252 COBDAITEAE [CH. 



the bordered pits are sometimes separate and circular, and opposite 

 pits occasionally replace the usual alternate arrangement. Another 

 feature on which stress has been laid is that in Cordaites the pits 

 occupy the whole breadth of the tracheal wall ; but this, though 

 frequently the case, is by no means a constant feature. In 

 Dadoxylon Newberryi^ the pits tend to form groups, leaving 

 unpitted areas, as in the genus Coenoxylon^. In the stem of 

 Dadoxylon materiarum Daws, represented in fig. 475 the pits do 

 not always cover the whole of the tracheid- walls : this stem is 

 also instructive as an example of the different appearance presented 

 by pitted tracheids according to the state of preservation. In 

 some places an obhque pore is well shown while in others only 

 the outer border of the pit is seen. Gothan^ has described a 

 specimen in which some of the pits are circular and occupy only 

 the central area of the xylem elements : separate circular pits 

 occur also in D. Pedroi Zeill.* (fig. 476). Similar departures from 

 the normal are illustrated by recent species of Araucarineae. 

 The absence of a torus is another feature shared by Dadoxylon 

 and true Araucarian wood. Annual rings other than incomplete 

 and spasmodically formed rows of narrower tracheids are not as 

 a rule present, and in this respect also Araucaria affords a close 

 analogy. Thomson^ has figured a transverse section of a root 

 from English Coal Measures in which rings of growth are well 

 defined; and other instances are recorded. In an Australian 

 species named by Arber D. australe^, there are well-marked rings 

 of growth, and this is equally the case in some Indian wood' of 

 Permo-Carboniferous age, more nearly allied to Mesoxylon than 

 to Cordaites, and in a Dadoxylon of similar age from South Africa. 

 On the other hand the statement that annual rings occur in 

 Palaeozoic wood is often incorrect, partial rings having been 

 confused with regular concentric cyUnders of summer elements. 

 Dawson and Matthew^ described rings in D. ouangondianum, 

 and Goeppert and StenzeP, who examined the Canadian material, 



1 PenhaUow (00) p. 64, fig. 12. 2 See page 293. 



" Gothan (05) p. 16. • See page 259. 



= Thomson (13) p. 19. « Arber (05) B. p. 191. See postea, p. 255. 



' An account of this wood is being published by Miss Holdeu. 

 « Matthew (10) p, 77, Pis. i., 11. 

 ^ Goeppert and Stenzel (88) p. 10. 



