320 Dr. G. Ogilvie on the Vascular 
the rhizome, and their internal dark tissue disappears, while the 
cortical is much thickened and, by fusion with that of the neigh- 
bouring petioles, forms a mass of dark-brown prosenchyma on 
the exterior of the rhizome, very hard and tough, and of such 
thickness as to make up the great bulk of the -stem,—the only 
other constituents beg a slender medullary tract of pale tissue, 
in which the vascular cylinder is imbedded, and a cuticular film 
of white spongy substance, derived from that investing the bases 
of the petioles (Pl. V. fig. 1). It is the thick and tough cortical 
layer of dark indurated tissue that gives the tenacity to the stem 
which is so remarkable in this species, and is probably connected 
with the great age and size it occasionally attains. 
In this toughness of texture, and in the preponderance of 
dark tissue on which it depends, there is a great resemblance 
between the stems of Osmunda and Blechnum, notwithstanding 
the larger dimensions of the former; but they differ in this— 
that the induration extends to the medullary region of Blechnum, 
where we find a dense axial column of dark tissue; while in 
Osmunda the axis, though reduced to slender dimensions, is di- 
stinctly formed of a pale parenchyma consisting of delicate cells 
with less than the usual amount of starchy deposit in their in- 
terior. The thick cortical layer of dark substance which sur- 
rounds it is marked on a horizontal section with white spots, 
indicating. the points of passage of the vascular bundles of the 
petioles and rootlets. 
Another point of difference is, that in Osmunda the vascular 
cylinder has not (at least to the naked eye) the beautiful netted 
appearance so common in the rhizomes of Ferns, from the close- 
ness with which the component fasciculi are set together. Hach 
fasciculus has the same crescentic section as in the petiole; and 
a transverse division of the stem shows about eight crescents 
placed in a circle near the outer margin of the pale medulla, 
with their concavities all turned inwards, and encircled in turn 
by the thick cortical layer of dark tissue. The vascular cylinder 
as a whole forms a cord of some thickness, cellular within, 
where the medullary parenchyma is not separated from the ves- 
sels by any cambium-layer, and fibrous externally, without any 
apparent interstices, but imbricated with the fasciculi given off 
to the petioles. On microscopical examination, a real interlace- 
ment of the vascular bundles may be detected; but it may be 
observed at the same time that the whole cord is surrounded by 
one continuous cambium-layer on its exterior, which dips par- 
tially between the several fasciculi, but never passes through the 
vascular circle to form an internal investment to the component 
bundles, as in other Ferns. Hence, while it is not difficult to 
dissect off from the vessels the stratum of pale parenchyma in- 
