Senile Changes in Leaves of Certain Plants 353 



seedlings capable of replacing them. Such seedHngs having been 

 developed, their propagation by cuttings may then be continued until 

 physiological deterioration again appears and recourse to a new seedling 

 variety is necessary. It may thus be possible to so classify and control 

 cultivated fruits that all loss from degeneration may be accurately fore- 

 seen and prevented. 



Before leaving the subject of the effect of vegetative propagation on 

 different varieties, it may be well to point out that in those cases in which 

 senile degeneration has progressed so far that the conduction of nutritive 

 materials to the leaves is markedly interfered with, the removal of a scion 

 and its insertion into a seedling stock should result in an improvement 

 in its nutritive supply. This may produce a certain increase in activity, 

 but not rejuvenescence since the cells of the scion are not changed. 



The effect of vegetative propagation on plants other than fruits, potatoes, 

 hops, And the like, is also of great importance. The general opinion among 

 growers seems to be that those varieties which have been long propagated 

 in this way are showing decrease in yield and in resistance to disease. 

 No preliminary applications of the venation method have been made in 

 the case of these plants. Where such evident adaptations for vegetative 

 propagation as tubers, bulbils, and the Uke are present in plants, the 

 possibility of the presence of other than sexual methods of rejuvenescence 

 seems greater than in plants without such special equipment. An ex- 

 amination of the leaves borne by trees and root shoots of Lombardy 

 poplar {Populus nigra italica Du Roi) ^ — which has lost its power to produce 

 seeds but sends up multitudes of root shoots, and which has been propa- 

 gated artificially by cuttings for about seven hundred years — showed the 

 vein islets to be so divided by minute veinlets of procambial' tissue as to 

 make measurement very difficult. The application of the venation test 

 to other cultivated plants vegetatively propagated should give interesting 

 results and will be undertaken. 



BEARING OF THESE FACTS ON THEORIES OF SENILITY 



The steady encroachment with age of the vascular tissue of the leaf 

 on the space originally occupied bj^ the photosynthetic cells, and the 

 accompanying decrease in the photosynthetic power of the leaf, must 

 be forced by some physical or chemical change occurring in the cells. 



