PHYSIOLOGICAL ASPECTS OF AGING IN PLANTS 381 



disappears within a week after the seeds have matured. On the other hand, 

 seeds of lotus, Nelumbo nucijera, which were estimated to be a thousand years 

 old, on the basis of radio-carbon dating, have been found capable of germinat- 

 ing. The length of life of the seeds of other kinds of plants lies between these 

 extremes. Some, like those of Hevea, are short-lived; some are intermediate, 

 retaining their viability for years ; others have life spans of a century or more. 



From the standpoint of the aim of this discussion, we may ask several ques- 

 tions: Does the embryo in a seed age? Is there a gradual deterioration in the 

 potential vigor of a dormant embryo with time, and does the age of seed 

 affect yield? What are the causes for the loss of viability of seeds? Why does 

 an embryo of one kind of seed live for a period measured in days and that of 

 others for centuries? 



Various hypotheses have been explored from time to time on the causes 

 for the loss in the viability of seeds. These include the exhaustion of food 

 reserves, the gradual and spontaneous denaturing of protoplasmic proteins, 

 the deterioration of enzyme systems, and the accumulation of injurious prod- 

 ucts formed in the metabolism which continues slowly, even in a dormant 

 seed. 



One of the notable advances in this area is the demonstration that the 

 frequency of mutations in plants grown from old seeds is, in some instances, 

 greater than can be accounted for by the effects of natural radiation to which 

 the seeds were subjected during the period of storage. These spontaneous 

 mutations apparently occur before seed germination and are associated with 

 structural changes in the chromosomes. Such age-induced mutations have 

 some importance if old seeds are used for propagation and may be of signifi- 

 cance in evolution. While they may result through the spontaneous deteriora- 

 tion of chromosomal material, there is some evidence that the chromosomal 

 changes are actually caused by the accumulation of mutagenic substances in 

 the seeds, and it has been suggested that these substances and perhaps others 

 eventually reach a level which destroys the vitality of the embryo, perhaps 

 through their action on the chromosomes. The automutagenic substances and 

 the autotoxins assumed to be responsible for the mutations in old seeds and 

 for the eventual loss of viability have not been identified. 



We might expect also a gradual deterioration in the vigor of seed embryos 

 with time. However, the evidence on the correlation of the vigor of plants 

 from seeds of various ages is conflicting. Certainly there is no strict relation- 

 ship. For many seeds rather wide age limits appear to have little, if any, 

 effect on their performance. The few plants grown from thousand-year-old 

 Nelumbo seeds showed no marked reduction in vigor and appeared to be 

 normal. 



When we consider the differences between species in the morphology and 

 physiology of their seeds, in their length of life under normal conditions, and 

 in the effects of various storage conditions on survival, it seems probable that 



