POROXYLEAE 509 



one, so that the whole leaf has been compared to a 

 single leaflet of a Cycad such as Bowenia. 



In the fact that the vascular bundles retain collateral 

 structure throughout the leaf, Poroxylon agrees with 

 Medullosa but differs strikingly from Lyginodendron, 

 in which, as described above (pp. 367 and 375), the 

 structure becomes concentric as the petiole is entered. 

 We shall see presently that the leaves of Poroxylon 

 present the closest analogies with those of the 

 Cordaiteae. 



An interesting point in the morphology of the 

 genus Poroxylon is the fact that the stem bore axillary 

 branches, thus presenting a further analogy with 

 some forms of Lyginodendron. The vascular system 

 of the branch was inserted on the two bundles of the 

 main axis, between which the trace of the subtending 

 leaf passed out. Thus, in the section shown in Fig. 

 181, A, the branch, if present, would have been in 

 vascular connection with the bundles marked 6 and 9. 

 The first few internodes of the branch were short, and 

 the first leaves probably rudimentary, judging from the 

 small development of the leaf-traces supplying them. 

 In its upper part, the branch assumed the same 

 structure as the main stem. 



Poroxylon appears to have possessed a complex 

 branch-system, for shoots are found of very different 

 calibre, independently of differences due to age. 



Roots and rootlets, which have been found in 

 association with Poroxylon Boyssetii and Edwardsii, 

 agree so exactly, in the structure of their histological 

 elements, with the stems of those plants, as to leave 

 no reasonable doubt that MM. Bertrand and Renault 



