Senile Changes in Leaves of Certain Plants 329 



active parts of the plant. The faihng activity of each would then hasten 

 the senile degeneration of the others, but would not be the primary cause. 



It seems to be true, from the facts brought out in the foregoing dis- 

 cussion, that the change with age in the size of the vein islets in the leaves 

 is not the result of any changes that have occurred in the old specialized 

 tissues of the plant. The evidence demonstrates that neither decrease 

 in the efficiency of excretion nor toxin production in the old speciaUzed 

 cells can account for the change in venation. There remains for investi- 

 gation the possibiHty that the changes in size of vein islets may be due to 

 the increasing size of the plant and the consequent change in the position 

 of the leaves. 



In the case of the paired vines of Vitis vulpina, the external conditions 

 were much the same, and leaves were picked which had been growing 

 under similar conditions of exposure to Hght. Care was taken also that 

 the leaves selected were growing at the same height above the ground and 

 at approximately equal distances from the bases of the vines. The leaves 

 of the paired vines were therefore growing under much the same external 

 conditions in both the young and the old vines. In spite of this fact 

 they showed the differences in venation that have been tabulated above. 

 Even more striking is the evidence from root shoots and water sprouts 

 of various plants, the leaves of which, although borne close to the ground, 

 possessed vein islets of a size typical of the age of the stump. The sizes 

 of the vein islets in the leaves borne by cuttings raised under identical 

 conditions furnish the final conclusive evidence that differences in external 

 conditions are not the cause of the change of venation with age. 



It may be concluded, therefore, that the progressive decrease in size 

 of vein islets which accompanies the increasing age of the perennial plants 

 examined is caused neither by changes in the older parts of the plant nor 

 by the increasing size of the plant. The visible change in venation is 

 therefore the result of a progressive change in the cells of the meristem 

 and of the leaf which is inherent in the nature of their protoplasin. Such 

 inherent progressive change in animal protoplasm is called senihty. 

 Animal senihty is marked by an initial rapidity and later slowing of its 

 onset, and may be represented by a curve which corresponds to the lower 

 half of a parabola with the equation of the general nature of (Y-A)2= BX. 

 In such a curve Y represents the condition of function or structure, X rep- 

 resents the age, and A and B are specific constants for the case in question. 



