308 THE ANATOMY OF WOODY PLANTS 



a cordon completely around the phloem. Outside the cells of nar- 

 row lumen is found a second zone, composed of elements larger in 

 diameter. Both outer and inner zones consist of tracheary elements 



FIG. 219. Stereoscopic view of a cordaitean wood from Lancashire, England 



and constitute the so-called double transfusion sheath. In b, the 

 longitudinal view, the various elements of the bundle are shown 

 in the median section. The protoxylem is continuous with tra- 

 cheids which by the usual transitions gradually pass into the 



