136 PLANT-ANIMALS [CH. 



cells, become, as it were, infected, and commence to 

 divide. Then cells neighbouring these begin to divide 

 and play the part of cambium, till finally a complete 

 ring or hollow cylinder of actively dividing cells is 

 formed in the stem ; and from this ring, which lasts 

 as long as the plant lives, are produced new wood 

 and new bast. Though no definite, chemical stimu- 

 lator has been discovered in this case, we may feel 

 sure that that is due to the fact that it has not been 

 sought. 



Applying the conception of chemical stimulators 

 or hormones to the case of arrested development 

 exhibited by C. roscoffensis, we may suppose that in 

 this animal, the signal for the commencement of the 

 later phases of development owes its origin to the 

 presence, within the body of the animal, of the green 

 algal cells, that, in the absence of these cells, the 

 signal is not given, and that, consequently, develop- 

 ment does not proceed. 



Hence it would follow that no amount of feeding, 

 either with diatoms or any other elements of the natural 

 micro-flora and fauna existing in the environment of 

 C. roscoffensis, can compensate for the lack of the 

 hormone entrusted by custom with the task of sig- 

 nalling to the animal to proceed with the business of 

 ordered development. On this view, the failure 

 which has been complete to rear C. roscoffensis on 

 artificial food, starch, sugar, peptone, protein, milk 



