Problems in Aquatic Biology. 167 
z yg°sp°r e -f or mation was going on. Judging from analogy, it 
probably disappeared at the commencement of the next year. 1 
In this pond we must therefore distinguish at least three 
species of Spirogyra and two of Oedogonium. The first species of 
Spirogyra, S. insignis (Hass.) Ktz., which flourished in December— 
January, 1902-3, disappeared completely and has not subsequently 
been observed. It must remain an open question as to whether it 
has developed zygospores, which are passing through a prolonged 
resting-period, or whether owing to some cause or other it became 
completely exterminated in the early part of the year, 1903. The 
second species,—S. gracilis (Hass.) Ktz.—was first observed in the 
winter of 1904 and had possibly flourished during the last months 
of 1903 (cf. September to December, 1904) ; it then disappeared 
from March to August, 1904, reappearing again in September, from 
which time onwards it flourished till the last of the samples on which 
these observations are based was taken. The third species of 
Spirogyra (S. condensata) was first observed in the spring of 1904, 
attained its maximum in May, and disappeared again by July. With 
reference to the two species of Ocdogoniuin, the first (0. inversion) 
occurred abundantly in the winter of 1903, but not at the same period 
in 1904, whereas it flourished and fruited in the early part of the 
summer of 1904; after that it again disappeared. The second 
species, (0. crispulum), occurred in slight quantity during the whole 
period of observation, and was observed to become sexual in 
September, 1904, (possibly also in September, 1903?), after which 
it practically disappeared. 
The observations recorded in the preceding paragraphs seem 
to me to bring out two important facts. In the first place the 
three species of Spirogyra attained their maximum at different 
times of the year, one in mid-winter, one in summer and the third 
in autumn. This may be due to the inherent tendency of each 
species or to reproduction in the three cases depending on different 
combinations of external factors. Comparison with the times 
and conditions of reproduction of the same species in other pieces 
of water will help us to settle this point and a number of data of 
this kind will be submitted in the later publication. The second 
and more important point is the fact that each time a Spirogyra 
became abundant in the pond a species of Oedogoniuni followed 
suit (see especially the chart); and still more striking is the point 
that on two distinct occasions during 1904 (viz. May 15th and 
1 It was unfortunately impossible for Mr. Boodle to continue 
collecting after 1904. 
