488 OPHIOGLOSSALES 
axial system by foliar lacunae, after the manner of the phyllosiphonic type. 
But in its lower parts, Zmesipderis is typically cladosiphonic: it is thus 
seen that a phyllosiphonic structure may arise in a cladosiphonic stock, 
and the illustration is taken from that group of plants which show analogies 
with the Ophioglossaceae in other respects: on comparison of Fig. 268 
with Fig. 258 of O. Bergianum, the essential similarity of the two cases 
is evident. But in the Ophioglossaceae the structural dominance of the 
leaf is on our hypothesis a consequence of the advance of the leaf towards. 
megaphylly, combined perhaps with weakening of the axis which bears 
it. It matters little how the balance between the parts of the shoot is 
disturbed: the progression would be essentially the same in either case. 
These considerations show, in the first place, that it cannot be assumed 
that all phyllosiphonic plants are necessarily derived from a distinct and 
primitive phyllosiphonic stock, such as Jeffrey assumed for his Pteropsida : 
and secondly, that analogy of their structure with Zmesipéeris indicates a 
possible origin of the phyllosiphonic type in the Ophioglossaceae, phyletically 
quite distinct from that shown by the true Ferns.! 
The leaf-trace in the Ophioglossaceae is typically a single strand, which 
branches within the cortex into strands which vary according to the pro- 
portions of the leaf which they serve:? these facts accord with a theory of 
origin of the leaf from the simpler type. It is interesting to note that 
“the branching of the leaf-traces within the cortex is very characteristic 
of Sphenophyllum.”*® As regards the structure of the collateral strands of 
the leaf, those of the larger forms show similarity to the Eusporangiate 
Ferns, the smaller correspond rather to those of the larger-leaved strobiloid 
Pteridophytes. 
The occurrence of secondary thickening in the Ophioglossaceae is. 
occasional rather than typical of them. In Helminthostachys it is absent: in 
Ophioglossum a feeble growth has been occasionally seen; but in Botrychium 
it is a marked feature, and extends from the axis onwards to the base 
of the root. This inconstant occurrence of secondary activity, sometimes 
feebly shown, has its parallel in other affinities, both of Filicales on the 
one hand, as in the Marattiaceae,* and of the Psilotaceae, where a develop- 
ment very like that of Ophioglossum has been seen in Psilotum® The 
1It will be seen below that certain Ferns, for instance the Botryopterideae, are not 
phyllosiphonic: thus the anatomical distinction of Jeffrey breaks down on both sides. 
? The only known exceptions to this are in O. pendulum and simplex, where the leaf- 
trace is inserted on the cauline system as several distinct strands. These species belong, 
however, to a section of the genus believed to be highly specialised rather than primitive 
types: and this character itself must, by analogy with the similar cases in the Ferns, be held 
to be derivative (see Aun. of Bot., xviii., pp. 209, 215). 
3 Scott, Studies, p. gt. ‘Farmer, Azmn. of Bot., xiii., p. 440. 
5 Boodle, Zc. Scott (Journ. Roy. Micr. Soc., 1906, pp. 519-521) has described 
under the name of Botrychioxylon a new genus from the Lower Coal Measures, with 
‘radially seriated wood, apparently of secondary character.” It is related anatomically 
to Zygopteris somewhat as Botrychium is to Ophioglossum. 
