i 9 o8] THIESSEN—DIOON EDULE 377 



at points not definitely located but well-distributed; two strands of each 

 organ leave the cylinder approximately on the same side as the leaf for 

 which they are destined, and run more or less directly through the 

 cortex into the central or abaxial part of the petiole without branch- 

 ing; while the other two strands of each organ leave the cylinder 

 approximately on the opposite side and describe a wide curve around 

 it, the one in one direction and the other in the other, and finally 

 ascend in the dorsal or adaxial part of the petiole, branching repeat- 

 edly. The girdle on the side toward which the spiral may be said to 

 turn is generally the longer one. 



5. When the cotyledonary vascular strands leave the vascular 

 cylinder they are endarch, gradually become mesarch in their upward 

 course, and finally approach the exarch condition. 



6. The foliar vascular strands also are endarch at their separation 

 from the vascular cylinder, and in their upward course become 

 mesarch and finally exarch. In the very young leaf this transition 

 extends through the whole base and petiole uniformly; but in the 

 adult leaf it is comparatively rapid through the base up to the lower 

 part of the petiole, where only a few centrifugal elements remain, and 

 which remain uniform until in the rachis, where the transition is com- 

 pleted, so that in the pinna only centripetal xylem is left. 



7. In the lower stretches of the foliar strands a considerable 

 amount of the centrifugal wood is primary xylem. 



8. In the foliar strands of the embryo and seedling, the xylem 

 tissue, so far as developed, from below upward passes from an inner 

 to a central position in reference to the whole bundle. 



9. The girdle is established very early, and is horizontal from the 

 beginning. 



United States Geological Survey 

 Washington, D. C. 



LITERATURE CITED 



1. Von Mohl, Hugo, Ueber den Bau des Cycadeen-Stammes und sein Ver- 

 haltniss zu dem Stamme der Coniferen und Baumfarn. Abhandl. Kgl. Bay. 

 Akad. Wiss. 1:399-442. pis. 18-20. 1832. 



2. Karsten, G., Organographische Betrachtung der Zamia muricata Willd. 

 Abhandl. Kgl. Akad. Wiss. Berlin 1856:193-219. pis. 1-3. 



