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 being 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 (PI. 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. Each 

 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- 



