CH. X] THE CONDITIONS OF LIFE IN THE SEA 249 



for their protoplasm possesses irritability, and so may possibly 

 react towards light stimuli. But the main influence of light on 

 the metabolism of the diatoms, and other protophyta, is expressed 

 in the power of photo-synthesis which these organisms possess. 

 This influence of light is manifested in the elaboration of carbo- 

 hydrate from the carbon dioxide and water of the medium, and in 

 the building-up of proteid from these substances and from the 

 inorganic food-salts of the sea. We shall see that the tendency of 

 all metabolic processes in the sea is towards the resolution of the 

 tissue substances of organisms into the most highly organised 

 compounds of nitrogen and carbon — that is into nitric acid, 

 water, and carbon dioxide. These substances can only be utilised 

 by organisms when the energy of sunlight is intercepted and used 

 in the process of photo-synthesis by the green plant. Thus all 

 the energy manifested in the organic world is ultimately derived 

 from the energy of solar radiation. 



What precisely are the relations of the metabolism of marine 

 protophyta towards the intensity of sunlight, or radiant energy of 

 different wave-length, have not been sufficiently investigated. 

 Much work of this kind has been done by the plant physiologists, 

 in the case of terrestrial plants, but we cannot assume that the 

 results so obtained hold good with regard to the unicellular plants 

 living in the sea ; and this caution is all the more necessary when 

 we remember that the experiments of Putter on the metabolism 

 of marine organisms have led us to suspect that the life-processes 

 which are characteristic of the higher land animals do not 

 necessarily hold good with regard to the lower animals living 

 in the sea. We have seen that the Plankton-Expedition found 

 Halosphaer^a living at a depth of over 1000 metres, and this 

 would apparently indicate that the amount of light necessary for 

 the carrying on of photo-synthetical processes may be very small ; 

 for at this depth there must be almost total darkness, unless 

 the light of the phosphorescent organisms is sufficient for the 

 needs of the plants. Pantanelli^ has shewn that there is an 

 " optimum " intensity of light in the case of the photo-synthesis 

 of starch b}^ the land plants, which is about one-quarter of the 

 intensity of full sunlight ; and it is not improbable that there may 



1 See Jost's Plant Physiology , Clarendon Press, 1907. 



