Hknky and Fi-oou — The Hutorn of the. London Phnie. 23 



Age of vine. 



Average iige of the vein 

 islets of tlie leaf. 



3 years old, 



. 0-515 



sq. 



, mill. 



6 



. 0-394 







14 



. 0-330 







28 



. 0-204 







50 



. 0-173 







70 



. 0-137 







Benedict was thus able to determine the age of a plant, by the average 

 area of the vein-islets of the leaves which it bore. The progressive diminu- 

 tion of the soft tissue with age indicates senile decay. Benedict believes that 

 a twig cut from a mature tree, having uiidei-gone senile cliange, will not 

 produce when propagated a new tree endowed with tlie youthful vigour of a 

 seedling. It will possess merely the lessened vigour of the adult tree from 

 which it was taken. Tliis agrees with the view held by practical gardeners, 

 that varieties which are propagated vegetatively (by cuttings, etc.) ultimately 

 lose their vigour and gradually die out. If this view is correct, it is important 

 to renew varieties of seed. Even the most vigorous lirst cross would eventually 

 require to be produced again by cross-pollination. The plant breeder is 

 obliged, when old varieties, whether hybrids or sports, become enfeebled, to 

 develop new varieties from seed to take their place. 



Measurements of the vein-islets of the different planes have perhaps con- 

 firmed Benedict's views to some extent ; but further research is required. 

 The results obtained, though not capable in many eases of satisfactory 

 explanation, are now given for what tiiey are worth. 



1. In a series of leaves from trees of /'. occidentalis growing in the United 

 States and in no case originating from cuttings, the average areas of the 

 vein-islets (PI. IX, fig, 10) were: — 



Average area of the vein- 

 P. occidentalis. islets of the leaf. 



Seedling, 1 year old 0-26 sq. mm. 



Seedling, 2 years- old, ..... 0-17 „ 



Two trees, 5 feet high, and probably 5 years old, 0-07 „ 



Tree, 10 feet high, 0-05 



Tree, 30 feet high, 0-03 



Tree, 50 feet high, 0-03 



Other trees, size not stated, .... 0-03-0-05 sq. mm. 



It would appear from these figures that the size of the vein-islets is of 



