468 Boodle.—Anatomy of the Hymenophyllaceae. 
a practically continuous ring ; the xylem also forms a ring, 
and shows a differentiation similar to the upper and lower 
bands of the rhizome, so that a slight shifting of the protoxylem 
would make the structure identical with that of the rhizome. 
Close above the axillary branch, as has been described, the 
petiolar bundle is still more like the stele of the rhizome. 
Fig. 11 shows the further fusion of the two steles of the pre¬ 
vious figure. Here the right and left of the last figure are 
reversed. The metaxylem (x 1 ) of the leaf-trace has become 
attached to the upper band of the rhizome ; px x is the proto¬ 
xylem of the leaf-trace. Fig. 12, Plate XXVI (orientated as 
in Fig. 10), is a further stage, in which the metaxylem of the 
leaf-trace and the upper xylem-band of the rhizome form 
a single curved arc, while the protoxylem of the leaf-trace 
(px *) is turning in towards the central parenchyma. It after¬ 
wards passes further in and joins the other protoxylem-ele- 
ments [px) already present in the central parenchyma of the 
stele, which, no doubt, are continuous with the protoxylem of 
other leaf-traces. In sub-collateral species the leaf-trace and 
the stele come into lateral contact, and fuse. The two proto- 
xylems may remain distinct for a short distance. 
The fact that the basal region of the petiole approaches 
stem-structure, may be due to physiological requirements. It 
has to connect two similarly constructed organs, the rhizome 
and axillary branch, so that a closed xylem-ring and con¬ 
tinuous phloem, resembling their structure, is probably well 
suited for connecting the vascular elements of the two. 
As to the water supply of the leaf in a species like the 
above : the protoxylem-elements appear to be all in contact 
with one another at some points, and some of them with the 
lower xylem-band as seen in Fig. 12, so conduction may go 
on from the roots through the lower xylem-band to the proto¬ 
xylem in the rhizome, and the protoxylem in the leaf-trace. 
The rest of the water must pass from the lower band to 
the upper (either at occasional points of union, or across the 
central parenchyma) and from there to the metaxylem of the 
leaf-trace. In H. demissum , var. nitens , where the two bands 
