44 
Lang—Studies in the Morphology and 
thickening known in the stems of some Zygopterideae. The marked 
secondary thickening in Botrychioxylon appears more closely comparable 
to what occurs in Botrychium , but the stem of Ankyropteris corrugata 
appears to offer features suggesting a comparison with Helminthostachys. 
It may not be without significance that the only specimen of A . corrugata 
in which secondary thickening has been described and figured was branching. 
The secondary growth, described as ‘ only a local formation superadded to 
the normal woody zone’, 1 is most marked in the larger of the two steles 
shown in the cross-section of the branching specimen. The few existing 
Ophioglossaceae afford examples of all grades of replacement of the 
primary wood, with both centripetal and centrifugal xylem, by the 
secondary wood, and raise the same difficulties in homology and nomenclature 
as are met with in the case of Cycadofilices and Gymnosperms. 
( c ) The peculiarities of the departing leaf-trace of Helminthostachys 
afford data for comparison with long-extinct plants, which are remarkable 
features to find in an existing plant with no known fossil history. These 
features make it easier to compare the leaf-trace of Helminthostachys with 
the Zygopterideae and such Cycadofilices as Calamopitys and Lygmodendron 
than with other existing Ferns, except perhaps the Osmundaceae. No 
detailed comparisons will be made here, and this is the less necessary since 
the main points to which reference must be made are raised in recent 
works on the Zygopterideae, and were summarized by Dr. Paul Bertrand in 
the ‘ Progressus Rei Botanicae 5 for 1912. 
It must be borne in mind that while the leaf-trace as it arises from the 
stele is monarch in the Ophioglossaceae, Osmundaceae, and the Cycadofilices 
mentioned, it already possesses two groups of protoxylem where it separates 
from the stele in those Zygopterideae in which the leaf-trace departure is 
best known. This is not an essential distinction, since the two protoxylem 
poles in the Zygopterideae may be regarded in some cases at least as 
derived by division of one, and since further there is some reason to regard 
the traces of the simplest known Zygopterid trace ( Clepsydropsis) as having 
been monarch at departure. It is a question of how far down into the stem 
the double nature of the leaf-trace is continued. 
Dr. Paul Bertrand and Kidston and Gwynne-Vaughan agree in com¬ 
paring a stage in which the departing leaf-trace of such an Osmundaceous 
plant as Thamnopteris exhibits two protoxylem groups immersed in the 
metaxylem of the trace, with the structure exhibited by the petiolar bundle 
of Clepsydropsis . The traces of Asterochlaena , Metaclepsydropsis , and 
Diplolabis pass through a similar stage. Those of Ankyropteris Grayi and 
A. corrugata only differ in having a small central extension of the inner 
xylem or mixed pith of the stele. This soon dies out, and the central 
portion of the trace xylem becomes solid. This clepsydroid stage is of 
1 Scott: Trans. Linn. Soc., vii, 1912, p. 376, PI. XL, Fig. 19. 
