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 Rosacea?. 



