Manchester Mciiiohs, Voi. Ivi. {i()i2), Xo. 1*4. ii 



structure of which was typical a specimen was found 

 bearing two lateral branches. As is usual in such cases 

 the growth of the parent plant had been arrested. The 

 study of the structure of the specimen {Fig. 6) revealed 

 the existence of centripetal xylem, mixed with the 

 parenchyma of the pith and apparently derived by the 

 conversion of some of the medullary cells into tracheides. 

 Thus, under exceptional circumstances, centripetal xylem 

 forming a mixed pith may be developed in Ophioglossnni, 

 as has been seen above to be the case for Botrychiiiin.j 



GO 



Fig. 6. Transverse section of the stele of a plant of 

 Ophioglosswn which bore two buds. Short tracheides have 

 developed throughout the pith forming a centripetal xylem. 

 ( X 6o. ) 



The occurrence of an internal endodermis in the stele 

 of the Ophioglossacea; and the view to be taken of the 

 nature of the pith in this group have still to be considered. 

 An internal endodermis is found in strong rhizomes of 

 HelviintJiostacJiys and becomes continuous with the ex- 

 ternal endodermis at the leaf-gaps. If Hebniiithostachys 

 be considered by itself the gradual development of the 

 medullated stele as described above and the irregular 

 occurrence of an internal endodermis in large plants 



+ In both cases the stimulus to the development of the tracheides in the 

 pith may have been traumatic. 



