326 Harris M. Benedict 



from which all are developed in their time. If this change in the size 

 of the vein islets is the effect of progressing senihty, then the constant 

 changes must be occurring in the meristematic tissue of the plant, and 

 it follows that this tissue must no longer be considered immune from 

 senile degeneration and from death. 



There remains to be examined the possibility that the change in size 

 of the vein islets may be due to some cause other than the progressive 

 senile degeneration of the meristematic tissue. The possible causes may 

 be grouped into two divisions: first, changes in the old speciahzed tissues 

 of perennial plants; second, differences in external conditions resulting 

 from increasing size and subsequent difference in position of leaves. 



Is it possible that some progressive change occurring in the early 

 specialized tissues of the root or the stem produces the decrease in size of 

 the vein islets, and therefore no senile degeneration of meristematic 

 tissue need be considered ? Any such progressive change either must 

 interfere with the transfer of materials to and from the meristem and 

 the developing leaf, or must act through the production of toxic products 

 that poison the meristem and the leaf. 



If the decrease in size of the vein islets is caused by a progressive decrease 

 in the efficiency of the root or the stem as absorbing and conducting 

 organs, then not only the venation, but the size and development of the 

 leaf as weU, should be affected. The leaves and the shoots of Vitis vulpina, 

 however, show absolutely no signs of impaired vigor of growth. In fact, 

 the leaves and the shoots of very young vines often appear smaller and less 

 luxuriant than those of older vines, due, no doubt, to differences of 

 nutrition received. Furthermore, data given show no relationship between 

 the sizes of the leaves and the sizes of vein islets. The w^ater shoots 

 from the stumps of cut trees and the root shoots of Tecoma, mentioned 

 above, must have had much more than the normal supply of nutritive 

 materials, but this did not markedly change the size of the vein islets. 

 It was found also that leaves borne by cuttings grown under identical 

 conditions showed the same differences in venation as were present on the 

 original vines in their natural habitat. 



It should not be overlooked in this connection that the leaves are not 

 the only new structures being constantly produced in perennials. The 

 increase in number of root hairs keeps pace with the number of leaves, 

 and the increasing circumference of the newly formed zones of xylem 



