Periodicity of Freshwater Algce in Nature 19 
although Eudorina developed apparently equally well with either a 
falling or a rising concentration, pronounced growth of Pandorina 
always coincided with a rising and never a falling concentration. 
I his may or may not be important, but on the whole it appears 
that mutual competition best explains the observed facts; and thus, 
in accordance with the general rule enunciated at the end of Section 
XII, little relation to the meteorological data, and to the concentra¬ 
tion of the water, is apparent. 
Volvox aureus has its occurrence in the pond given in the table 
at the beginning of this section, from which it is obvious that the 
species is a pronounced spring-form, generally developing during 
March and April, culminating in a well-marked maximum in May, 
and after producing oospores disappearing at the end of May or in 
early June. Temperature is undoubtedly an important factor, a 
monthly mean temperature between 6° and 12 0 C. being most 
favourable, while growth always took place in this pond with a rising 
temperature. The exceptional development of the species towards 
the end of December 1918 (cf. table) is correlated with the fact that 
this month was abnormally mild, its mean temperature being 6-8° C. 
(2*9° above normal). 
The relation of this alga to the concentration of the water is 
exceptionally well shown. From a comparison with the data in 
Fig. 3, it is evident that the development of Volvox aureus coincides 
with the lowest concentrations recorded during the period of observation. 
Information is not available for the April—May maximum of 1918, 
but growth of the species, whether slight or pronounced, at other 
times always occurred with a low concentration (falling or rising! 
between 8 and 14 degrees. The complete absence of Volvox from the 
pond during the dry spring of 1921 was obviously due to the fact 
that a favourable low concentration was never attained during that 
season (cf. Fig. 3). 
Abundant bright sunshine is not essential for the purely vege¬ 
tative growth of V. aureus, as shown in December 1918 (extremely 
dull, but very mild), February 1919 (very dull) and April 1920 
(rather dull), but it is very different when we consider sexual 
reproduction in this species. As will be noticed from the frequency- 
table sexual organs were regularly produced at the end of the 
spring-maximum ( i.e. in May) in each of the three complete years, 
and it has been mentioned several times that May was the sunniest 
month in each of these years (cf. Fig. 2). Thus abundant sunshine, 
with a moderate temperature, appears to be an essential condition for 
