PELAGIC PLANT LIFE 341 



that sink to the bottom in the shallow coastal seas, where they Resting 

 rest until conditions of development become favourable again, ^p^""^^- 

 This has been observed by many naturalists since Schlitt first 

 noticed in the Western Baltic that a species which begins to 

 form resting- spores disappears shortly afterwards from the 

 surface-layers. He showed, too, that the resting-spores sink 

 down to the bottom, and, although their germination has not 

 been carefully studied, we may be sure, all the same, that it 

 does take place ; further, when we subsequently find the same 

 species once more in abundance, we have every reason for 

 surmising that the resting-spores on the bottom were the 

 principal source from which these forms have been derived. 



Ability to form resting-spores must be of the utmost 

 importance for the existence of the species in coastal waters. 

 The chief difference between coastal seas and the ocean, so 

 far as hydrographical conditions are concerned, lies in the 

 extreme and rapid changes in such fundamental conditions 

 of existence as salinity and temperature in coastal waters, 

 Resting-spores, therefore, must be the means by which many 

 species continue in coastal seas, notwithstanding the fact that 

 there conditions of existence are only favourable for a limited 

 portion of the year. The arctic diatoms, for instance, which 

 are sometimes to be found in the plankton of the Skagerrack, 

 are very easily affected by a rise in temperature, but their 

 development takes place during the winter months from 

 February to April, when the temperature is at its minimum. 

 In the summer they are not to be seen, but their resting-spores 

 are then most probably on the bottom. In the same way a 

 whole series of warmth-loving species pass through the winter 

 as resting-spores, and are to be found along our shores only 

 in the warmest months of summer and autumn. 



The neritic species may often be met with a long way Neritk 

 out at sea, still continuing to increase, though they are ^i^t°i"- 



• -y^ rir- 1 P^ ^^' 



seldom m any great quantity. One of the few mstances that 

 I know of, where we regularly find an immense production of 

 neritic diatoms in the open sea, is in the Gulf Stream north 

 of Shetland and the Faroe Islands during May. I made this 

 discovery as long ago as 1895, and it has often been confirmed 

 since then during the international investigations. When the 

 snows begin to melt in the spring, the surface-layers of water 

 are carried far away out from the land, and the neritic algae are 

 taken with them. I shall presently show that it just happens 

 to be in the spring that conditions of nourishment favourable 



diatoms in the 



