Periodicity of Freshwater Algce in Nature 31 
Volvox aureus (Section XVII) is a spring-form with a maximum 
usually in May. Moderate temperatures (a monthly mean tempera¬ 
ture between 6° and 12 0 C.) and a low or very low concentration of 
the water are essential conditions for development. Abundant bright 
sunshine is not at all necessary for purely vegetative growth, but is 
very essential for oospore-production. 
Anabcena oscillarioides (Section XVIII) is a summer-form favoured 
by the higher temperatures (a monthly mean between 12 0 and 
16*5° C.) and also by high or very high (17 to 27 degrees) concentra¬ 
tions of the water. Its nutrition is probably mixotrophic. A sudden 
decrease in the concentration is very unfavourable for this species. 
Little or no relation towards bright sunshine is shown. Cylindro- 
spermum majus is also a summer-form adapted to rather lower 
temperatures than Anabeena, and prefers a monthly mean tem¬ 
perature between 12 0 and 14 0 C. It is adapted to high or rather 
high concentrations, but its growth in the pond was adversely 
affected by Anabeena. 
Oscillatoria, Phormidium and Arthrospira Jenneri (Section XVIII) 
show a marked relation to the concentration of the water, the 
position of a maximum generally coinciding with a high concen¬ 
tration, so that these algae are doubtless mixotrophic. High or 
moderately high temperatures (a monthly mean between 8° and 
16° C.) appear to be best, the maxima usually occurring in the 
autumn. No relation to bright sunshine is apparent. 
Cryptomonas ovata (Section XIX) is a hardy form, but generally 
more abundant during July—September (and when the monthly 
mean temperature is between 13 0 and 16-5° C.). High or rather high 
concentrations (between 17 and 21 degrees) are decidedly favourable; 
while no relation is shown towards bright sunshine. 
Trachelomonas volvocina (Section XIX) shows a periodicity which 
appears to be best explained by reference to bright sunshine and 
the concentration-conditions, since the conjunction of abundant 
sunshine with a high (or rather high) concentration, and during the 
warmer months of the year, appears to supply the best conditions 
for growth. It is suggested that this species may to some extent be 
able to vary its mode of nutrition in nature, being under certain 
conditions mainly autotrophic and under other circumstances 
mixotrophic. T. hispida agrees closely in its mode of occurrence 
with T. volvocina but its development always occurred with moderate 
concentrations. 
In conclusion I wish to acknowledge my indebtedness to the late 
