Periodicity of Freshwater Algce in Nature 201 
resulted in the maximum of 1921 falling very early (January— 
March) 1 . 
This species of Zygnema is much more hardy than even the 
hardiest species of Spirogyra. It became really abundant only when 
the temperature was moderate (a monthly mean temperature 
between 5 0 and 12 0 C.). The falling off of the species which usually 
commences in May (April in 1921) is due apparently to the combined 
influence of several factors, of which the increased temperature and 
competition with (Edogonium (see below, Section X) appear to be 
most active. The higher summer temperatures are particularly 
unfavourable, while the lower ones of winter may be tolerated: 
for instance in the winter 1919-1920 this Alga was “rather rare,” 
while in that of 1918-1919 it was “rather common,” apparently as 
a result of the mild November—December of 1918. 
There seems little relation between the growth of Zygnema and 
the sunshine-data (cf. Figs. 2 and 5), unless it is that abundant bright 
sunshine is harmful. The pronounced development during March— 
April, and in October 1918, during April 1919 and 1920, and again 
in January 1921, in every case coincided with a rather dull or even 
a very dull month. The decrease in each of three complete years 
commenced in May (the sunniest month), so that excessive insolation 
may be a harmful factor, but it is difficult to separate its influence 
from that of the rising temperature. 
A comparison with the concentration-curve in F'ig. 3 shows that 
the most pronounced growth of this Zygnema coincides with low and 
moderately low concentrations of the water between 10 and 17 
degrees, although it is clear that the form is very tolerant of other 
concentrations, both lower and higher. The conditions requisite for 
this species to conjugate were apparently not realised in the pond 
during the period of observation; or it may be that the species is 
one which conjugates very rarely, relying upon the production of 
“cysts” for tiding over unfavourable periods. 
Filaments of Mougeotia , although almost always present in the 
pond, are generally rather scarce and more or less isolated. Two 
species (M. sc atari s and M. viridis) have conjugated during the 
period of observation, but possibfy other species have been present 
in a purely vegetative state in addition to these. The most abundant 
species of the genus in the pond was M. scaiaris , a form which tends 
1 West (30) observes that the maximum growth of Zygnema in the upland 
lakes of the British Isles “usually occurs in the late summer and early autumn 
as the temperature is gradually declining.” This obviously is not applicable to 
the species now under consideration. 
5—9 
