462 OPHIOGLOSSALES 
The facts thus stated relating to the vascular structure of the stock in 
the three genera are all consistent with a theory of origin from a type 
with primarily a solid protostele, and subsequently a medullated monostele : 
for the structure of the vascular system in the mature shoots of them all 
is in point of fact a hollow cylinder perforated by the leaf-gaps: in 
Botrychium and Helminthostachys, where these are less closely grouped 
than in Ophzoglossum, the fundamental structure as a vascular cylinder is 
plainly seen. The opening of the cylinder to give exit to the leaf-trace 
is a characteristic of that type designated by Jeffrey ‘‘ phyllosiphonic,” and 
he distinguishes it from the “cladosiphonic type,” in which the leaf-trace 
passes off from the axial system without any opening. It has already 
been pointed out that these two types are the anatomical expression of 
the relative prevalence in the whole shoot of the axis in the cladosiphonic, 
and of the leaf in the phyllosiphonic type. Supposing in any phyletic 
series there should be an increasing dominance of the leaf, it would be 
reasonable to expect evidence in the individual of a transition from the 
one vascular type to the other. In the young plants of the Ophioglossaceae 
themselves there is no indication of. any such transition, for the young 
plants are phyllosiphonic from the first. It will, however, be shown 
later that on comparative grounds there is reason to think the origin 
of the phyllosiphonic state in the Ophioglossaceae was from the clado- 
siphonic, following upon an increase of proportion and importance of the 
leaf. 
The leaf-trace itself is typically a single strand of the collateral type. 
This is seen in Botrychium and Helminthostachys, and in most species of 
Ophioglossum. The collateral strand may have its margins curved together 
on the adaxial side, so that in the petiole of large leaves it may approximate 
to a concentric structure, as in &. wirgintanum; but this is merely a 
modification of the coll&teral structure. Even in the large-leaved He/min- 
thostachys the leaf-trace comes off as a single strand, though it branches 
very soon, in fact before the cortex is traversed, to form the numerous 
strands of the petiole (Fig. 2578). The condition seen in some species 
of Ophioglossum is interesting for comparison with this, forming as it does 
an exception to the rest of the family., In the section Awophioglossum 
the leaf-trace comes off, as in other Ophioglossaceae, as a single strand, 
which soon branches into three; and this fact is embodied in Prantl’s 
diagnosis as amended by myself.) But in the section Ophioderma the 
numerous strands of the petiole are not united into a single strand at 
the base: they are inserted as separate strands upon the vascular system 
of the stock. It is still uncertain whether or not §Cheiroglossa shares this 
character. A comparison with other forms of Op/zoglossum shows this con- 
dition to be exceptional, and it is probably a derivative state, the separation 
of the strands shown in other species only in the upper leaf having been 
continued in §$Ophioderma down to their actual base of insertion on the 
lAnn. of Bot., xviii., p 215. 
