VERTICAL DISTRIBUTION OF VOLVOX l8l 
force of gravity, or light may cause a change in the position of colonies. 
The following table shows the temperature variations for the upper 
seven meters of the lake on the morning of July 6. Thermotaxis is 
not a factor in the problem before us since the difference between the 
upper and lower range is comparatively slight, while the differences 
between day and night temperatures are negligible. 
The variable factors of the chemical environment are the gases 
in solution in the water. Birge and Juday (i) have studied this 
problem of dissolved gases particularly on Lake Mendota, which flows 
into Lake Monona, and at that time the conditions in the two lakes 
differed but little. They find that 'the carbon dioxide occurs in two 
forms, the fixed (CaCOs or MgCOs) which is not available for the 
photosynthetic activities of the plant, and the half-bound (CaCOs. 
H2CO3 or MgC03.H2C03) which is such a loose combination that it is 
available for the use of algae. When the free carbon dioxide of the 
water has been exhausted the algae draw upon this supply of half- 
bound carbon dioxide and this produces an alkalinity of the water. 
The degree of alkalinity in Table IV is measured by the number of 
cubic centimeters of carbon dioxide that would be required to convert 
the normal carbonates into bicarbonates and thus give the water a 
neutral reaction. This difference, or degree of alkalinity, is the 
measure of the amount of the half-bound carbonates that have been 
utilized by the algae. The data of Birge and Juday for July 10, 1906, 
may be taken as typical for midsummer conditions in Lake Mendota 
(Table IV). The table shows that the oxygen was practically constant 
Table IV 
Gases Dissolved in the Water of Lake Mendota {after Birge and Juday, j, p. 158). The 
Data Are in the Form of Number of Cc. of Dissolved Gas per Liter 
Depth in Meters 
Temp. 
Half-bound CO2 
Fixed CO2 
Oxygen 
0 
24.2 
-6.6 
35-9 
7.0 
5 
20.1 
-6.1 
36.2 
6.9 
8 
17.7 
—2.0 
364 
4.0 
in the upper five meters, the region from which Volvox was collected, 
while the extent to which the available supply of carbon dioxide had 
been utilized shows that there was comparatively little difference in 
the first five meters. There is then no especially localized density in 
the amount of photosynthetic material or oxygen to cause a localiza- 
