F. E. Fritsch. 
168 
September 18th) were species of these two genera observed to be 
passing through the sexual stage of their life-cycle at the same 
time. This seems to indicate, that the same conditions influence 
the appearance of the species in question and the occurrence of the 
sexual process in each. This point is so important, that it may be 
well to define a little more clearly the exact condition of the pond 
previous to May 1 and September, 1904, i.e. previous to the occurence 
of the sexual process in these species. Previous to collecting the 
May sample the weather was very warm for the time of the year 
and very fine with little or no rain ; the water must have become 
fairly warm on most days (on the day of collecting for instance 
temperature=70 Q F.) and the concentration of the dissolved salts 
probably steadily increased. These may possibly be the more 
important causes, which led to sexual reproduction ; the Ocdogoniuin, 
it will be remembered, already exhibited sexual organs on April 20th, 
and the April of 1904 was also very mild and fine (temperature of 
water on April 20th=61°F). It may be pointed out in this 
connection that the previous year (1903) was very abnormal, being 
unusually wet and relatively cold (involving the occurrence of a 
number of irregular factors) and this may possibly be the reason 
why the phenomena above recorded for 1904 were not also 
observed in 1903. The weather during the first half of September, 
1904, was very cool (temperature of water on September 18th 
=58°F) and there was a good deal of rainfall; so that in the second 
case it would seem as though the sudden cooling was one of the 
stimuli which gave rise to the second sexual process then observed 
(in Oedogonium crispulum and Spirogyra gracilis). An important 
factor in this case was probably also the increase in amount of 
organic substance dissolved in the water. Alteration in the 
intensity of the light is also a possible factor in both cases 
(cf. general discussion above). 
We have so far considered only the dominant members of the 
algal vegetation of this pond, but there are a number of other species, 
which play a fairly important part in its constitution and we may quite 
briefly notice them. Oscillaria tenuis , Ag. is practically wanting 
during the early months of the year, but becomes more abundant 
with the commencement of the warm weather in June (1903 and 
1904) when it forms a thin film covering the mud at the bottom of the 
pond ; its maximum development however falls into September and 
1 It should be noted that on this occasion two other species 
were in the sexual stage (viz. Coleuchcute scutata and 
Chara sp.) 
