Periodicity of Freshwater Algce in Nature 27 
It has been shown by certain workers that many algae in cultures 
prefer a mixotrophic method of nutrition, and there is no doubt that 
further research on the lines suggested by the present paper will 
show that the same obtains with many algal species (especially 
certain of the Volvocales 1 , and many Blue-green Algae, and also 
Flagellates) in nature, more particularly those which at times develop 
in great abundance in the form of “water-bloom 2 .” 
XXI. SUMMARY OF THE CHIEF RESULTS 
Spirogyra (Section VII) is best represented in the spring-phase 
(maximum in May) although S. infiata, S. protecta, S. colligata and 
S. varians appeared and conjugated in the autumnal phase (October 
—November). A monthly mean temperature between 6° and 12 0 C. 
appears to be best for the growth of this genus, although the hardier 
species ( 5 . infiata, S. protecta) can tolerate much lower temperatures. 
S. bellis appears to prefer rather higher temperatures. Abundant 
bright sunshine is very essential for growth of the more sensitive 
species of Spirogyra, but S. infiata and S'. protecta may develop in 
months rather poor in sunshine. A moderately low concentration of 
the water between 12 and 18 degrees in the pond is best suited for 
Spirogyra, although again S. infiata and 5 . protecta are more tolerant 
in this connection than the other more sensitive species (S. catence- 
formis, Grevilleana, condensata, Jurgensii, mirabilis, longata, maxima, 
Weberi) ; very high concentrations are particularly harmful, and 
prevented the development of an autumnal phase of Spirogyra in 
1919. A sudden increase in the concentration is unfavourable 
towards the more sensitive species of the genus. 
Zygnema (Section VIII) has its maximum at the end of April or 
in early May, but may be rather common again in the autumn. It 
is very hardy but became really abundant only when the temperature 
was moderate (a monthly mean between 5 0 and 12 0 C.), and the 
concentration of the water low or moderately low (10 to 17 degrees). 
Mougeotia scalaris (Section VIII) is a hardy form as regards its 
ability to withstand extremes of temperature, and varying con¬ 
centrations of the water. Abundant bright sunshine is not necessary 
for good vegetative growth, but is very essential for zygospore- 
1 Although certainly not in the case of Volvox, the highest member of this 
order (cf. Section XVII). 
* Cf. in this connection the observations of Pliimecke on Gonium pectorale 
(in Ber. Deutsch. Bot. Ges. 32, 1914, pp. 131-136); and also the paper of 
Kolkwitz and Marsson (23). 
