president’s address. 
171 
one square metre of surface, calculated for the Grosser 
Ploner See. 
In this table you will notice that some of the figures are 
almost astounding ; e.g., it is stated that on the 31st day of 
July, 1892, there were about 3,811,400,000 individuals of 
the diatom Fragillaria crotonensis under 1 square m. 
Of this detailed work on the abundance and periodicity 
of plankton, Wesenberg-Lund, who is at the head of a Danish 
fresh-water laboratory, says there is now no need, except 
perhaps for lakes, the physical conditions of which are very 
much out of the ordinary, and he characterises the elaborate 
counting work as superfluous, and erring on the side of too 
great accuracy ; for the results give us information which 
from lack of knowledge in other directions we are not in 
a position to make use of. 
He himself has studied the seasonal differences in growth 
of species, and finds in general that this takes one particular 
form, viz., a growth of the longitudinal axis and a greater 
development of the floating apparatus in correlation with 
the seasonal changes which take place in the specific gravity 
of water. 
German writers lay stress upon the value of plankton 
researches to the scientific cultivation of fish ; but I think 
more has been made of this point than is necessary, especially 
as the cultivation of plankton to feed the fish has not yet 
been suggested. 
Apstein has elaborate calculations to show the weight of 
organic substance and ash in various species, and gives the 
following figures amongst others : — 1 Bosmina has 0.00079 
mgr. of organic substance in its composition, 1 Diaptomus 
graciloides 0.00449 mgr., 1 Cyclops oithonoides 0.00308 mgr., 
1 Daphnia pulex 0.0272 mgr. He calls attention to the 
fact that on this calculation a fish which swallows 
1000 Daphnias in a day takes in 27.2 grs. of food. Now 
Regnard says that a Pike devours 30 kilos, of fish to increase 
VOL. VIII. 
N 
