486 OPHIOGLOSSALES 



always well marked : it is next the pith in Ophioglossum and Botrychium, 

 but mesoxylic in Helminthostachys. The central protoxylem in the stele 

 of the seedling is in a position corresponding to that in the medullated 

 stele of the older stem of Ophioglossum and Botrychium ; consequently, 

 the mature state appears to be a natural amplification of the centroxylic 

 protostele. 



The mesarch xylem of Helminthostachys presents a difference from 

 the rest, and it raises a question as to the importance attaching to the 

 exact position which the protoxylem holds, for purposes of comparison. 

 The stele of Tmesipteris is mesarch also in its upper region (Fig. 268), and 

 this is stated to be so also locally in Psilotum, though the position of the 

 protoxylem in both is peripheral in the lower parts. Again, in Selaginella 

 spinulosa it fluctuates in the individual stem (Fig. 173): in the seedling all 

 conditions from the endarch below, to the mesarch, and finally to the exarch 

 above, may be seen in sections taken successively from the same plant. The 

 Psilotaceae and Ophioglossaceae thus show a similar instability within 

 their respective families, and in less degree in the individual plants also, 

 and this instability is shared by Selaginella. This deprives comparisons 

 based on the exact position of the protoxylem of much of their cogency, 

 so far as they relate to these families. Too much weight has been 

 attached to the position of the protoxylem in the comparative study of 

 the Pteridophytes. It is a well-known principle of taxonomy that 

 diagnostic characters which may be good in one alliance may be so 

 fluctuating as to be useless in another. This appears to be so in 

 respect of the position of the protoxylem in many of the strobi- 

 loid Pteridophytes. Accordingly, a prevailing, though not constant 

 central position of the protoxylem in any given family cannot be held 

 as in itself invalidating comparisons on other grounds with types where 

 the protoxylem is usually though not always peripheral. The conditions, in 

 point of fact, overlap within certain families, or even in the individual ; 

 the position of the protoxylem as a comparative or diagnostic character 

 must therefore be held as suspect. In the present case the prevalent 

 centroxylic state of the Ophioglossaceae cannot in itself be held to 

 dissociate them anatomically from the strobiloid Pteridophytes (and particu- 

 larly from the Psilotaceae), since both meet on common ground in showing 

 at times a mesoxylic condition. 



The stele of the Ophioglossaceae, amplified as described, does not 

 remain a closed cylinder: its continuity is interrupted by foliar gaps, the 

 vascular ring opening at the point of exit of each leaf-trace. The structure 

 is that described as phyllosiphonic by Jeffrey, and distinguished by him 

 from the cladosiphonic type, where the leaf-trace passes off from the stele 

 without any opening. Jeffrey has laid this distinction down as separating 

 his Lycopsida from his Pteropsida. According to their structure, the 

 Ophioglossales would then fall into the Pteropsida. Jeffrey remarks x that 



1 Phil. Trans. , vol. cxcv., p. 144. 



