Problems in Aquatic Biology. 165 
inversion, Wittr., and Spirogyra insignis (Hass.), Kiitz. var Brnunii, 
Rabh. Of these the former species formed flocculent coverings on 
leaves and stems of Potamogeton and Myriophyllum , the filaments 
of the Alga bearing large quantities of Gomphoneina olivaceum and 
containing numerous well-developed oogonia. The Spirogyra 
formed floating masses round the edge of 'the pond, and was not in 
quite a healthy condition. No smaller forms were included amongst 
the masses of these two Algae, which were practically pure. 
The second sample was collected about a month later (Jan. 25th, 
1903), and shewed that considerable changes had taken place. The 
Oedogoniuiu (and its epiphytic Gomphoneina) had practically dis¬ 
appeared, together with the Potamogeton on which it was growing ; 
this may, in part, have been the result of the severe frost which 
occurred in the weeks previous to the collecting of this sample, but 
the occurrence of oogonia in the previous month indicated the 
probable subsequent disappearance of the Alga. The Spirogyra, on 
the other hand, had evidently withstood the frost quite well, and 
was, if possible, more abundant than before ; it now, however (as 
Mr. Boodle informed me), occurred as a large floating mass, round 
about Myriophyllum in the middle of the pond, and presented 
the same slightly unhealthy appearance as in the previous month. 
A week or so previous to collecting the third sample (March 8 th, 
1903), Mr. Boodle paid a visit to the pond and observed that the 
Spirogyra had decreased very much in amount, and an examination 
of the third sample showed that it was entirely wanting. The 
exact causes which led to its disappearance are not quite obvious ; 
it has already been pointed out that it was not in a perfectly 
healthy condition on the previous occasions and it is possible (even 
probable) that it underwent conjugation in the (unfortunately pro¬ 
longed) interim, which elapsed between taking the second and third 
samples. No trace of either species was observed in the subse¬ 
quent samples, taken in 1903, although of course either or both 
may have flourished in the second half of the year, in which no 
samples were taken. During the whole period another species of 
Oedogonium (0. crispulum) was observed in small quantity in the 
monthly samples, although it never became common and re¬ 
mained perfectly sterile ; from analogy with what happened in 1904, 
it probably developed sexual organs in the latter half of 1903. 
The first sample collected in 1904 (Jan. 23rd), again revealed a 
little of this same Oedogonium on Myriophyllum. There were just 
traces of a Spirogyra (S. gracilis) which, however, was a much 
