6 Lang, Vasadar Anatomy of i he OpJuoglossncav. 



developed on the dorsal side of the stele from which the 

 leaf-traces arise than on the ventral side. 



The mesarch xylem is usually altogether primary, but 

 in rhizomes which bear lateral branches a considerable 

 d-evelopment of accessory xylem, which appears to be 

 comparable to secondary thickening, may take place. 

 This is due to active tangential divisions in the parenchy- 

 matous cells between the phloem and the xylem. The 



Fii^. 4. Transverse section of a large slelc of Helmin- 

 thostachys without internal endodermis, which has borne a 

 branch at the preceding node. In addition to the centripetal 

 and centiifugal xylem a zone of accessory or secondary xylem 

 (cross-hatched) has developed on the outside of the ordinary 

 primary xylem. A root is being given oft" at R. ( x 30.) 



zone of accessory or secondary wood may either simply 

 supply the branch or may be present all round the stele 

 for some distance before the departure of the branch and 

 persist for some distance beyond its departure {^Fig. 4). 

 Thus centripetal primary xylem, centrifugal primary 

 xylem and (in exceptional cases) secondary xylem can 

 be distinguished in Hchninthostnchys. 



