354 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



which was often discoidal as a consequence of the rapid elongation 

 of the stem, so that casts yield characteristic forms described by 

 the early writers as Artisia or Sternbergia. Structurally the wood, 



commonly known as Araucarioxylon 

 or Dadoxylon, is centrifugal, with the 

 narrow spiral protoxylem lying at the 

 periphery of the pith, succeeded radi- 

 ally by spiral tracheids, soon passing 

 into scalariform elements, which after 

 an interval pass into pitted tracheids 

 greatly resembling those of the modern 

 Araucariales. The bordered pits, 

 which were limited to the radial walls 

 of the tracheids, are usually in two or 

 more rows and so densely crowded in 

 alternating scries that they tend to be 

 hexagonal in outline. The rays are 

 narrow ; originally they may be three 

 cells wide, but those of the secondary 

 wood are usually one or two cells wide 

 at the most. The phloem, of sieve 

 tube:;, parenchyma, and some bast 

 fibers, is like that of modern conifers. 

 The leaf traces were often double, as 

 in some Pteridosperms and Ginkgo. 

 The bark was thick, but is usually not 

 well preserved. The root system was 

 feebly developed for such large trees, 

 diarch in structure, with a broad zone 

 of secondary wood, conforming in its 

 histology with that of the stem. The 

 leaf structure is comparable to that of 

 ;i cvcad pinnule, with collateral bun- 

 dles, and the stomata are confined to 

 the lower surface. 



The fructifications sIioav consider- 

 able variation. The catkins, com- 

 monly preserved as impressions under 

 the name of Cordaianthus, show an 

 elongated axis with bracteate pedicels 

 bearing the alate bilaterally symmetri- 

 cal cordate seeds. Petrified material 

 shows the staminate catkins to consist of a thick axis bearing spirally 

 arranged bracts, between which the microsporophylls are inserted, 

 either singly or massed near the apex. Each microsporoph}!! consists 



Fig. 20.— Cordaianthus from the lower Alle- 

 gheny formation of Maryland. 



