352 Harris M. Benedict 



The leaves used for these data were picked at approximately the same 

 height from the ground and care was taken that all had the same light 

 exposure; so that conditions in each were as nearly ahke as possible. The 

 soil and the cultivation were apparently the same for each. Ten leaves were 

 picked from each tree and the venation count given in the table is the 

 average of the ten. At the time the venation count was made the writer 

 did not know the respective ages of the varieties. An arrangement of 

 the varieties in the orcler of their venation count proved an accurate fore- 

 cast of the respective varietal ages as subsequently learned. 



In so far as this preliminary test goes, therefore, it supports the view 

 that propagation by cuttings does not prevent the progress of senile degen- 

 eration in the tissue of the cuttings. A number of interesting points 

 suggest themselves on examination of table 59. In the case of the peach, 

 probably the shortest-lived fruit represented in the table, it seems signif- 

 icant that the rate of decrease in size of vein islets is more rapid than in 

 any of the others. Some species seem to have an original size of vein 

 islets greater than closely related varieties. It is possible that such an 

 initial large size of islet is a mark of physiological vigor, and indicates 

 greater constitutional strength and a longer productive life than is the 

 case with those plants with smaller islets. This point might well be con- 

 sidered by plant breeders in selecting breeding strains. On the other hand, 

 some varieties — the grape, for example — seem to show a marked slowing 

 of the rate of decrease in size of islets after maturity, and thus attain a 

 considerable age in spite of the rather small initial size of the islets. 



The possibility of determining the age of a plant by the size of the 

 vein islets in the leaves opens the way to a comprehensive determination 

 of the present physiological condition of the different varieties of fruits. 

 The results obtained from the study of Vitis vulpina and the preliminary 

 tests of cultivated fruits seem to indicate that all varieties of fruits now 

 vegetatively propagated must in time run out. A wise provision for the 

 future of the fruit industry must therefore include an investigation into 

 the physiological condition, as regards senile degeneration, of each variety. 

 Such a study may show some varieties to be capable of some degree of 

 rejuvenescence through processes similar to those found in sexless plants, 

 or it may show that all are doomed to extinction. Certainly, the deter- 

 mination of the fact is important. If certain varieties must shortly show 

 deterioration, plant breeders must pay particular attention to developing 



