CH. X] THE CONDITIONS OF LIFE IN THE SEA 251 



that there can be no question of an increase of energy due to the 

 stimulation of a sensory surface by light, and of a consequent 

 greater amount of movement producing an increased respiratory 

 interchange. Further there is no differentiated sensory, or central 

 nervous system. We must apparently conclude that the change 

 in the intensity of light produces a change in the rate of 

 metabolism, and that the increased oxygen consumption is the 

 indicator of these changes. 



There is a relation between the colour and transparency of 

 sea water and the density of the plankton. Sea water varies in 

 colour from cobalt-blue to yellow, and these variations are due to 

 the mixture of the blue colour, which is due to the water itself, 

 with the green-yellow to brown-yellow of the chromatophores of 

 the plant organisms. The transparency also varies, as we have 

 seen, and the changes are due to the variable amount of inorganic 

 particles in suspension, and to the varying opacity of the organisms 

 of the plankton. There is a parallelism between the density of 

 planktonic life, the transparency, and the colour. "The yellow 

 Baltic only allows us to see the white plankton-net at a few metres 

 beneath the surface ; the green Arctic water transmits light rays 

 from a greater depth ; and what visitor to the Mediterranean has 

 not been impressed by the blue colour of that pellucid sea ? " 

 The Baltic shews us a " colossal wealth " of plankton life ; the 

 Arctic waters, as in the Irminger Sea, or the fjords of Greenland, 

 are not much poorer in vegetable microscopic organisms ; the 

 plankton fauna and flora of the Mediterranean is still more 

 meagre ; and that of the oceanic Sargasso Sea is apparently the 

 poorest of all. " Observations of transparency, colour, and 

 plankton-contents, all shew a parallelism, and all lead to the same 

 conclusion, that the pure blue is the colour of desolation of the 

 high seas. The colour of the Arctic seas is comparable with the 

 green of the meadows, but the colour of the most luxuriant 

 plankton-flora is the yellow of the Baltic ^" 



We have considered only the coarser factors of temperature, 

 salinity and light, but it would probably be wrong to conclude that 



1 Schiitt, Das PJlanzenlehen der Hochsee, p. 76, Kiel u. Leipzig, 1893. 



