176 CONIFBRALES [CH. 



XIV. THYLLOXYLON. 



Tracheal pitting partially Araucarian. 



Xylem-parenchyma at the end of a year's growth. No true 

 resin-canals, but the central parenchyma of some of the broader 

 rays is replaced by a canal-like space often filled with tyloses. 



Medullary rays uniseriate ; all the walls pitted. 



Jurassic. 



XV. PLANOXYLON. 



Wood with well-marked annual rings, resin-canals usually 

 absent ; tracheids with 1 — 3 rows of alternate hexagonal bordered 

 pits on the radial walls and in the late wood there may be a single 

 row of separate pits. Xylem-parenchyma occurs only between the 

 spring elements and the last-formed tracheids of the previous year. 

 Medullary rays almost entirely uniseriate ; all the walls pitted. 

 Lias to Cretaceous. 



I. DADOXYLON [and Araucarioxylon]. 



The anatomical characters implied by the expression 'wood of 

 the Araucarian type' are enumerated in the Chapter on Recent 

 Conifers, and in Chapter xxxiii. reference is made to the impossi- 

 bility of drawing a clear line of division between the wood of 

 Araucarian plants and that of certain members of the Cordaitales^. 

 The name Araucarites was used by Goeppert^ for fossil wood of 

 the Araucarian type, but the previous use of this name by Presl 

 for impressions of foliage-shoots and cones renders unsuitable its 

 application to wood apart from the fact that Araucarites at once 

 commits an author to a determination implying an affinity which in 

 many cases cannot be demonstrated. Endlicher's non-committal 

 genus Dadoxylon^ has been widely used, especially for Palaeozoic 

 wood having the characters of Araucana or • Cordaites. This 

 designation leaves open the question of precise systematic position. 

 In 1882 Kraus* instituted the genus Araucarioxylon, a name which 

 has been widely adopted for fossil wood both from Palaeozoic and 

 later formations. The practice of limiting Dadoxylon to Palaeozoic 

 species and reserving Araucarioxylon for Mesozoic and Tertiary 



1 See p. 248, Vol. m. = Goeppert (45). 



' Endlicher (47). * Kraus in Schimper (72) A. p. 370. 



