2 i8 William J. Hodgetts 
Section IV), although culture experiments would perhaps be neces¬ 
sary to prove the matter definitely. Others have noted this preference 
of T. affine, for peaty water: West for example ( 31 ) stated that 
T. affine “is not uncommon in peaty ponds and ditches” (l.c. p. 430). 
Doubtless the fact that this species has usually only two small, very 
pale chromatophores lends support to the view that its nutrition in 
nature is mixotrophic (cf. CMamydomonas, Section XVII, and the 
Cyanophyceae, Section XVIII). 
There was doubtless a certain amount of competition between 
the two species of Tribonema in the pond, as a comparison of the 
curves suggests, and in the following section it will be seen that 
there was also fairly obvious competition between Tribonema and 
Microspora—ge nera showing very similar periodicities and usually 
found growing together in the shallow-water marginal region of the 
pond. Competition between these two genera was also noted by 
Fritsch and Rich ( 18 ) in Barton’s pond. 
XII. MICROSPORA 
This genus was never very abundant in the pond, although 
represented by two species, M.floccosa and M. stagnorum, the former 
being the more important. From the frequency-curve of M.floccosa , 
given in Fig. 9, it will be seen that the maxima tend to fall regularly 
in the coldest months of the year, when the monthly mean tem¬ 
perature is between 1-5° and 5-5° C., and there is no doubt that a 
low temperature is the most important factor determining the 
occurrence of this species. This agrees with the observations of 
Fritsch and Rich ( 18 ) who found that the maximum development of 
M. amcena var. gracilis in Barton’s pond was determined by the 
lowest winter-temperatures. The rather late attainment of the 
maximum of M.floccosa in 1918, as compared with 1919, is correlated 
with the fact that November and December 1918 were rather mild 
(cf. Fig. 2); while the general scarcity of the species in the winter 
1920-1921 was doubtless due to this season being very mild. 
As will be seen from Fig. 9, Microspora and Tribonema tend to 
arrive at a maximum at the same time of the year, and since the 
two generally grew together it is fairly obvious that the former genus 
suffered by competition with the latter. Exactly why Tribonema 
always succeeded in getting the upper hand, and operated as a 
limiting factor checking the growth of Microspora, it is difficult to 
say; the actual conditions favouring the last-named genus would 
have to be studied more fully in some pond where its development 
is not restrained by competition with other species. The case of Micro - 
