Periodicity of Freshwater Algce in Nature 227 
this Alga, but in each of the three complete years the decline after 
the maximum coincided exactly with a marked decrease in the 
concentration, a coincidence which can hardly be considered as 
accidental. 
Pediastrum Boryanum and P. tetras are both favoured by the 
higher summer temperatures, a monthly mean temperature (of the 
air) between 13 0 and 16 0 C. being requisite for pronounced growth; 
and in no case did a high temperature ever influence these two 
species adversely. Little relation is shown by Pediastrum towards 
the concentration of the water. In the case of P. Boryanum the 
increase to the normal maximum may take place either when the 
concentration is very high (August 1919), or moderately high 
(June—July 1918 and July—August 1920); while the moderately 
low concentrations during September—October 1920 apparently 
affected neither species of this genus adversely, the decline at the 
end of October 1920 being due to the lowered temperature. Tem¬ 
perature thus appears to be far more important than concentration 
of the water in determining the development of Pediastrum. 
In concluding this section it may be noted that these five members 
of the Protococcales, in regard to the temperatures most favourable 
for their development, form a sort of series, at one end of which is 
Ankistrodesmus , for which a monthly mean temperature of the air 
between n° and 14 0 C. is most suitable and the higher temperatures 
unfavourable, while at the other extreme there are the two species 
of Pediastrum , adapted to the highest summer-temperatures, while 
the two remaining forms are intermediate as regards their behaviour 
towards temperature. 
(To be continued) 
