GERMAN PLANKTON INVESTIGATIONS. 319 



bulk of the ocean is unproductive of vegetable life, since 

 it lies to a greater depth than 400 metres. It is certain 

 that the Diatoms and Peridinese play a far greater part in 

 the cycle of matter in the sea than do the attached Algse. 

 The animals which inhabit the greater depths may be 

 ultimately indebted to the spores of Diatoms and Peri- 

 dinese for their food supply, which spores are set free in 

 enormous numbers, and contain, as it were, a concentrated 

 extract of the organic substance of the plant. It is certain 

 that Copepoda do not use Diatoms as food to any large 

 extent, and it is as yet unknown what, if any, groups of 

 animals habitually live on Diatoms, so that apparently the 

 greater mass of the vegetable plankton is useless, at any 

 rate directly, as a food for the animal part of the same. 

 The Diatoms must consequently die and putrify, and it is 

 known that the bottom of some of the shallower seas is 

 covered by a large quantity of decomposing material. 

 Behrens discovered that the percentage of combined nitro- 

 gen at the bottom of the sea varied from 0T8 to 0"4, which 

 exceeds the percentage of the soil of the land considerably. 

 The soil of the continents is likewise enriched by a large 

 quantity of vegetable material, which is not directly used 

 up by animals, but which is utilised by dispersion and 

 decomposition. 



Hensen has determined how much plankton is daily 

 generated in the Baltic. The estimation bristled 

 with difficulties, and the result must be considered 

 as a minimum one. The conclusion arrived at was 

 that for every square metre of surface water, omitting 

 from consideration matter devoured by the animals of 

 the plankton, there is daily generated at least 18 ccm. of 

 plankton, giving for a year (j,57() ccm. This mass con- 

 sisted principally of Diatoms, and according to the weight 

 estimation method would contain from 14*8 to 177 grams 



