248 CORDAITEAE 



CH. 



Artisia alternans is quoted by authors as evidence of the persistence 

 of Cordaites into the Jurassic period ; but in view of the fact 

 that the discoid type of pith is not by any means confined to 

 Cordaites or even to the Cordaitales the occurrence of Artisia is 

 in itself of no great botanical significance. 



It is also true that a discoid pith is not an invariable attribute of 

 stems closely allied to the genus Cordaites ; but if these reservations 

 are made the use of the generic term Artisia serves a useful purpose. 



ii. Petrified stems. 

 Dadoxylon. Endlicher. 



Palaeobotanical literature contains numerous descriptions of 

 Palaeozoic petrified wood occasionally enclosing an Artisia pith- 

 cast described under such names as Dadoxylon, Cordaioxylon, 

 Araucarioxylon, etc., and regarded as portions of Cordaitean 

 stems. It is, however, certain that much of this material belonged 

 to stems other than those of Cordaites. Recent research has 

 demonstrated the insufficiency of the secondary xylem alone, 

 however well preserved, as a safe guide to generic position : stems 

 identical in the structure of the secondary xylem differ in that 

 of the primary portion of the stele, and it is on the characters of 

 the latter tissues that several genera have recently been founded. 

 Mesoxylon affords a striking example of the importance of the 

 primary xylem as a distinctive feature. As Gothan 1 points out, 

 the species of Calamopitys recently made the type of a new genus 

 Eristophyton 2 would, in the absence of the primary xylem, probably 

 be regarded as Cordaitean. It is important to recognise the 

 limitations imposed by the imperfection of the material; we 

 cannot in most cases determine whether a specimen should be 

 referred to Cordaites or Mesoxylon, and while it may be described 

 as probably Cordaitean in affinity there remains the possibility 

 that some of the Palaeozoic plants with secondary wood like that 

 of Cordaites, if their reproductive organs were known, would not 

 be included in the Cordaitales. Goeppert's species Araucarites 

 Tchihatchejfi, which Renault 3 quotes as Cordaites, has recently 

 been assigned to a new genus Mesopitys* because of certain 



1 Gothan (05) p. 28. 2 See page 199. 



3 Renault (79) B. p. 288. * See page 295. 



