382 ROBBINS 



no single explanation for the effects of age can be applied to all kinds of 

 seeds. The physiological changes which occur in the aging of seeds offer a 

 profitable area for further research. 



Individual plants. Just as seeds vary greatly in length of life, so do indi- 

 vidual plants. Annuals live for a few months and die; perennials may live 

 for years, some of them for thousands of years, though parts of them — for 

 example, leaves, flowers, and fruits — age, die, and are discarded. 



The aging of annuals and their senescence and death is associated in part 

 with the production of flowers, fruits, and seeds, a condition which exists also 

 for some perennials like the so-called century plant. Agave americana. For 

 these plants the prevention of flowering may extend their length of life con- 

 siderably. The Agave in its native habitat blooms and dies within a period of 

 eight or ten years. In less favorable areas, parts of southern Europe for ex- 

 ample, it may grow for as much as 100 years before its life span is terminated 

 by flowering. 



Loss of magnesium by the chlorophyll, a decrease in ability to retain water 

 with consequent wilting and withering, and other causes for the death of 

 annuals have been suggested, all of which are more probably results of senility 

 than causes. The transfer of food reserves, especially nitrogenous substances 

 to the flowers, fruits, and seeds, with a resulting starvation of the vegetative 

 parts, seems a more likely explanation, though the possibility has not been 

 eliminated that the movement of hormonal-like accessory foods to the floral 

 parts is also involved. There is no adequate explanation which tells us why 

 food reserves move from the vegetative to the floral parts. 



Physiological changes of various kinds associated with aging may perhaps 

 occur in an annual plant even before flowering. The root system of some 

 annuals has been reported to be less effective in absorption in the latter part 

 of the growth period, and Frank found a loss of nitrogen in older plants of 

 Oenothera and Arabinopsis amounting to as much as 30 per cent of the maxi- 

 mum. I am not sure how closely these changes are correlated with the forma- 

 tion of flowers. 



Various explanations have been offered for the differences in the length of 

 life of various kinds of trees. These include resistance to decay, decrease in 

 the absorption and conduction of water, changes in photosynthetic power, 

 deterioration of protoplasmic proteins, accumulation of ash, etc. 



Last summer, as Mrs. Robbins and I visited Windsor Great Park with 

 Sir Eric Savill, the Estate Manager, I asked him why an English oak lives 

 so much longer than the European beech. He said if I would tell him why a 

 birch lives for 100 years he would tell me why the beech lives for 300 years 

 and an oak may survive for 1000 years or more. This expresses more or less 

 faithfully our inability to assign specific causes for the great differences in the 

 normal length of life of various kinds of trees. 



Not only do plants become senescent and die, some of them go through 



