322 Dr. G. Ogilvie on the Vascular 
woody, which may in all essential points be compared with those 
of Ferns, and which really owe their induration to a deposit in 
the cells of ligneous matter undistinguishable from that of the 
true wood of the stem—such as the veins of leaves, nut-shells, 
and various husky tissues. But between the dark tracts of the 
rhizomes of Ferns and the proper wood of the stems of Phanero- 
gamia, there are at least two points of difference, both of con- 
siderable importance :— 
1st. Unlike the woody tissue of the stems of the higher plants, 
the cells of the dark-coloured tracts of the Fern, even when they 
assume a distinctly fibrous character, never occur in the same 
fasciculus or layer with the vascular tissue, but are always sepa- 
rated from the ducts by the cambium-layer which encircles each 
vascular bundle,—and this even when in the closest relation, as 
in the sheath of dark tissue round a fasciculus. 
2nd. The hard tissues of Ferns, even when they put on most 
distinctly a woody character, do not seem to be formed out of a 
superincumbent layer of cambium-cells, like the true wood of 
the phanerogamic stem, but simply by an induration of the 
parenchyma, with occasional elongation of its cells. Hence, 
while the vascular bundles—lubricated, as it were, by their cam- 
bium-coat—may with a little pains be dissected clean out of the 
cellular tissue of the stem, the coloured tracts adhere so inti- 
mately to the surrounding parenchyma, that, with every care, the 
denuded surface has a rough or villose appearance, from adhering 
particles. 
I have found these points constant in all the British Ferns I 
have examined ; and I have reason to believe that they hold also 
in Tree-ferns, though my opportunities of examining the latter 
have been too limited to allow me to speak very positively on 
this point. 
It may be observed, further, that while the woody fibres of the 
ribs of leaves and of their footstalks in all Phanerogamic plants 
are continuous with those of the stem or trunk, the dark lines 
of the petioles of Ferns are rarely to be traced into those of the 
rhizome. Among our native species, the Braken (Pteris aquilina) 
is perhaps the only instance. 
The variability in the development and disposition of these 
dark tissues seems of itself an argument in the same direction, 
as tending to assimilate them rather to the capsular indurations 
and the husky tissues generally of the higher plants, which we 
observe to vary much, even in allied species*, than to the true 
stem-wood, which possesses so constant and uniform a structure. 
Mr. Berkeley takes the same view of the relations of the 
* Compare, for instance, the fruit of the Apple, Pear, and Mountain 
Ash, and, more strikingly, of the different suborders of Rosacez. 
