2 SENESCENCE AND REJUVENESCENCE 



is left behind. In such cases there is, as Weismann has aptly put 

 it, no death because there is no corpse. 



We see, however, that in certain other forms of reproduction, as 

 in various types of sporulation in plants and budding in lower 

 animals, and in sexual reproduction also if we take the facts at 

 their face value without reference to the germ-plasm theory, only 

 a circumscribed part of the parent organism is directly involved in 

 reproduction. In such cases the parent organism either remains 

 alive for a longer or shorter time, perhaps with periodic or con- 

 tinuous reproduction, or it dies almost at once. In short, death 

 of the non-reproductive or somatic parts of the organism is ap- 

 parently the final result in at least most of these cases. 



In the higher animals which reproduce only sexually, and in at 

 least many of the higher plants, certain physiological and morpho- 

 logical changes accompany growth and development of the somatic 

 parts. The rate of growth decreases, in many cases irritabiHty 

 and the rate of metaboHsm have also been found to decrease, a 

 relative and later an absolute decrease in the percentage of water 

 occurs, the structural elements become less plastic and in some 

 cases undergo more or less atrophy in later stages, and the organism 

 in general appears to be gradually losing its vigor. In many plants 

 these changes occur rapidly in certain parts and may be long, perhaps 

 indefinitely, delayed in others, and it will be shown in Part III 

 that the same is true for many of the lower animals; but in the 

 higher animals the whole of the body is apparently involved, though 

 even here the facts indicate that these changes may occur more 

 rapidly in one part or another according to various conditions. 



These changes, which constitute a gradual deterioration of the 

 organism, a gradual decrease in the intensity of its Hving, are com- 

 monly designated as aging or senescence. The question whether 

 certain changes are properly to be regarded as senescence or not 

 may often be raised with respect to particular cases, but in general 

 there can be no doubt that in at least many organisms a process 

 of senescence does occur: the organism grows old. Moreover, 

 there is no doubt that in at least many forms this process of senes- 

 cence leads to the cessation of the processes of life, i.e., to what 

 we call death. 



