i59 
Problems in Aquatic Biology. 
period of prevalence is past they disappear either suddenly or 
gradually. They may remain in small quantity for some time or 
during the whole year or vanish completely. Apart from the 
Cladophoraceae very few of the filamentous Algas are found to be 
present all the year round ; in the case of the smaller forms (Proto- 
coccales, Diatoms, etc.) there are more examples of this kind of 
thing. Even the few forms, however, which are always present, 
often show a well-marked period of maximum abundance and 
during the rest of the year they exist in much diminished amount. 
During the period of complete absence a form is of course repre¬ 
sented by its resting-spores, but they sink to the bottom of the pond, 
are lost to view, and as far as the aspect of the flora is concerned, 
they are momentarily gone. Aquatic vegetation thus often shews 
a varying composition according to the time of the year at which it 
is examined and hence the necessity of caution in drawing con¬ 
clusions regarding the vegetation of a piece of water from one or 
two casual examinations, until periodical observation of a number of 
cases has established a basis to work upon. In the case of an algal 
formation the character-form or forms may be altogether absent at 
some times of the year and it will only be when we know something 
about the subordinate members of the formation that we shall be 
able to recognise it at these periods. The different phases of a 
formation require far more consideration in the case of an aquatic 
flora than of a terrestrial flora. An excellent method of obtaining 
a picture of the aspects of a pond during a prolonged period of 
observation is the plotting out for each species of a curve repre¬ 
senting its relative abundance (cf. the chart on p. 160). 
In a recent paper on “Pond Vegetation,” by Norman Walker, 1 
eight associations are enumerated as occurring in the Bramhope 
ponds in the course of a year. Prior to the publication of a more 
detailed communication on this subject it is perhaps a little premature 
to criticise ; but, from what I have seen in other cases of periodicity, 
I should not be inclined to describe the successive kinds of algal 
growth as associations, but rather as phases of one and the same 
association. I should add that it seems possible that the way in 
which the different phases succeed one another and their degree 
of relative importance may often be of considerable value in 
discriminating between allied algal formations. 
Periodicity in aquatic vegetation may be of two kinds, viz., 
seasonal and irregular. Seasonal periodicity is the result of 
1 Naturalist, October, 1905. No. 585, p. 311. 
