Anatomy of the Ophioglossaceae. III. 5 
rhizomes, and often does not form a continuous zone ; a portion of 
a vascular tube of this type is shown in PI. I, Photo 2. In such rhizomes 
the parenchyma of the pith may at places abut directly on the protoxylem 
(PL I, Photo 3). Such steles present an appearance in transverse section 
not unlike some steles of Botrychium lunaria , 1 in that the outline of the pith 
is rendered irregular by the scattered elements of centripetal xylem. The 
analysis of the primary xylem into outer and inner is, however, much easier 
in the case of Helminthost achys, since protoxylem elements are usually 
present at intervals in the tube of xylem, and are not confined to the region 
of a departing leaf-trace. 
It is not necessary to enter into the histology of the xylem further 
than to say that, in longitudinal section, the tracheides of the inner and 
outer metaxylem are similar, with narrow and often multiseriate pits. The 
protoxylem elements are narrower and stain less deeply ; they show spiral 
marking passing into a reticulum, in the meshes of which bordered pits 
appear. 
The vascular attachment of the roots. The roots are developed close 
to the growing point of the rhizome, and the early growth of the root thus 
coincides with the cortex of the rhizome attaining its full thickness. In the 
light of this we can understand how it is that only a comparatively thin 
zone of the cortex is actually broken through by the emerging root; this 
zone corresponds in thickness to the preceding leaf-sheath. The root is 
thus borne on a pedicel, the inner portion of the root-stele having no cortex 
of its own distinct from that of the rhizome. Since the superficial layer 
of cortex which is broken through often wears off later, the roots on mature 
portions of the rhizome may appear as if exogenous. The general relations 
will be clear without further description from Text-fig. 1, A and B. These 
figures represent transverse sections close to the tip of a rhizome (a), and 
through a mature region (b), in both cases passing through the attachment 
of roots. 
As Farmer and Freeman state, the pith of the base of the root may be 
indirectly continuous with the pith of the rhizome by means of the xylem 
parenchyma. This statement may be somewhat amplified. The root, 
especially at its base, has a wide pith, the cells of which resemble histo¬ 
logically those of the pith of the stem. The xylem of the root is continuous 
with the outer xylem of the stem-stele, and the xylem parenchyma is well 
marked beneath the base of the root. The pith of the root is continuous 
with this parenchyma, the position of which is usually between the inner 
and outer xylems of the rhizome, though sometimes a thin zone of outer 
xylem persists across the base of the root. When the inner xylem is well 
developed, it separates the pith of the root from that of the rhizome (cf. 
Text-figs. 1, B, 3, B, C, and PI. I, Photo 4). When, however, the inner xylem 
1 Cf. Ann. of Bot., xxvi, PI. XX, Photos 16, 17. 
