486 OPHIOGLOSSALES 
always well marked: it is next the pith in Ophzoglossum and Botrychium, 
but mesoxylic in Aelminthostachys. 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 Ofhzioglossum 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 Zmesipieris is mesarch also in its upper region (Fig. 268), and 
this is stated to be so also locally in Ps¢/otum, 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 Sedaginella. 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 usudlly 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? that 
1 Phil, Trans., vol. cxcv., p. 144. 
