v] NATURE OF PLANT- ANIMALS 137 



and prepared "human foods" of various kinds, is 

 intelligible ; nor may we expect success to attend our 

 attempts to raise a colourless race of C. roscoifensis 

 till we have discovered the signalling substance pro- 

 duced by the green cells. 



We turn now to another phenomenon exhibited by 

 larval C. roscoffensis. Considered attentively, the 

 rapid development of the green tissue in the infected 

 animal is no less remarkable than the arrest of develop- 

 ment in the uninfected animal. How comes it that 

 an alien organism, intruding itself among the tissues 

 of a young animal, is able to multiply so rapidly and 

 extensively therein? It might be supposed that it 

 was but a case of simple parasitism ; that the green 

 cell lives and multiplies directly at the expense of 

 the animal's cells. This, however, can scarcely be the 

 case, for, in their early stages at least, the green 

 cells keep themselves to themselves. They lie in 

 vacuolar spaces out of direct contact with the animal 

 cells. Hence any food-materials which they obtain 

 from the body of the animal must be in a state of 

 solution. Again, there is no evidence whatever that 

 the green cells obtain access to any soluble food- 

 substances which the animal has prepared for its 

 own use. The time during which the increase of the 

 green cells is greatest soon after infection has taken 

 place is also the time when the animal itself is 

 growing most rapidly. It is true that during this 



