154 PLANT- ANIMALS [OH. 



It is noteworthy that the interpretation, in terms 

 of the hypo thesis of nitrogen-hunger, of the relation 

 between animal and algal cell throws light on the 

 facts, already referred to, concerning the distribution 

 of algal cells in various marine animals. Analyses 

 have demonstrated (Johnstone, 1907) that the amount 

 of combined nitrogen present in sea-water is less 

 during the warm months (e.g. August) than during 

 the cold months of the year, and that it is less 

 in the warmer seas (Mediterranean) than in the 

 colder seas (Baltic and North Sea). Now, as we 

 have mentioned, certain animals possess green or 

 brown algal cells in one part of their range of dis- 

 tribution but lack them in other parts. Thus 

 Noctiluca, colourless in the North Atlantic, is green 

 in the Indian Ocean. Whence it would appear to 

 follow that where the stress of nitrogen-hunger is 

 more acute, there the association between algal cells 

 and animals manifests itself. 



One word more and one more speculation and our 

 work is done. The colourless phase in the life-history 

 of the infecting organism of C. roscoffensis, the colour- 

 less state of the just-ingested algal cells both in 

 C. roscoffensis and C. paradoxa, and the rapid as- 

 sumption of their proper pigments by the infecting 

 cells after they are established in their respective 

 animal quarters suggest that the colourless phase is 

 itself the outcome of nitrogen hunger. Such colour- 



