576 DR. D. H. SCOTT ON BOTRYCHIOXYLON PARADOXUM. 
almost wedged in between the strands of secondary xylem. At any rate, such indications 
of protoxylem as we find point to a centripetal development of the internal xylem. 
The longitudinal section passing through the “mixed pith” shows that the larger 
internal tracheides are long scalariform elements. The part figured (Pl. 41. fig. 22) 
shows three of the smaller tracheides, about 20 in diameter; some of them appear to be 
spirally thickened. 
The large tracheides of the secondary wood have scalariform markings, the bars being 
often short, so that there are two or three series on the same wall (Pl. 41. fig. 21). There 
are no tangential sections, but, judging from oblique parts of the transverse sections, it 
appears that the sculpturing was present on the tangential as well as on the radial walls. 
The cells of the ground-tissue accompanying the internal xylem are shown in longi- 
tudinal section to have been of moderate length, with square ends (Pl. 41. fig. 22). Some 
of them have dense brown contents. The small-celled phloem is fairly shown in places 
in the best transverse section (Pl. 38. fig. 7), and in the longitudinal section the elements 
are seen to be elongated. No definite pericycle or endodermis has been identified, 
It will be noticed that the structure of the stele agrees closely with that of such a 
member of the Zygopterideze as Ankyropteris corrugata, with the one important difference 
that the peripheral zone of xylem, which is part of the primary structure in the Ankyro- 
pteris, is here a secondary tissue. It is true that in 4. corrugata formation of secondary 
xylem is not unknown. Jn a small dichotomizing stem in my collection there is very 
distinet secondary wood in one of the two steles resulting from the bifurcation (Pl. 40. 
fig.19,2?). It is, however, only a local formation, superadded to the normal woody zone, 
and it is only represented to a small extent in the sister stele. In Botrychioxylon the 
main part of the wood is secondary, only thinning in the upper part of the stem. In 
fact, so far as the peripheral xylem-zone is concerned, the condition is the same as in 
Botrychium, where the primary wood of related Ferns is mostly or even wholly replaced 
by radially seriated tissue with cambial growth. In Botrychium, of course, there is, as à 
rule, no internal xylem, so that in some cases practically nothing primary is left. Recent 
work, however, has shown that internal tracheides occur normally in the first-formed 
part of the stem as well as occasionally in advanced specimens and abnormally as a 
traumatic effect (B. ternatum) (Bower, ’112). The analogy with Botrychioxylon, as 
Prof. Bower has pointed out, thus becomes extremely close, even closer than I realized 
when the comparison first occurred to me. Internal tracheides have also been observed 
in the other genera of Ophioglossacez, both in the stem of the sporeling and in small 
stocks. In these genera the woody zone is primary. Thus there is a complete analogy 
in these respects between the Zygopterideze (including Botrychioxylon) and the modern 
Ophioglossacez, with the distinction that in the former the internal tracheides are 
constantly present, while in the latter they have come to be only an occasional feature 
of the structure. 
THE Lear-Trace. 
The stele in the transverse section shown in Pl. 37. fig. 2 is evidently preparing to 
give off a large lateral strand to some appendage. The protrusion of the wood (Lt.) is 
1:5 mm. wide at the base; a strand of nearly horizontal tracheides passes out into it 
