THE PERIODICITY OF FRESHWATER ALGAE 
3. Four distinct periods may be recognized in the life history of 
most fresh-water algae: germination, vegetative development, re- 
production, and dormancy. 
4. The great majority of zygospores, oospores, and aplanospores 
germinate in the spring. There is germination going on however at 
all times, and a secondary maximum occurs in the autumn. 
5. The factors involved in the germination of the spores are 
probably as numerous as those initiating germination in seeds. The 
importance of temperature has probably been overestimated. 
6. The length of the vegetative period in some forms is quite 
indefinite. In the Zygnemales and Oedogoniales it probably has a 
definite length under normal conditions. 
7. Temperature, light intensity, concentration and mineral con- 
tent of the water accelerate or retard the approach of the reproductive 
period. 
8. The normal length of the vegetative cycle in Spirogyra is an 
inverse function of the specific surface of the cells. This is possibly 
also true in Zygnema and Oedogonium. 
9. The normal length of the vegetative cycle in species of Spirogyra 
is approximately equal to a constant (65) divided by the specific sur- 
face times the temperature. 
10. The concentrations of the waters in pools, ponds and surface 
streams attain their maxima in early spring and autumn, corresponding 
in general with the periods of heavier rainfall. 
11. The lowest concentrations occur in late winter, and at the end 
of a prolonged drought in summer. 
12. The periods of most abundant fruiting of algae correspond 
with the periods of high water levels. 
13. The concentration of natural waters at their maximum is so 
small in comparison with the concentrations of the cell sap that it is 
doubtful whether it is of any significance in initiating reproduction. 
14. In the Zygnemales, lateral conjugation, scalariform conju- 
gation and aplanospore production appear to be hereditary tendencies 
rather than the result of environmental conditions. 
Ohio State University, 
Columbus, Ohio 
