305 
The importance of light for the photosynthesis of chlorophyll- 
bearing plants is unquestioned. The liberation of oxygen by the 
plant declines as the light fades, and is at its lowest ebb in darkness. 
The access of light to the phytoplankton is limited by several factors 
of the environment, principally by silt, which increases the turbidity, 
and by clouds, which interfere with the penetration of the sun’s rays. 
The fluctuations of the silt are chiefly the result of floods, and, as 
above stated, the floods do not exhibit a rhythmic pulse which can 
be correlated with that of the phytoplankton; much less do the 
periods of rising water which are most silt-laden. The cloudiness 
of the sky varies greatly at different seasons of the year, being 
predominant at times in the autumn or winter months. It is sub- 
ject to pulse-like occurrences of variable duration, but an examina- 
tion of the records for central Illinois for the years under discussion 
does not disclose any periodic rhythm which can be correlated 
continuously with that revealed in the statistical records of the 
growth of the phytoplankton. 
Another factor of the environment which modifies the quantity 
of light which impinges upon the chlorophyll-bearing organisms 
of the plankton is the light from the moon. The amount of light, 
both absolute and relative, derived from this source is not great. 
According to the calculations of Zéllner, the light from the sun is 
618,000 times as bright as that from the full moon. In the pre- 
sent connection it is only important to know whether the moon- 
light contains an amount of solar energy sufficient to appreciably 
affect the photosynthesis of the phytoplankton. The amount of 
such energy utilized in photosynthesis is relatively a small propor- 
tion of the total, so that there is a possibility that moonlight may 
contribute to the process to an appreciable extent. 
This matter was investigated by Knauthe (’98), who determined 
the fluctuations in the gaseous contents of the waters of carp ponds 
rich in Euglena. While this author does not report upon the plank- 
ton of the ponds investigated, it seems quite probable that carp 
ponds rich in Euglena would present conditions very similar to those 
found in the Illinois River, which has a remarkably well-developed 
Euglena water-bloom, and abounds also in carp. 
The following table presents the results of his work bearing 
upon the point in discussion. 
