CORDAITALES AND GINKGOALES 



313 



at the present time an almost inexcusable ignorance in regard to 

 the nature and distribution of the structures here designated 

 bars of Sanio. They are clearly correlated with opposite pitting 

 and are a feature of the secondary wood, not appearing in the 

 organization of the tracheids of the primary xylem. Evidently 

 the structures in question are of considerable value in the identi- 

 fication of gymnospermous woods and consequently must rank 

 high as a diagnostic criterion among competent anatomists. 

 The mature vegetative leaf in Ginkgo supplies very little evidence 



FIG. 223. Foliar bundle of Ginkgo, showing transfusion tissue (after Sprecher) 



of the presence of centripetal elements in the strict sense of the 

 term. In the terminal region of the blade of the leaf a well- 

 marked zone of transfusion tissue manifests itself, as is shown in 

 Fig. 223; but typical centripetal tracheids are usually conspicuous 

 by their absence. In the cotyledon, however, the centripetal or 

 cryptogamic wood is present in a much clearer manner in accord- 

 ance with the principle of recapitulation discussed in an earlier 

 chapter. The reproductive leaves, both ovuliferous and staminate, 

 also show the centripetal elements in a good condition of develop- 

 ment, although even here they more nearly resemble transfusion 

 tissue. In the stalk which supports the pair of ovules centrip- 

 etal elements and ordinary transfusion cells are seen in the upper 

 region in great abundance and are likewise found in the collar sur- 

 rounding the base of the seeds. The situation presented by the 



