50 SENESCENCE AND REJUVENESCENCE 



temperature, time of exposure, and occurrence of coagulation as 

 indicated by death is the same in living plant cells as in proteid 

 sols outside the organism, and he therefore concludes that the pro- 

 toplasmic sol is slowly undergoing changes in the direction of coagu- 

 lation even at temperatures where continued life is possible. If 

 this view is correct, then a slow increase in aggregation is occurring 

 continuously in protoplasm, but the formation of new sol and the 

 gradual chemical breakdown of the older partially coagulated sub- 

 stance may serve to delay the final result for a long time, or indefi- 

 nitely. 



The accumulation and apparent gelification of protoplasm in 

 the course of growth and differentiation suggest that changes of 

 this sort are characteristic of the developmental history of all 

 organisms. If this is true, they must result in increasing physio- 

 logical stabihty of the protoplasm or parts of it, and so lead to 

 decrease in the rate of metabolism, and the decrease in metabolic 

 rate may in time lead to changes in the character of the metabolic 

 complex and so to further changes in structure which may again 

 alter metaboHc conditions, and so on. 



It is probable then that mere continued existence may in many 

 cases result in gradual progressive changes in protoplasm which 

 become evident sooner or later as some degree and kind of differ- 

 entiation. Such a process is a self -differentiation in the strictest 

 sense. Its occurrence or non-occurrence must depend upon the 

 absence or presence of changes which balance or compensate in 

 some way the progressive changes, and these are the changes which 

 lead to dedifferentiation (see following section). 



Where all cells or parts are alike, self-differentiation must pro- 

 duce the same result in all, but where differences of any sort exist, 

 such, for example, as differences in metaboHc rate between external 

 surface and interior or between other parts, then the different parts 

 may influence each other and differentiation becomes a correlative 

 process which may result in the production of many different parts. 

 In correlative differentiation the parts may influence each other in 

 various ways. Dynamic changes of one kind or another may be 

 transmitted from one part to another; quantitative or qualita- 

 tive differences in the chemical substances produced by different 



