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even more impressive, Johnstone (1908, p. 163) has calculated that on the basis 
of this haul "every drop of sea water from this part of Kiel Bay contained some 
200 diatoms;” and though by Hensen’s (1887) calculations less than one-tenth as 
many diatoms as this are present on the average in the West Baltic, their numbers 
are sufficiently appalling when extended to any considerable sea area. 35 
During the years that have passed since Hensen’s pioneer studies in this field 
many similar counts have been made in the Baltic and in various parts of the North 
Sea, with the details of which it is unnecessary to delay here. 36 Lohmann, for in- 
stance (1908, Table B), has recorded some very large counts by the centrifuge 
method, including 7,800,000,000 Skeletonema per cubic meter in June, 1906, with 
another individual catch of about 2,000,000,000 diatoms in Kiel Bay on April 11, 1906. 
As still another example of the results of this modern method, the accuracy of which 
leaves little to be desired, though, as Gran (1915) himself points out, it is not of 
universal application, I may quote his own average of about 228,000,000 diatoms 
per cubic meter in the surface waters of the Skager-Rak for February, 1912. 
The centrifuge, however, is not the "last word” in quantitative determination of 
the phytoplankton, for E. J. Allen (1919) has recently essayed the following totally 
novel procedure: To a small sample of sea water (0.5 cubic centimeters) he added a 
large amount (1,500 cubic centimeters) of a nutrient solution that had previously been 
found suited for the cultivation of marine diatoms (Allen and Nelson, 1910; E. J. 
Allen 1914). The culture was then examined after a period of incubation, where- 
upon he found a total of 232 different kinds of organisms. A second experiment 
yielded similar results. Since now it is obvious, to use his own words (E. J. Allen, 
1919, p. 4), that each of these organisms "must have been represented by at least 
one individual or unit, either cell or spore, in the original Yz cubic centimeter of 
sea water from which the experiment was started,” the latter must have contained 
at least 464 organisms (mostly diatoms) per cubic centimeter — -that is, 464,000 per 
liter — and probably, as he calculates, as much as 1,000,000 per liter for the part of 
the English Channel whence his sea-water sample was taken. How much more 
effective this method is than centrifuging, even for such comparatively large organ- 
isms as diatoms (for which the culture method is particularly well adapted, as 
indicated by their great predominance in the final product), is illustrated by the fact 
that whereas the two culture experiments call, respectively, for 378,000 and 290,000 
diatoms as the absolute minimum per liter, centrifuging a similar sea-water sample 
at the beginning of the experiment revealed only about one-thirtieth as many. Nor 
can even the method of the culture medium be relied on to give a total census of the 
phytoplankton, because it is by no means certain that the nutritive fluid employed 
was as suitable for the growth and reproduction of peridinians, infusorians, coccoli- 
thophorids, etc., as it was for diatoms. In short, as Herdman says (1920, p. 819), 
"every new method devised seems to multiply many times the probable total 
population of the sea.” 
35 There has been much discussion as to the reliability of numerical results yielded by nets of the “Hensen” type, owing to 
uncertainty as to their coefficient of filtration. In the present connection it is enough to point out that in any case the ostensible 
results are always smaller, never larger, than they should be 
36 For details of such I may refer the reader to Hensen (1887) himself, Driver (1908), Lohmann (1903 and 1908), and Oran (1915). 
