152 PL ANT- ANIMALS [CH. 



increase, for only in the presence of plentiful sup- 

 plies of nitrogen does the chlorophyll-apparatus work 

 well. Large quantities of carbohydrate material 

 are produced in the algal cells enough for the needs 

 of these cells and also for those of the animal. All 

 goes well, so well indeed that C. roscoffensis, less 

 conservative than its ally, contents itself entirely 

 with the supplies of food-material, of fat and also 

 of organic nitrogen compounds, provided by its green 

 cells and abandons the practice of fending for itself. 

 The ample tribute which it receives suffices for its 

 needs and also for the provision of its eggs. But the 

 weakness of the system here discloses itself. This 

 handing of nitrogen-containing substances to and fro 

 from animal to plant and from plant again to animal 

 cannot go on indefinitely or without loss. Sooner or 

 later, the animal finds itself lacking in essential, 

 nitrogen-containing food-materials. Supply fails to 

 equal the demand. Then the animal is under the dire 

 necessity of digesting its algal cells. To satisfy an 

 imperious, present need, the plant-animal destroys 

 the source of its supplies. 



Thus the animal repudiates the association and, 

 having digested its green cells, C. roscoffensis dies 

 of the very complaint nitrogen-hunger which the 

 green cells sought to avoid by their intrusion into 

 the body of the animal. To dismiss the association 

 between animal- and plant-constituent of the plant- 



