Manchester Memoirs, Vol. hi. {\g\2\ No. \% 3 



The chief features of the vascular anatomy of the 

 stem and leaf-trace must now be briefly described for the 

 three genera of Ophioglossaceae, Heljninlhostachys, which 

 is in some respects the most instructive, being taken first. 

 Most of the facts are common knowledge, but some 

 additional ones will be added which aid in the interpreta- 

 tion of the vascular structure ; it is with the interpretation 

 rather than with the detailed description of the facts that 

 this paper is concerned. 



On following up the development of the stele of 



Fig. I. Transverse sections of steles showing the tran- 

 sition from the centrarch (a), through the endarch (b) to the 

 mesarch (c) condition in a young plant of Helniiiitho'itachys. 

 (Purely diagrammatic.) 



[In this and the f(jllowing figures the outer line represents 

 the endodermis. The phloem is indicated by a dotted line, 

 and the protoxylem is black. The metaxylem is indicated by 

 linear shading, but where the centripetal tracheides form a 

 mixed pith they are individually indicated.] 



Hehninthostachys from the base of a sporeling plant or 

 from the base of an axillary branch, the earliest stage is 

 one with a solid xylem, the protoxylem elements being 

 situated in the centre (centrarch) and the development of 

 all the metaxylem centrifugal {Fig. ia). Parenchymatous 

 cells are usually mixed with the tracheides, and if they 

 are numerous the stele may have a small pith from the 

 first. In any case by the gradual increase in the number 

 of parenchymatous cells in the centre of the xylem, a 



