THE PERIODICITY OF FRESHWATER ALGAE 
127 
which the species occurring in a particular pond will fruit. If we 
take one of the larger genera like Oedogonium, Spirogyra or Zygnema, 
we find a remarkably regular annual succession of species. In a 
general way there is an evident correlation between the length of 
the vegetative phase and size of the cells. In the case of the Spiro- 
gyras I have attempted to analyze this relationship more definitely. 
If we arrange the species of this genus in the order of their time of 
Fig. 2. Above is the curve for the specific surface against the time from March 
fruiting. Below is the curve for the temperature against the time to fruiting, 
fruiting, we can compare this order with that of the cell diameters, 
cell volume, and total cell surface. There is no very close coincidence 
between the order of any of these dimensions and the order of lengths 
of the vegetative cycle. When, however, the total surface is divided 
by the volume and these quotients are compared, i. e., when the 
specific surfaces of the cells are arranged in the order of their magni- 
tude the correlation is very striking, and we are justified in assuming 
