204 SCIENCE PROGRESS 



and it should be noted that the abundance and extent of varia- 

 tion of the diatoms are so much more considerable than those of 

 other organisms, that the variations in the amount of plankton 

 obtained are due almost entirely to these organisms. Note, 

 then, that there are two maximal periods of reproduction of 

 the diatoms — a principal maximum in April and a secondary 

 maximum in October. 



The thick line shows the variation in the amount of silica in 

 the water of the Baltic. This substance is most abundant in 

 February and least so in May. From February to the latter 

 month the amount diminishes very rapidly — just as the abun- 

 dance of diatoms falls off very rapidly. Then we have a 

 secondary increase in the amount of silica, and also an increase 

 about the same time in the abundance of diatoms. During the 

 winter the amount of silica in the sea has been accumulating, 

 and in the spring, when the increased sunlight affords the 

 stimulus for the increased propagation of vegetable life in the 

 sea, this accumulation of foodstuff is made use of and there is 

 a rapid increase in the mass of vegetable life. But this reduces 

 the proportion of silica, and the minimum values for the amount 

 of the latter are'found in May, just after the diatoms have been so 

 abundant. From May to August the silica again accumulates, and 

 we find, about the latter period, that there is usually a second 

 period of diatom abundance. 



We can consider the silica, then, as a substance which is 

 sometimes present in the sea in minimal proportion. This is 

 certainly the case during those times in the year when the 

 diatoms are present in maximum abundance. Brandt has esti- 

 mated the weight of silica which is contained in the sea in the 

 form of the skeletons of diatoms during the time when these 

 organisms are most abundant. 1 In one catch, made in the Bay 

 of Kiel on May 15, 1893, there were 3,173 millions of diatoms 

 (mainly Chcctoceros). The estimated dry weight of this mass of 

 diatom substance was 0*99 grm., and this contained o"54 grm. 

 of silica. This catch was contained in a volume of water which 

 was estimated as 1*5 cubic metres. Now it is well known that 

 the quantitative plankton net does not catch all the diatoms 

 which pass through the opening, for a number of these are so 

 small as to slip through the meshes of the silk employed in the 

 construction of the net. Brandt estimated that about one-third 

 1 Brandt, " Beitr. Chem. Zusamm. Planktons," loc. cit. 



