180 CONIFERALES [CH. 



they afford with regard to the phylogeny of the Araucarineae an< 

 'the Abietineae must be left to the individual student. 



Annual rings occasionally well marked but not infrequentb 

 absent or indistinct. Tracheids with uniseriate and more or 

 flattened bordered pits or with two or more rows of alternate 

 polygonal pits on the radial walls. The alternate disposition, 

 even if unaccompanied by flattening and the polygonal contour 

 of the pits, is a Dadoxylon feature if it is the dominant arrangement 

 and not a sporadic occurrence. Bordered pits occasionally occur 

 on the tangential walls but they are smaller and comparatively 

 rare. Rims of Sanio absent. Xylem-parenchyma usually absent 

 or feebly developed ; resiniferous tracheids occasionally present. 

 Medullary rays uniseriate, very rarely double, homogeneous, 1 30 

 or as many as 50 cells deep ; walls comparatively thin and without 

 pits on the horizontal and vertical walls ; the radial walls may show 

 1 15 small pits, the oblique pore being occasionally enclosed in a 

 feebly developed border. In view of the entire absence of pits on 

 the ray cells of at least some recent Araucarias the structure of the 

 ray cells in fossil stems requires careful revision 1 . 



In the following brief descriptions of species of Dadoxylon a few 

 examples are chosen to illustrate the wide geological and geo- 

 graphical range of fossil wood of this type, but it must be remem- 

 bered that in many cases no positive statement can be made with 

 regard to the nature of the parent-plant beyond the facts afforded 

 by the anatomical characters of the stem. Evidence bearing on 

 the geological age of the Araucarineae is discussed in the course 

 of the description of genera founded on vegetative shoots and 

 reproductive organs. Species of Dadoxylon from Carboniferous 

 and Permian strata have already been described in Chapter xxxm. 

 as more probably referable to Cordaites or at least to the Cordai- 

 tales, and it is not by any means impossible that some of the 

 Dadoxylons recorded from Triassic or even higher strata may 

 belong to Cordaitalean species rather than to the Araucarineae. 

 The evidence afforded by petrified wood in conjunction with that 

 derived from vegetative remains lends probability to the view that 

 Araucarian plants existed at least as early as the later Palaeozoic 



1 See, in addition to Kraus and other authorities, Lignier (07 2 ). 



