and 1897 Petersen took 3 series of plankton samples, beginning in the North Sea 
and extending through the Limfjord and out into the Kattegat, in one case right 
down into the Baltic. The plankton samples were taken with Hensen's quantita- 
tive net. The quantity of plankton was determined, partly from its volume and 
partly from its weight. It appears from these investigations, that the North Sea 
is poor in plankton in comparison with the Limfjord and the Kattegat. The Lim- 
fjord at certain times of the year is rich in plankton organisms, chiefly diatoms. 
Two plankton maxima occur, one in the spring and one in the autumn. The 
diatom plankton arises in the Limfjord itself and does not enter from outside. I 
shall return later to these plankton investigations. 
The sources for the organic materials in the sea are mainly the vegetation 
there. The vegetation in the sea falls again into two divisions, the vegetation of 
the Zostera region and the plankton organisms. The question is now, whether 
these two groups are of equal importance in quantitative regards, or whether one 
of them plays the predominant role as source for the organic matter and as food 
for the ånimal world of the sea. To this question, the vegetation of the 
Lostera belt or the plankton orgaåanisms, we shall endeavour to give an answer in 
the following chapter. 

II Deposits of organic matter on the sea-bottom. 
l. Introduction. The organic matter produced by the plants is at last, 
in so far as it is not previously destroyed, deposited on the sea-bottom. Late in 
the autumn the Zostera leaf-blades are thrown off, whilst the leaf-stalk, as Osten- 
feld has explained, remains. The loosened leaves drift about for some time on 
the surface of the water, supported by the air-vesicles the Zostera leaves contain. 
At last they sink to the bottom and remain sometimes in quiet localities, in inlets, 
channels and the like, as the »dead-weed«. More frequently, however, the Zostera 
leaves are torn in two, partly mechanically, partly by the destructive agency of 
various bacteria, and are scattered over the sea-hottom in the form of small, more 
or less unrecognizable particles. The algae, so far as they are attached, are also 
torn loose and carried away from the place where they grow. They do not contain 
air-vesicles like the Zostera (with exception of Fucus and Halidrys) and they are 
not met with therefore floating on the surface. The plankton organisms also, in 
so far as they are not destroyed, must end on the bottom. 
It is thus characteristic of the metabolism of the sea, that the 
organic materials do not remain at the place where they were produced 
but are distributed more or less uniformly over large areas. It is easy 
to see that this condition has quite a supreme importance. As mentioned in the 
previous chapter, the vegetation in the sea (apart from the plankton) is greatly 
localised in contrast to the vegetation on land, as only an inappreciable part of 
