PHYSIOLOGICAL ASPECTS OF AGING IN PLANTS 389 



To account for the facts I have presented briefly and inadequately is, of 

 course, to account for one of the most fundamental problems in biology. We 

 may talk of metabolic gradients, polarity, electrical potentials, macromolecu- 

 lar structure, and similar generalities, but their contribution to clarifying the 

 questions in which we are interested is limited unless we can define their 

 relationship in more than general terms. 



We know that the metabolism of young cells differs from that of older 

 cells. For example, the apical meristem of the stem produces auxins; the 

 older and distal cells produce little or none. Gibbs and Beevers found imma- 

 ture tissues to respire glucose exclusively, or to a large extent by the Embden- 

 Meyerhof-Parnas glycolitic sequence. As tissues aged and differentiated, a 

 direct oxidation pathway assumed more importance, accounting for at least 

 50 per cent of the respiration in many adult tissues. While we might suspect 

 the relative amounts of critical enzymes to determine the extent to which 

 one pathway or another occurs, it is not known whether this or some other 

 cause is responsible for the difference in the type of respiration in young and 

 old tissues. But even if we distinguish specific metabolic differences between 

 young and older cells, we cannot be sure whether they are the causes or the 

 results of aging. 



The observations of Riker and his colleagues on the growth of single 

 isolated plant cells may be significant for our understanding of the persist- 

 ence of meristems. You will remember that they found that single cells from 

 tissue cultures of marigold and of tobacco grew if cultivated on filter paper 

 which rested on young and actively growing cultures of tissue. However, as 

 the "host" culture became old and senescent, the young culture it supported 

 lost its vigor and it was necessary to transfer the filter paper and the culture 

 on it to a fresh and active mass of tissue. These observations suggest that 

 young cells in an active growing condition excrete substances which induce 

 other cells to divide and that the continued existence of a meristem is the 

 result in part of a kind of autocatalysis ; dividing cells induce neighboring 

 cells to divide by the products produced in division. We might even assume 

 that these products diffuse slowly or are evanescent, with the result that they 

 are only effective near the cells which produce them and that their absence or 

 deficiency initiates the aging changes. This is probably far too simple an 

 explanation for the complications of differential aging shown in the cells of a 

 root tip or stem tip, but it is a hypothesis which might well be pursued 

 further. 



I am sure that my discussion has given you very little firm information on 

 the physiological aspects of aging in plants. I have raised more questions 

 than I have answered. I hope I have said enough to demonstrate that there 

 are problems in the aging of plants which can be fruitfully investigated. The 

 rapidly developing field of macromolecules and the relation of their structure 

 to biological activity, the greater information on the modifications of devel- 



