Senile Changes in Leaves of Certain Plants 291 



For several reasons the organ selected was the leaf, not the meristematic 

 tissue itself. In the first place, not only must the presence of any change 

 in the meristematic cells effect changes in the speciahzecl cells produced 

 from them, but the effects must be more evident than tiie conditions 

 that produced them, for the same reason as that accounting for the fact 

 that any internal differences between two seeds will be displayed much 

 more plainly in the plants that are produced from them than in the seeds 

 themselves. Thus, changes too minute to be perceived in the meristematic 

 cells might produce very obvious differences in the mature leaf. In the 

 second place, the leaves are renewed each year, and therefore, according 

 to the commonly accepted conception, should be entirely immune from 

 any sign of age; or, to express this in another way, any sign of senility 

 found would be of exceptional importance in its bearing on the problem. 

 In the third place, the leaf is more accessible than other organs. 



The many careful researches that are on record regarding the effect 

 of all factors excepting age on the structure and functions of the leaf 

 seemed to justify the opinion that if any differences were found it would 

 be possible to decide whether they were due to one of these factors or 

 whether they must be referred to age. 



Leaves from young and old vines of Vitis vulpina were collected and 

 were subjected to a preliminary examination by means of a hand lens. 

 The most striking difference apparent was observed in the venation. On 

 holding the leaf blades to the hght, the meshes formed by the most minute 

 veinlets appeared larger "in the leaves of young vines than in those of 

 old ones. A difference so marked that it could be discerned with a hand 

 lens would be capable of utilization in the field and of practical appH- 

 cation in orchard and nursery. Therefore an investigation was begun 

 of the size of the meshes in leaves of vines of different ages. 



Collection and preparation of material 

 In order that the effects of differences in exposure to light and wind 

 and of differences in nutrition might be reduced as much as possible, 

 the country in the vicinity of Ithaca, New York, and of Cincinnati, Ohio, 

 was searched for vines of different ages growing near one another under 

 the greatest possible similarity of external conditions. It proved to be 

 more difficult than was expected to find such paired vines, but at last 

 about twenty pairs were found. 



