224 Lang . — - Studies in the Morphology and 
been observed. The interpretation of these leaf-traces suggested here is 
supported by the fact that when truly mesarch leaf-traces are met with in 
Helminthostachys , the adaxial completion of the centrifugal xylem may also 
take place and the two processes be clearly distinguished. 1 
The interpretation of the leaf-traces of Botrychium Lunar ia^ as derived 
from the outer xylem of the stele only, is supported by the distribution of 
the xylem in small steles, which are giving off leaf-traces (cf. Phots. 10-11, 
12-15). It need only be added to what was said above regarding these 
figures that the xylem of such small leaf-traces may show a solid and 
apparently mesarch structure due to the completion of the metaxylem 
adaxially. 
It is not uncommon to find that the adaxial xylem becomes separated 
from the rest of the leaf-trace on nearing the periphery of the stem, as is 
shown in Text-fig. 9. 21. The main portion of the leaf-trace then passes 
out into the base of the petiole, while the adaxial xylem appears to die out 
in the cortex. The separation appears to be due to the active divisions at 
the periphery of the rhizome, associated with the development of periderm, 
affecting the old leaf-trace as it traverses this region. I was at first inclined 
to regard this disposition of the adaxial xylem as indicating a vestigial 
vascular supply to the vestigial axillary buds to be described below. While, 
however, the adaxial xylem of the trace would naturally be concerned in 
the supply of these buds, and may in a sense be related to their presence, 
the separation of the adaxial xylem from the main body of the trace is 
brought about secondarily, and it seems impossible to attach significance 
to it. 
There remains for description the occasional development of xylem 
elements as a result of cambial activity starting in the pericycle. This was 
found both in specimens that had branched and in a normal unbranched 
rhizome. In PI. XXI, Phot. 23 a portion of the stem stele of the rhizome 
bearing the branch represented in Phots. 32-34 is more highly magnified. 
The xylem is primary and only about four tracheides deep ; outside it and 
separated by conjunctive parenchyma comes the phloem, the small sieve-tubes 
of which are clearly seen. Outside the sieve-tubes radial divisions, which had 
resulted in the development of a number of short lignified tracheides, are 
seen to have taken place in the pericyclic cells. The same thing had hap- 
pened in the adult rhizome, a portion of which in radial section is represented 
in Phot. 22. From right to left this shows the pith, the primary xylem, 
secondary xylem, and cambium ; outside this lies a crushed and darkly 
stained sieve-tube, and just outside this a zone of tissue derived from active 
periclinal divisions in the pericycle. From the inner segments thus cut off 
groups of short tracheides have developed. In this case there appears to 
be no disturbing reason for the development of pericyclic xylem, as there 
1 Cf. Lang, Proc. Manch. Lit. and Phil. Soc., vol. lvi, Pt. II, p. 5, Figs. 3, 4. 
