THE LAW OF THE MINIMUM IN THE SEA 205 



of the diatoms were caught in the net. Therefore, the 1*5 cubic 

 metres of water contained about 1*5 grms. of silica in the form 

 of diatoms ; that is, the water contained, roughly, about one part 

 in a million of organic silica. Now Raben's results showed that 

 Baltic water contained from 1*45 to 0*65 parts in the million of 

 inorganic silica. Since it is not possible that diatoms can 

 absorb more than a large fraction of the dissolved silica, we 

 must conclude, then, that this substance was present in minimal 

 proportion. 



But this was an extraordinarily large catch of diatoms, and 

 although the silica was certainly present in minimal quantity 

 during the period of the year when the diatoms were at their 

 maximum of abundance, it must still be remembered that this 

 period is only a short one, and that at other times in the year 

 the silica may be present in such proportion that it need not 

 necessarily be regarded as minimal in amount. We have, there- 

 fore, to consider also the nitrogen compounds, and indeed the 

 problem is a very complex one. One is usually inclined in such 

 discussions as these to emphasise the importance of some one 

 particular factor, and to forget that there are usually many 

 others which must also be taken into account. If, as seems 

 clearly the case, the inorganic nitrates, nitrites, and ammonia 

 are also present in less abundance during the autumn than in 

 the spring, then we have to consider the probability that the 

 abundance of these foodstuffs may also influence the production. 



Are the nitrogenous foodstuffs present in the sea in minimal 

 quantity ? We can answer this question if we know how much 

 nitrogen is present in the sea in the form of living matter, and 

 if we compare this amount with the amount which is present 

 in the same volume of water as inorganic salts in a dissolved 

 form. That is, the chemical composition of the plankton, as 

 well as that of the sea water, must be determined. Brandt 

 made a number of such determinations of the composition of 

 the plankton taken in a quantitative net in Kiel Bay in 1892 

 and 1893, and his principal results which bear on this question 

 are as follows : the material collected was washed, so as to 

 get rid of the sea-water, and dried at ioo° C, and the nitrogen 

 was determined by the ordinary processes of organic analysis. 

 The percentage of nitrogen in the dry substance varied with 

 the nature of the organisms caught ; thus diatoms are poor in 

 albumen, while the naked protozoa are richer. The average 



