GINKGOALES 



189 



observed is a reduction from the tetrarch condition. In the case of 

 three cotyledons, which sometimes occurs in Ginkgo, the vascular 

 situation is modified, so that the root is ultimately triarch. 



The testimony of the vascular anatomy shows that Ginkgo has 

 departed farther from the primitive fern stock in this regard than 

 have the Cycadales and the Cordaitales, only the foliar gaps and the 

 mesarch bundles of the cotyledons remaining to indicate the connec- 

 tion in vascular structure (36). 



THE LEAF 



There is great variation in the size and in the lobing of the leaves 

 of Ginkgo. The blades are often deeply cut and with more than two 

 lobes, and the same tree may show every gradation between deeply 



Fig. 214. — Baicra gracilis: leaf showing bilobed character, but each of the two 

 principal lobes repeatedly dichotomous; regarded as a transition between Baiera and 

 Ginkgo; natural size. — After Renault (9a). 



lobed leaves and those with nearly entire margins. The leaves of 

 mesozoic species were much more divided, extreme forms being 

 palmately dissected into numerous very narrow lobes (28) (fig. 214). 

 In the surviving genus Ginkgo, the lobed condition is always found 

 in the leaves of seedlings (fig. 2^15), and usually on the long shoots, 

 leaves from the top of a tree being particularly deeply cleft. The 

 leaves on the dwarf shoots are usually nearly entire, but when the bud 

 of a dwarf shoot develops into a long shoot, the lobed character of 



