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 (PL 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 indurated 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 strise 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 a very 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 



