CORDAITALES AND GINKGOALES 311 



living genus is extremely meager and in fact is confined to the 

 structure of woods which have been referred to the group. 



The stem in Ginkgo is characterized by the presence of clear 

 annual rings which terminate with tracheids provided with tan- 

 gential pits and in this respect reveal a marked contrast to the 

 tracheary elements constituting the remainder of the annual 

 increment. It is obvious that as regards the organization of the 

 annual ring the group under discussion is distinct from those rare 

 cordaitean stems in which yearly zones of growth can be dis- 

 tinguished, by the presence of tangential terminal pitting. In 

 other respects, however, the structure of the wood is clearly archaic, 

 for there are no parenchymatous elements present other than those 

 related to the rays. The pith and cortex in the group possess 

 secretory canals which are comparable to those found in certain 

 Abietineae. 



The longitudinal aspect of the secondary xylem in Ginkgo 

 is very different from that of the Cordaitales. In the more ancient 

 group the radial pits are often extremely numerous and they are then 

 angular by mutual contact. In Ginkgo the pores of the tracheids 

 are not so abundant as to be described as crowded and, moreover, 

 instead of being alternating and angular as in the older tribe 

 are round and opposite. Another equally striking feature offers 

 itself in the presence of transverse bars of pectic cellulose in the 

 walls of the tracheids between the pairs of opposite pits. These 

 may conveniently be designated bars of Sanio, to distinguish 

 them from the trabeculae of Sanio, structures which are found 

 not uncommonly in all woods of secondary origin from (and 

 including) those of the Cycadales to those of the dicotyledons. 

 The latter structures consist of ligneous processes crossing the 

 cavity of the tracheid, possibly due to the activity of parasitic 

 fungi, while the true bars of Sanio are concealed in the wall itself 

 and consist of pectic cellulose. Bars of Sanio are found only in 

 the walls of tracheids of secondary origin, and statements as to 

 their occurrence in any elements of the primary wood are erroneous. 



An interesting condition appears in the organization of the 

 secondary wood of the peduncle of the seed. In Fig. 222 is 

 shown the transition region in the xylem. It will be observed 



