ly: 



GYMNOSPERMS 



eter, still with the spiral markings; then come scalariform tracheids, 

 and finally the pitted tracheids of the secondary wood; so that there 

 is a gradual transition from the earliest spiral elements to the pitted 

 tracheids of the secondary wood (fig. 183). Such a sequence can be 

 seen in seedlings of Dioon spimdosum, but not in older plants, where 

 even the earliest protoxylem has scalariform tracheids. 



Other genera, as Mesoxylon, 

 Pitys, Callixylon, and others, 

 have various amounts of centrip- 

 etal xylem. In Poroxylon, all of 

 the metaxylem is centripetal, so 

 that the bundle is exarch (fig. 

 184). The general tendency is to 

 reduce the amount of centripetal 

 xylem, so that the mesarch con- 

 dition changes to the endarch. 



The multiseriate pitting, re- 

 sembling that of the living genus 

 Araucaria, is characteristic. The 

 pits are bordered, in two, three, 

 and sometimes even five rows, 

 and are usually so crowded that 

 they have a hexagonal outline. 

 The pits are almost entirely re- 

 stricted to the radial walls, but 

 tracheids of the older regions 

 have pits on the tangential walls. 

 The medullary rays are narrow, usually only one cell wide; but in 

 Pilys and its allies they are wider, the middle of the ray reaching a 

 width of several cells. 



The pith, in the Cordaitcs section of the order, is very characteris- 

 tic. It cracks transversely, but remains intact at the periphery, 

 where it borders on the xylem, giving the whole pith the appearance 

 of a pile of concave discs. So it was called a discoid pith. In many 

 fossils, the pith drops out, and was first discovered and described as 

 a distinct genus, Artis describing it as Slcrnbcrgia, and Sternberg 

 returning the compliment by naming it Arlisia.^^^ 



Fig. 182. — Cordaitessp.: transverse sec- 

 tion of stem with large zone of wood, small 

 pith and cortex, and numerous growth- 

 rings. — From a photograph by Land of a 

 section made by Lomax. Natural size. 

 From Coulter and Chamberlain, Mor- 

 phology of Gymnos perms'^ (University of 

 Chicago Press). 



