THE GYMNOSPERMAE : CYCADALES, ETC. 



735 



and in Cordaites. On entering the blade the traces separate to right and left 

 and then fork repeatedly, each half of the blade having thus an independent 



Fig. 737. — Ginkgo bilobo. A branch showing the 

 characteristic leaf form. 



venation. There are no side connections, so that if a vein is severed near the 

 base of the leaf, the strip of tissue above, which should have been supplied 



Fig. 738. — Ginkgo biloba. Transverse section of 

 the petiole showing the double bundle. 



by its branches, withers and dies. This has often been cited as evidence 

 of the physiological superiority of net-venation, which gives ample provision 

 of alternative passages for the water supply. No doubt theoretically this is 

 true, but it must be pointed out that transverse tearing of the Ginkgo leaf is 

 very difficult owing to its leathery texture, and that in Nature it does not 



