STUDIES OF BRITISH FRESHWATER ALG^. 
49 
conditions, which are liable to change in spring, that is the cause of the 
prevalent vernal reproduction of species of Spirogyra. It remains to 
support these theoretical conclusions by experiment, and it is hoped 
to do this in the near future. 
Apart from Spirogyra we have observed sexual reproduction only in 
Vaucheria, which was found with sexual organs from August — 
November, 1905. The genus was also observed, though not in a repro- 
ductive condition, in 1904, but has not been seen since 1905. 
Although, as mentioned above, we consider that it may have been 
overlooked in collecting, it is not likely that it was ever present in any 
quantity in the pond after 1905. The year 1905 was altogether a 
phenomenal one for the algal flora of the Fish Pond (cf. p. 34), 
(Edogonium, for instance, also attaining its most prominent develop- 
ment, during the whole period of observation, in that year. It seems, 
therefore, that the conditions were particularly favourable for algal 
growth, and these may have called forth a more abundant development 
of Vaucheria, and led to its reproducing sexually, but it is quite 
impossible at present to say anything as to what the influencing 
conditions were ; this occasional occurrence of a particular form 
. (sometimes even in great quantity), followed by complete disappearance 
for a prolonged period, is a feature we have frequently observed in 
different ponds, and is at present one of the most puzzling problems 
confronting the student of algal biology. 
It remains to say a few words about the reproduction of Melosira, 
the only Diatom in which we have noted anything but vegetative 
division. 1 Auxospores were found on the Melosira in March, 1904 ; 
February, March, and June, of 1905 ; April and November, 1906 ; 
January, 1908. As there were usually not a very large number of 
individuals bearing auxospores, it is quite possible that such may have 
been present in other months also ; nevertheless, the evidence that we 
have points quite clearly to the fact that auxospores tend to form during 
the winter-months, and especially towards the end {i.e. February and 
March) of the period of chief abundance of the Melosira. The presence 
of auxospores in November of 1906 tallies with the fact that this was 
the period of maximum development of Melosira during the winter of 
1906-7 (cf. table on p. 43), and that after November, 1906, there was a 
marked decrease in the amount of this Diatom present (see especially 
January of 1907). The development of auxospores in the June of 
1905 is thus the only abnormality, and is another of the special 
peculiarities which were so evident in the algal flora of the Fish Pond 
during 1905. It may be noticed that there was a slight increase of 
Melosira in June of 1905 (note the exceptionally heavy rainfall during 
that month !) as compared with the preceding two months, and that 
auxospore-formation went hand in hand with this increase, while it was 
followed by temporary complete disappearance of the Diatom. 
^ All the other Diatoms appear to persist all the year round in small 
numbers, so that their reappearance is due to rapid division of these 
persisting individuals. 
D 
