i66 



MORPHOLOGY OF GYMNOSPERMS 



combination; and as the bundles approach their terminations in the 

 leaf the centrifugal xylem may "die out" (8), resulting in an exarch 

 bundle. In fact, Miss Stopes (13) states that the exarch bundle 

 is apparently characteristic of. the majority of cordaitean leaves. 

 In modern cycads this dying out of centrifugal xylem may occur at 



any [level in the leaf, so that 

 exarch bundles are not necessarily 

 restricted to vascular termina- 

 tions. 



The vascular anatomy of Cor- 

 daitales evidently connects the 

 group, by descent or common 

 origin, with such Cycadofilicales 

 as the Lyginodendrineae; but 

 this connection is made still more 

 evident by the occurrence of 

 whole series of anastomosing 

 forms (24). One of the most 

 notable of these is Poroxylon, 

 whose beautifully preserved ana- 

 tomical structure has been de- 

 scribed in detail by Renault and 



Fig. 197. — Cordaites sp. (?): trans- 

 verse section of stem; the pith is small and 

 the extensive wood shows rings that re- 

 semble growth rings; the structures out- 

 side the secondary xylem have not been 

 preserved; natural size. — From photograph 

 of section made bv Lomax. 



Bertrand (3, 4). It was origi- 



nally found in the Permo-Car- 



boniferous of France, a period 



much too recent to be regarded 



as containing members of the phylogenetic series leading to Cor- 



daitales. 



The stems of Poroxylon are slender (approximately i . 5 cm. in 

 diameter), with a complex branch system, and bear spirally arranged 

 and simple leaves separated by rather long internodes. The pith is 

 relatively large and continuous, and the vascular cylinder is collateral 

 exarch according to Bernard and Renault. Furthermore, the 

 leaf trace is double, the two strands being separate and mesarch to 

 the central cylinder; and they remain collateral mesarch in both 

 cortex and leaf. In Lyginodendron, it will be remembered, the col- 

 ateral strands of the leaf trace become concentric when they enter the 



