GILBERT MORGAN SMITH 
Table I shows that during the morning of the 7th there was a 
continued upward migration of the colonies until at noon practically- 
all were in the upper meter. These results are wholly inconsistent 
with those of the previous day. The colonies were in the region of 
maximal sunlight where the amount of light was undoubtedly far in 
excess of the optimum. The measurements of the total illumination 
for the two days are practically the same, so that this movement cannot 
be accounted for on the basis of heliotaxis. It is a well-known fact 
that filamentous algae like Spirogyra rise and fall during the day be- 
cause of a buoyancy from the adherent oxygen bubbles fprmed during 
photosynthesis. The suggestion that a buoyancy of this type may 
have carried the colonies to the surface is inadequate since the illumi- 
nation of the two days was constant. The variation in the gases 
dissolved in the waters of Lake Mendota has been discussed above 
and the assumption made that present-day conditions are the same. 
During recent years the "sludge" from the city sewage disposal plant 
has been emptied into Lake Monona, and it is just possible that the 
oxygen requirements of the undecomposed sewage coupled with the 
lack of aeration in the upper five meters by wave action produced a 
deficiency of oxygen that caused the migration of the colonies into 
the region of intense illumination the second day. 
The third day's investigations showed but few colonies. The 
length of the hose did not permit sampling below eight meters, but all 
depths to the bottom were sampled where the water was less than 
eight meters deep and few colonies were found where the previous day 
the shallow water had been a bright green. This disappearance of 
the alga overnight, while very remarkable, is not inexplicable. We 
know from the studies of Marshall Ward (3) and others that direct 
sunlight kills many algae. Mast (5) has shown that this is also true 
for Volvox. Since the colonies were in full sunlight on the 7th, it is 
not at all improbable that they were killed by this strong light and 
gradually sank to the bottom. We should therefore find no colonies 
on the 8th or on succeeding days. 
Department of Botany, 
University of Wisconsin. 
BIBLIOGRAPHY 
I. Birge, E. A., and Juday, C. The Inland Lakes of Wisconsin. The Dissolved 
Gases of the Water and their Biological Significance. Wis. Geol. Nat. Hist. 
Sur. Bull. 22. 1911. 
