Senile Changes in Leaves of Certain Plants 347 



similar; the second is the fact that such physiological activities as are 

 common to both plants and animals- — respiration, for example — show the 

 same changes with age; the third is the nature of the morphological changes. 

 Although the structure of these plants is so different from that of animals 

 that detailed comparisons in but few specific organs can be made, never- 

 theless the recognized theories of the causes of the senile changes in animal 

 structure, if applied to plant organs, would call for such morphological 

 degenerations as have been shown in this paper to exist in the leaves. 

 Since these theories are given later, their application to this point will 

 not be repeated here. In the case of senile changes in cells, while the 

 proportion between the mass of nucleus and that of cytoplasm in young 

 and in old leaf cells of Vitis vulpina was not determined without possible 

 error, the considerable change in the proportion found with age seems too 

 great to be ignored. The nature of the change in nuclear mass in these 

 leaf cells is the same as that found in animal cells. 



When considered in its entirety, the evidence that these changes are 

 visible expressions of senile degeneration in the meristematic protoplasm 

 seems to be reasonably substantial. At any rate, any other explanation 

 of these changes in the leaf would be much more difficult to maintain. 



If this be granted, it will be interesting to apply these tests of age to 

 the long-disputed problem of the effect of continued vegetative propaga- 

 tion as compared with that of seed propagation. Heretofore no evidence 

 of senility could be used that was not open to the objection that it might be 

 merely the effect of some external agency. 



significance of senile changes to the problem of the running-out 

 of vegetatively propagated fruits 



The first publication of importance regarding the possibility of senile 

 deterioration was that by Thomas Andrew Knight (1795), who attributed 

 to this cause the gradual failure of different varieties of apples and pears. 

 Having tried experiments in grafting scions from young and from old 

 seed-grown trees of different ages. Knight decided that the cuttings were 

 directly influenced by the age of the plant from which they were taken. 

 He concluded that a variety propagated by cuttings would not be vigorous 

 much beyond the age of the parent tree. He turns to nature for a most 

 interesting support of his theory regarding the inherent defect of vege- 

 tative reproduction. Certain trees, he says, such as the aspen and some 



