546 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and that the element belongs to the secondary wood is vouched for 

 by the presence, on the right and left, of cells of the wood rays in 

 transverse section. The dark transverse sections of the bars of Sanio, 

 embedded in the substance of the lignified tracheid wall, between the 

 radial pits, are easily distinguished. Figure /, Plate 6, illustrates the 

 conditions observable in the ordinary secondary wood of the vegeta- 

 tive axis. Obviously the pits as seen in profile are here in close contact 

 and are not separated by bars of Sanio. 



The stem of the seedling and the first annual ring of the adult 

 branches of various species of Araucaria, were examined in the region 

 of the primary xylem for the presence of bars of Sanio. Where any 

 evidence of their existence was apparent, however, they were ex- 

 tremely indistinct and ghostly and very evanescent. The same con- 

 ditions were observed in the root. In accordance with the now 

 widely accepted dictum of comparative anatomy that the leaf trace is 

 very apt to perpetuate ancestral conditions, the foliar traces of several 

 species of Araucaria were investigated, but on account mainly of the 

 small size of the tracheary elements, it was difficult to make out the 

 presence of bars of Sanio, with an^^ distinctness, although their 

 existence in these regions was indicated. 



The absence of bars of Sanio in the seedling, where the pits are 

 often widely separated from one another, is of particular significance, ■ 

 in view of the statement of Gothan, that their non-existence in Aura- 

 carian woods is to be explained by the close approximation of the pits. 

 Obviously such an explanation will not hold in the case of the undoubt- 

 edly Araucarian wood of Araucarian seedlings. ^^ It follows that woods 

 from the Mesozoic which are without typical Araucarian pitting, can 

 best be diagnosed as to their affinities not on the basis of their radial 

 pitting or even their ray structure, but by the presence or absence of 

 bars of Sanio, in well preserved material. Where the bars are present 

 in the mature wood, we may certainly assume that the wood is not 

 Araucarian. On the other hand, where the bars of Sanio are dis- 

 tinctly absent in well preserved Mesozoic woods, it may safely be con- 

 cluded that they are of Araucarian affinites, no matter what may be 

 the nature of their radial pitting or that of the cells of their rays. 



In conclusion of the descriptive part of the present article, it is 

 necessary to refer to the pitting and structure of the tracheids in the 

 living genus Agathis. It has been found here, that even in the cone, 

 the tracheids very quickly cease to show opposite pitting and the bars 



28 Gothan, Die fossilen Holzreste von Spitzbergen, Kung. Svensk. Veten- 

 ekap, Handlingar, Bd. 45. 



