330 



Harris M. Benedict 



By a comparison of the curve of decrease in size of vein islets and that 

 of some recognized senile degeneration in animals, some evidence of 

 the significance of the change in venation may be obtained. In figure 

 58 two such curves are shown, curve B representing the senile decrease 

 in rapidity of growth of guinea pigs from birth to the age of three months, 

 and curve A representing a decrease in size of vein islets in the leaves of 

 Vitis vulpina. Both curves can be expressed by the same equation with 



different constants, and are essen- 

 tially of the same type. Sunilarly, 

 figure 57 (page 317) exhibits the 

 common characters of a senile pro- 

 gressive change in an animal activity 

 and the progressive change in ve- 

 nation in the leaves of cuttings. 



The elimination of other possible 

 causes of the change wdth age in the 

 size of vein islets, and the marked 

 similarity to typical senility in ani- 

 mals shown by the rate of its prog- 



0.60 



S 50 



"^ 0.30 



° 0.20 



0.10 



«3 10 20 30 40 50 60 70 



Age in years of Vitis vulpina vines 



Fig. 58. — Curves shoiving (A) senile decrease in . i-i . 4.1 



size of vein islets in leaves of Vitis vulpina, TSSS, IS Strong evidence tnat tne 



and (B) senile decrease in rate of growth of real cause is Senile degeneration of 



guinea pigs , . . ^ , • , • 1 i- , i 



the meristematic tissue and oi the 

 cells of the leaf. That the increase with age of vascular tissue in the leaf 

 is a degeneration as regards physiological activity will be made plain. 



COMPARISON OF PHOTOSYNTHETIC ACTIVITY IN LEAVES OF YOUNG AND 



OF OLD PLANTS 



It has been shown by Schuster (1908) that the veinlets are of Httle 

 or no importance in the mechanical support of the leaf. If, therefore, the 

 increase in the proportion of vascular tissue and the consequent decrease 

 in photosynthetic cells result in a decrease of photosynthetic activity, 

 the change in venation with age is a loss without any compensation. 

 Such a loss in photosynthetic activity must obviously accompany the 

 decrease in photosynthetic cells unless the latter improve in efficiency 

 as rapidly as they decrease in amount. 



In animals the speciahzed cells persist for the entu'e length of fife of the 

 organism, and therefore an increase in efficiency of the specialized functions 



