata have been found in lakes which 
have very soft water. On the other hand, soft-water lakes, 
especially with very soft water, present very different condi- 
tions. No large supply of free COz is readily obtainable, and 
since only a small quantity of half-bound CO: will be present 
in soft water, the total available COe will be limited in amount. 
In general the soft-water lakes of Wisconsin, so far as ob- 
served, do not support so large a phytoplankton as the hard- 
water lakes, and no hard-water lakes have as yet been found 
which contained so little phytoplankton as many of the soft- 
water lakes. 
Wesenberg-Lund (’08), in his comparative study of the 
fresh-water lakes of Denmark and Scotland, found that those 
of Denmark, which are nearly all shallow and usually al- 
kaline, support a rich biota, while the lakes of Scotland, most 
of which are deep and acid on account of bogs and humic acid 
in the soil of the drainage basins, are very poor in plankton 
and submerged aquatics and, consequently, in the ordinary 
life of fresh-water lakes. The blue-green algee which form 
so important an element in the littoral region of the Danish 
lakes, often covering the stones and depositing on them a 
layer of greenish lime, are almost non-existent in the corre- 
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