DR. D. H. SCOTT ON BOTRYCHIOXYLON PARADOXUM. 385 
observed elsewhere among Paleozoic Ferns. No doubt, as our knowledge extends, 
we shall find plenty of transitional cases between this structure and the usual primary 
arrangement, j 
It is no longer possible to regard Botrychiorylon as a form of Ankyropteris corrugata, 
for, if we have rightly interpreted the structure of the petiolar bundle, there are no 
peripheral loops, and the plant is therefore not an Ankyropteris at all. As we have seen 
(p. 880), it cannot on petiolar characters be referred to any of the established genera of 
Zygopteridez. The branched spine-like aphlebiæ also form a peculiar character. 
On the whole of the evidence at present available we have no alternative but to keep 
up Botrychioxylon as a distinct genus; its place as a member of the Zygopterides is 
clearly established. In its stem-structure it is one of the simpler members of the 
family, the stele maintaining an almost cylindrical form. ‘The “ mixed pith” (internal 
xylem accompanied by parenchyma) indicates an affinity either with Ankyropleris or 
JMetaclepsydropsis, for Diplolabis has no internal parenchyma and the form of the stele 
excludes any near relation to Asterochlena. Provisionally, I am inclined to regard 
Metaclepsydropsis as the nearest genus. The two genera agree in :— 
1. The structure of the internal xylem, 
2. The form of the stele. 
3. The presence of secondary wood (constant in Botrychioxylon, exceptional in 
Metaclepsydropsis). 
4, The dichotomous branching. 
5. The open sinuses of the petiolar bundle. 
Botrychioxylon, then, is a member of the Zygopteridee, which, while retaining a 
simple form of stele, has advanced in the direction of substituting secondary for primary 
xylem, thus allowing of unrestricted wood-development in proportion to the needs of the 
plant. It appears that wherever there is a differentiation of internal and external 
xylem the latter tends to assume a secondary character. We see this in Lyginodendree, 
Calamopityexe, Mesoxylon, and Cycadean foliar bundles. Later on, the internal xylem 
may disappear altogether—we find it in a moribund condition in Botrychium. 
It is interesting to see in Botrychioxylon the same change ais « en né pr # 
- o imen it is evident that a of the xylem h: 
the leaf-trace. In the Dulesgate specim au vid wii. Rs 
been secondarily added (see p. 383). In the type specimen nearly 
à secondary character, assuming that the strand in question 1s rightly oye x 
leaf-trace, as to which, I think, there can be no real doubt — sut nium ie 
however, the whole of the bundle is still primary; the influence of the $ : 
does not reach so far as this. : ; . 
The anatomical evidence for an affinity between Zygop teridere crise aperi 
materially strengthened by our knowledge of A jene su m ent siii, 
earlier in the paper (p.976). The correspondence in the MOTETO no 378, p. 114) 
and Renault’s original view, based chiefly on sporangial ein owns dk a FN 
is now amply confirmed. Prof. Bower, who at one timo 6 Ta i u we may look for 
different view of Ophioglossaceous relationships, now believes | 
