4 88 STUDIES IN FOSSIL BOTANY 



cm. The external surface bears the marks of spirally 

 disposed appendages. The character of the genus 

 depends on the arrangement of the vascular tissues. 



Fig. 179. — Ptychoxylon Lcvyi. — Transverse section of a decorticated stem, x, f/i, normal 

 xylem and phloem ; x', ///, first inverted medullary bands of xylem and phloem, 

 continuous at the leaf-gaps with the normal zone ; x", pli ', second inverted zone ; br, 

 stele of a branch. Slightly magnified. After Renault. 



There is an outer, more or less continuous cylinder of 

 wood and bast, surrounding a very large pith, within 

 which are several secondary vascular bands, with their 

 wood developed centripetally, while that of the normal 

 outer ring is centrifugal (Fig. 179). The traces of both 



