346 Harris M. Benedict 



SUMMARY 



1. The size of vein islets in the leaves of Vitis vulpina is independent 

 of the area and thickness of the leaves, and is influenced only sHghtly 

 by differences of exposure to light. 



2. The vein islets in the leaves of Vitis vulpina become smaller as the 

 vine becomes older. This decrease in size of vein islets is due to en- 

 croachment of vascular tissue. 



3. Leaves borne on plants vegetatively reproduced show vein islets 

 similar to the plant from which the original cuttings were secured. Only 

 plants reproduced sexually show vein islets larger than those of the 

 parents. 



4. The same difference with age in the size of vein islets occurs in Vitis 

 hicolor, and apparently in a considerable number of other woody perennials. 



5. The decrease in size of vein islets means reduction ;n number of 

 photosynthesizing cells, and the evidence shows that for leaves of old vines 

 there is a decrease in the rate of photosynthesis. 



6. There is also a decrease in the rate of respiration (evolution of carbon 

 dioxide) in picked leaves. 



7. Leaves of young vines have a greater capacity for imbibing water 

 than have leaves of old vines, which may be the result of less vascular 

 tissue in the former. 



8. There is probably an increase with age in the number of stomata per 

 square millimeter. 



9. There is probably a decrease with age in size of stomatal aperture 

 and of guard cells. 



10. There is probably a decrease with age in size of palisade cells. 



11. There is probably a decrease with age in the mass of nuclei of border 



parenchyma cells. 



DISCUSSION 



It is behevecl that the evidence presented is fairly conclusive that the 

 changes discussed are due neither to external factors nor to internal factors 

 other than those produced in the meristematic tissues by age, and that 

 they must therefore be ascribed to the action of true senile degeneration 

 of the type found in animals. The positive evidence that these changes 

 are senile ones rests on three fundamental similarities which they show 

 to senile degenerations occurring in animals: the first marked similarity 

 is that the rate of change, as shown by tables and curves, is strildngly 



