316 THE ANATOMY OF WOODY PLANTS 



organs and regions. It has been pointed out in the foregoing 

 paragraphs that, although centripetal wood of the cryptogamic 

 type is represented almost exclusively by transfusion tissues in 

 the mature vegetative leaf of Ginkgo, it is present in a clearly 

 recognizable form in the cotyledon, in the microsporophylls, and 

 in the peduncle of the ovuliferous apparatus. In the case of the 

 microsporophyll the xylem, and more particularly the vestigial 

 centripetal xylem and transfusion tissue, are in clear relation to 

 the reticulately thickened opening mechanism of the microspo- 

 rangia. This feature is of value, not only as indicating the filiation 

 of the Ginkgoales with lower groups, but also as indicating the 

 morphological nature of the arrangements for dehiscence of the 

 spore sac in the seed plants above the Cycadales. The absence 

 of longitudinal parenchyma in the secondary wood is another 

 criterion of the relationship of the Ginkgoales with lower groups, 

 while the presence of tangential pitting in the terminal region of 

 the summer wood clearly relates the group with modern gymno- 

 spermous types. The radial pitting of the tracheids and associated 

 structures is also of importance as indicating the phylogenetic 

 position of the genus. As has been shown above, the radial pits of 

 Ginkgo are opposite in the mature wood, and often in the inter- 

 vals between them, particularly toward the ends of the tracheids, 

 have transverse bars of pectic cellulose imbedded in the tracheary 

 wall, and these are conveniently designated bars of Sanio. 

 The opposite pitting and the occurrence of bars of Sanio are 

 features which clearly co-ordinate the wood of the Ginkgoales 

 with that of the higher gymnosperms. However, in the primitive 

 regions and organs of the living Ginkgo we find both the pitting 

 of the Cordaitales and the absence of bars of Sanio which are 

 universally characteristic of the older gymnosperms from the 

 Cycadales downward. It seems quite obvious that Ginkgo is a 

 genus of the utmost importance from the standpoint of evolution- 

 ary anatomy, since it summarizes in such a remarkable manner 

 the anatomical characteristics of both Archigymnospermae and 

 Metagymnospermae. Its significance in the direction indicated 

 will be fully realized only after the next tribe, the Coniferales, 

 have been anatomically considered in the following chapter. 



