CHAP. XXXVIII PLANKTON-FORMATION 159 



have no sutures. Ground-diatoms, on the contrary, do not show the 

 various outgrowths described above. 



Quantity of plankton. Their power of rapid division seems 

 primarily responsible for the frequently enormous multiplication and 

 great abundance of plankton-organisms. Their quantity, however, 

 varies with time and place. ' Pure blue is the desert-colour of the high 

 sea. With the green of meadows we may compare the colour of the 

 vegetation of arctic waters ; yet the colour of the most vigorous vegeta- 

 tion, of the greatest abundance of plant-life, is the dirty greenish yellow 

 of the shallow Baltic Sea.' ^ Hensen ^ invented and employed methods 

 for estimating the quantity of plankton.^ His object was to determine 

 tlie amount of organic matter produced in the sea at a definite time and 

 place; and this object is of profound importance, since all marine animal- 

 life, both lowly and highly organized, is dependent upon those plankton 

 organisms that are plants, or at least assimilate carbon dioxide : plankton 

 is the ultimate source of food, which it perhaps supplies largely in the 

 form of fat, manufactured by diatoms and by Peridineae, from which 

 originate the great quantities of oil in marine animals (sea-gulls, whales, 

 and all zoo-plankton). 



It is of no slight interest to note that it is not, as in the land-flora, starch, 

 with its greater specific gravity, but oil, which is the main product of 

 assimilation in the floating plankton-community. 



The first to appreciate the significance of the microscopic plant-world 

 of the sea as the ultimate source of food-supply for animals was the 

 Danish botanist A. S. CErsted, who came to this conclusion as early as 

 1845-8, when on his journey to Central America.* 



Composition of plankton. A distinction may be made between 

 homogeneous and heterogeneous plankton. Plankton is sometimes 

 extremely rich in species, but at other times, particularly when the 

 organisms are so abundant as to colour the water, it is dominated by one 

 or a few species, as in the peridinia-plankton in western parts of the 

 Baltic, and in the diatom-district of arctic seas. It is especially the 

 Diatomaceae, Peridineae, and Cyanophyceae that lend a colour to water. 

 The colour of Baltic lakes is mainly determined by those of the chromato- 

 phores of the dominant plankton-organisms. In harmony with the 

 periodicity of the fresh-water plants, the colour of Baltic lakes under- 

 goes regular change. The season when plankton-organisms determine 

 the colour of the lake only to a slight degree is usually comprised 

 of the early days of June, when diatoms have vanished and Cyano- 

 phyceae not yet appeared. The colour of alpine lakes in arctic regions 

 is only slightly tinted by plankton, because this is present only in small 

 amount. 



Seasonal changes. Here it may be mentioned that plankton-asso- 

 ciations change with the season, just as does vegetation on land, because 

 they are dependent on' the temperature, illumination, and chemical 

 properties of the water. For instance, in Skager-Rak and Kattegat 

 during February and March there occurs a rich diatomaceous plankton 

 composed of species that later (in April and May) appear on the coasts 



' Schiitt, 1893. ' Hensen, 1887. 



* Sec also Haeckel, 1890, 1891. * Sec VVille, 1904 b. 



