BOOKS AND CURRENT LITERATURE 115 



while the corresponding range for a plant only 10 years old was from 

 0.015 to 0.260 sq. mm. While external conditions during leaf develop- 

 ment do influence the size of the vein islets, this effect appears to be 

 superimposed on the more pronounced influence exerted by internal 

 conditions, so that if the young and old plants grow with the same 

 environmental complex the age difference is still clear. 



For quick estimation is it sufficient to count the number of veins 

 intersected bj'^ a line drawn across the leaf. The count is made for a 

 line 2 cm. long. It is obvious that, the smaller the islets, the larger 

 is the number of veins this intersected. 



Evidence pointing in the same direction is presented for a number of 

 other plants, such as Vitis hicolor Le Conte, Tecoma radicans L., Salix 

 nigra Marsh, Castanea dentata Borkh., Quercus alba, L., etc. It was 

 found that the leaves borne out by sprouts coming from the stump of a 

 tree show intervenal areas of the size characteristic of the age of the 

 stump. 



In the case of cultivated plants propagated by cuttings the size of 

 the islets is related to the number of years that have elapsed since the 

 last reproduction by seed. Thus a Concord grape vine, propagated by 

 cuttings for 70 years bore leaves with 66 veins per 2 cm., while the 

 leaves of a Concord seedling 7 years old showed only 35 veins per 2 cm. 

 Similar^, a Hubbardston apple seedling 15 years old bore leaves with 

 30 veins per 2 cm. and the corresponding number of veins for a Hub- 

 bardston apple propagated by grafts for 82 years was 45. It thus 

 makes no difference whether the plant is propagated by cuttings or 

 whether the original stock is alone considered, the results are practi- 

 cally the same. Even after grafting the scion appears (by this cri- 

 terion) to be as old as the stock from which it was derived. Physio- 

 logical age is thus to be measured, in this connection, from the last 

 propagation by seed. 



Approximate determinations show that various other leaf characters 

 vary in a direction parallel to that shown by the variation in size of 

 vein islets. Thus, leaves of a vine with 6 growth rings gained 12.7% 

 of their green weight in a day, while the corresponding gain of leaves 

 of a vine having 20 rings was only 2.1%. The respiratory rate (CO2 

 elimination) also falls with increase in area of vein islets. Vines aver- 

 aging 7.3 years old, bore leaves that eliminated CO2 amounting to 

 0.0349% of green weight per hour, while this rate for vines averaging 

 25.4 years old eliminated 0.0297% per hour. If the leaves are dried 

 and powdered the powder from old plants has a much lower imbibing 



