v] NATURE OF PL ANT- ANIMALS 153 



animals by labelling it symbiosis is to miss the vary- 

 ing significance of the association. Looking at the 

 relationship from the standpoint of the animal, it is 

 one of obligate parasitism. Apart from their algal 

 cells, C. roscoffensis and C. paradoxa are unable to 

 live. The existence of either species depends upon 

 the infection of the individuals of each successive 

 generation. Where the infecting organism is absent, 

 there C. roscoffensis does not exist. Hence its re- 

 stricted range. From the standpoint of the species, 

 "infecting organism," the relation of certain of its 

 individuals with C. roscoffensis or C. paradoxa is an 

 episode without significance. Unlike the animal, 

 which bears the inherited impress of the relation 

 in lack of excretory system and in the habit of 

 patient waiting abiding the question of infection- 

 the alga is free. Of a swarm of flagellated green 

 cells, some small percentage meet the picturesque 

 fate of forming a tissue in the body of an animal. 

 The others pursue a less romantic adventure, either 

 as green, self-supporting organisms or as colourless 

 cells which batten on the offal of the sea. 



From the standpoint of the ingested algal cell, 

 association with the animal means a successful solu- 

 tion of the nitrogen problem. It sacrifices its 

 independence for a life of plenty. This universal 

 nitrogen-hunger is a misery which makes strange 

 bed-fellows. 



105 



