CONIFERALES 349 



characterized by alternating crowded pitting and the absence of 

 bars of Sanio as well as of parenchymatous storage elements. To 

 this type of wood occurring as a fossil the name Araucarioxylon 

 is given. A second genus of fossil woods is diagnosed by the 

 presence of spirals on the inner walls of the tracheids and by the 

 absence of wood parenchyma, under the generic designation 

 Taxoxylon. Those woods characterized by the possession of 

 ligneous resin canals in the horizontal and vertical planes are 

 included under the generic appellation Pityoxylon. In coniferous 

 wood without either resin canals or conspicuous storage paren- 

 chyma, the name Cedroxylon is adopted. Woods which, on the 

 contrary, are provided with abundant storage parenchyma are 

 designated Cupressinoxylon. In all genera of fossil woods except 

 the first the pitting is characteristically opposite. In the 

 Araucarioxylon type the pitting is frequently crowded and 

 alternation and bars of Sanio, universally present in other existing 

 coniferous woods, are conspicuous by their absence. 



Clearly the use of even the small number of genera of fossil 

 woods indicated in the preceding paragraph in connection with 

 evolutionary inferences must, in the light of the conditions de- 

 scribed for the various living conifers in the earlier part of the 

 present chapter, be a matter of great difficulty. This arises from 

 the fact that the organization of the mature wood in a given 

 conifer is by no means necessarily an indication of its true system- 

 atic position. The interpretation of the significance of wood 

 structure in fossil and existing conifers can be successfully attacked 

 only with a knowledge of the general principles of comparative 

 anatomy. A failure to realize this situation has led to very many 

 erroneous interpretations, both anatomical and paleobotanical. 

 For example, it is quite clear from the data supplied at an earlier 

 stage that the primitive condition of both the Araucarioxylon 

 and Taxoxylon type was a Cupressinoxylon, since abundant wood 

 parenchyma diffused throughout the annual ring was formerly a 

 feature of organization of these ligneous types, as is shown by 

 a consideration of conservative organs, experimental results, and 

 fossil evidence. Further, in some instances the Cupressinoxylon 

 type has been clearly derived from the complicated structure 



