CONDITIONS OF CELLULAR IMMORTALITY 75 



shoot, whose continuity with the base of its parent tree 

 has never been broken, makes a new tree after the old one 

 has died indeed in some cases the shoot has helped the 

 mortiferous process by the vigorous crowding of youth. 

 In this whole picture how fares any idea of the necessity 

 or inevitableness of cellular (somatic) senescence? Such 

 an idea plainly has no place in the realities of the con- 

 tinued existence of apple trees. 



From these facts it is a logically cogent induction to 

 infer that when cells show the characteristic senescent 

 changes which were discussed in the preceding chapter, 

 it is because they are reflecting in their morphology and 

 physiology a consequence of their mutually dependent 

 association in the body as a whole, and not any necessary 

 progressive process inherent in themselves. In other 

 words, may we not tentatively, in the light of our present 

 knowledge, regard senescence as a phenomenon appear- 

 ing in the multicellular body as a whole, as a result of 

 the fact that it is a differentiated and coiiferentiated (to 

 employ a useful term lately introduced by Ritter) mor- 

 phologic and dynamic organization. This phenomenon 

 is reflected morphologically in the component cells. But 

 it does not primarily originate in any particular cell 

 because of the fact that that cell is old in time, or because 

 that cell in and of itself has been alive ; nor does it occur 

 in the cells when they are removed from the mutually 

 dependent relationship of the organized body as a 

 whole and given appropriate physico-chemical condi- 

 tions. In short, senescence appears not to be a primary 

 attribute of the physiological economy of cells as such. 



If this conception of the phenomenon of senescence 

 is correct in its main features, it suggests the essential 

 futility of attempting to investigate its causes by purely 



