PHYSIOLOGICAL ASPECTS OF AGING IN PLANTS 387 



also held by many practical gardeners, to the effect that meristems not only 

 age but become senile, with the result that the length of life of vegetatively 

 propagated clones is the same as that of the seed-grown parent from which 

 the clones are derived. 



Without attempting to deal with ultimate causes or to settle the question of 

 juvenile vs. adult meristems, I suggest as a working h5T)othesis that juvenility 

 is an unstable metabolic state which exists in the meristem and which pro- 

 ceeds through a series of steps to a relatively stable metabolic-state char- 

 acteristic of the adult meristem. The change from unstable to stable may be 

 associated with the loss in ability to synthesize physiologically important 

 chemical substances and/or the development of the ability to synthesize 

 others. This means that the meristem of the juvenile stage may be character- 

 ized by the presence of particular substances of physiological importance 

 which are not present, or present to an appreciably lesser degree, in the 

 meristem of the adult. The steady state of the adult may be upset in the 

 direction of the unstable metabolism of the juvenile by cold, X rays, products 

 from the juvenile, the formation of adventitious meristems, and by the forma- 

 tion of zygotes or of asexual embryos. If these assumptions are correct, it 

 might be possible to isolate and identify substances from the juvenile stage 

 which, introduced into the adult, would cause it to become juvenile, a possi- 

 l^ility which is supported by the results on Hedera, by Doorenbos and by 

 Frank and Renner, which I mentioned earlier. 



Cells. For the purpose of our discussion, I am considering cells of the 

 apical meristem to be young cells and those of the mature tissues to be cells 

 which have aged; differentiation is considered to be a part of an aging process. 

 On this basis a number of facts may be pointed out and questions raised on 

 the physiological changes associated with the aging of the cells of a seed 

 plant. 



Young cells in the apical meristem divide, and for the sake of simplicity I 

 shall not distinguish between chromosomal division, nuclear division, and cell 

 division, though such distinctions are without question important in a full 

 consideration of aging of cells. The ability to divide is lost as the newly 

 formed cells are left behind the meristematic zone in the process of growth, 

 except for such tissues as the pericycle, cambiums, and buds. We may con- 

 clude, therefore, that loss of ability to divide is one of the physiological 

 changes which occurs in the aging of cells, though this loss may not be 

 permanent, as is pointed out below. But why do cells lose their ability to 

 divide? In fact, why are they able to divide in the first place? I do not in- 

 tend to answer these questions, but there are some facts to which I wish to 

 call attention. 



One of the curious characteristics of the young cells of apical meristems is 

 their stability. Apical meristems, the cells of which continue to divide, at least 

 periodically, persist through the life of the plant. Those of the oldest-known 



