and Woody Tissues of Ferns. 319 
stem, nearly the whole of the parenchyma within the vascular 
circle being converted into a hard woody core, of a deep brown 
colour, which is continued through all the ramifications of the 
rhizome (Pl. VI. fig. 6). It is as if the two longitudinal tracts 
of the Braken stem were fused into a solid central cord, to the 
obliteration of the intervening parenchyma and vascular bands. 
The induration of the stem reaches its maximum in the genera 
Blechnum and Osmunda. In the former, even in the petioles, 
near their origin from the rootstocks, the dark cortical layer 
becomes so much thickened at the expense of the pale paren- 
chyma, that the latter is reduced to a thin sheath investing the 
vascular fasciculi; and the same arrangement prevails through- 
out the whole rhizome, which consists, from its exterior to its 
centre, of hard tissue, formed of dark fusiform cells, except only 
a thin stratum of pale parenchyma surrounding the cambium- 
layer of the fasciculi of the netted cylinder (PI. V. fig. 5). Even 
the interstices of the latter are occupied in the centre by the 
dark material ; so that, as compared with some of the rhizomes 
which have been described, the white and dark substances seem 
to have changed places: instead of the vascular bundles and 
their sheaths forming a dark network on a pale field, we have 
here a general dark ground marked by a pattern of light reticu- 
lations. This great development of the durated brown tissue 
gives to the rootstock of Blechnum a very remarkable hardness 
as compared with others in which the pale parenchyma is the 
sole or preponderating element. In this respect Blechnum and 
Osmunda are peculiar among our Ferns, though at the same 
time they differ from each other in one or two important 
points. 
In Osmunda the vessels of the petiole are all collected into 
a single voluminous bundle, crescentic in horizontal section, 
with the concavity towards the common axis of the plant. A 
band of parenchymatous brown tissue, with a similar crescentic 
curve, lies in the concavity of the vascular bundle, separated 
from the scalariform vessels by the cambium-layer of the fasci- 
culus. The general parenchyma of the petiole is also marked 
on all sides with fusiform striz of brown tissue, like those before 
described as occurring in the rhizome of Pteris aquilina (Pl. V. 
fig. 2). The cuticular layers of cells form avery tough investment 
—green above, but passing at the lower part of the petiole into 
a dark brown. Towards the base it is covered on the exterior 
with a soft whitish film, forming lateral wings, by which the 
bases of the petioles overlap each other, as they become crowded 
upon the rhizome. This film becomes brown and chaffy when- 
ever it is left exposed. The bases of the petioles are at first 
somewhat dilated, and then taper away to their connexion with 
