GINKGOALES 187 



VASCULAR ANATOMY 



The transverse section of the stem of Ginkgo shows the general 

 features of Coniferales, namely, a comparatively narrow cortical 

 zone, a thick and compact cylinder of secondary wood developed 

 by a persistent primary cambium (in contrast with Cycadales), and 

 a relatively small pith (in contrast with Cordaitales). In the dwarf 

 shoots, however, the pith is large and the zone of wood comparatively 

 narrow. The primary xylem is endarch, and there is no distinct 



Fig. 212. — Ginkgo biloba: leaf showing bilobed character and dichotomous 

 venation; slightly enlarged. — After Coulter (54). 



mesarch structure except in the bundles of the cotyledons (20) 

 (fig. 213). The leaf trace is double, as in all the more primitive 

 gymnosperms, each of the strands forking at the base of the blade, 

 and the resulting four strands breaking up into the characteristic 

 dichotomous system of veins, which may show now and then slight 

 traces of centripetal wood, and therefore an indistinct mesarch struc- 

 ture. 



The vascular anatomy of the seedling has been reviewed by Hill 

 and De Fraine (51), whose summary contains the facts that Ginkgo 

 differs from the cycads in the rotation of the protoxylem of the cotyle- 

 donary traces; that a single vascular strand enters the base of each 



