Periodicity of Freshwater Algce in Nature 29 
production in these species; while medium concentrations of the 
water, between 16 and 20 degrees, are most favourable. Bulbochcete 
minor closely resembles the narrower species of CEdogonium as regards 
the conditions requisite for its development and reproduction. 
Tribonema bombycinum (Section XI) develops best with a falling 
temperature in autumn, and remains abundant during the lower 
temperatures of winter. It is very adversely affected by bright 
sunshine, and tends to develop in parts of the pond shaded by the 
leaves of various phanerogamic plants. It is very tolerant of almost 
all degrees of concentration of the water, the very highest concentra¬ 
tions being only slightly unfavourable; but a very abrupt fall in the 
concentration may be harmful. T. affine agrees closely with the 
former species in the matter of temperature and also sunshine, but 
prefers the very highest concentrations observed during the period 
of observation, and thus is probably mixotrophic in its mode of 
nutrition in nature. 
Microspora floccosa (Section XII) tends to develop during the 
colder months (with a mean monthly temperature between 1-5° and 
5-5° C.); and bright sunshine is not essential for its growth, and is 
probably harmful. Little or no relation is shown towards the varying 
concentration of the water, probably because competition with 
Tribonema tended to greatly limit its growth in the pond. M. 
stagnorum is also a cold-water species which was never abundant in 
the pond owing to competition with Tribonema and M . floccosa. 
It is a general rule that the occurrence of any alga which is 
struggling to develop under conditions in which it is adversely 
influenced by competition with other species (of animals or plants) 
will tend to show little relation to the various meteorological con¬ 
ditions or to the concentration of the water, since competition is the 
particular factor which sets a limit to the growth of the species, 
even though all other factors may be favourable towards its increased 
growth. 
Vaucheria aversa (Section XIII) prefers a low or moderately low 
temperature (a monthly mean between 4-5° and 8° C.), abundant 
aeration of the water, and probably also a low or rather low con¬ 
centration of the latter; while bright sunshine is not at all an 
essential factor for oospore-production. V. sessilis apparently differs 
in its requirements from V. aversa , but the evidence is incomplete. 
Aphanochcete repens (Section XIV) tends to be a perennial species 
in the pond, regularly showing a maximum in spring, and sometimes 
a secondary maximum in autumn. It prefers moderate temperatures 
