82 Forestry Quarterly 



of the plant on which the leaf was borne. Thus in plants 3 to 5 

 years old, the average size of the vein islets was 0.4914 square 

 m. m. from 14 to 25 years old, 0.2969 square m. m. ; 28 to 35 years 

 old 0.2211 square w. m., and in plants from 50 to 70 years old the 

 average size of the islets was 0.1638 square m. m. A similar 

 result was obtained when cuttings from plants of different ages 

 were grown in water cultures and in soil cultures; as the leaves 

 produced in these cultures were subjected to identical conditions, 

 the possibility that the result obtained in the native, field grown 

 plants may have been due to varying conditions of their environ- 

 ment was eliminated. Leaves of certain cultivated varieties of 

 grapes were examined, the age of the plants being the same, but 

 the duration of the cutting propagation being different. For 

 example, one variety of grape which had been vegetatively pro- 

 pagated for 70 years disclosed 66 intersecting veinlets in 2 cm. 

 While another variety which had been asexually propagated for 

 114 years, showed 82 intersecting veinlets in the standard length. 



The leaves of about a dozen species of trees were examined in 

 the same manner, and they contributed corroborative evidence as 

 to the effect of age in decreasing the size of the vein islets. For 

 example, in case of chestnut trees, on the leaf of a tree, 5.1 cm. 

 in diameter, the average size of the vein islets was 1 square cm.; 

 a tree 12.7 cm. in diameter, vein islets 0.7 square cm.; a tree 25.5 

 cm. in diameter, vein islets 0.5 square cm.; a tree 61.2 cw. in diam- 

 eter, vein islets 0.4 square cm., and the leaves of a tree 91.2 cm. 

 in diameter showed vein islets having an average area of 0.3 

 square cm. 



A decrease in the size of the vein islets indicates an increase in 

 the ntmiber of veinlets in a unit area, and an increase in the number 

 of veinlets means an increase in the amount of vascular tissue at 

 the expense of the photosynthetic tissue. It has been shown by 

 other investigators that the veinlets are of little or no importance 

 in the mechanical support of the leaf. Therefore, if the decrease 

 in the number of photosynthetic cells results in a decrease in 

 photosynthetic activity, the change of venation with age is a loss 

 without any compensation. The author performed some experi- 

 ments on this point and found that leaves of the sajne size and with 

 the same exposure to light from plants of different ages varied 

 according to age in their daily increase in weight through the 

 accimiulation of food products. For illustration, the average 



