318 Dr. G. Ogilvie on the Vascular 
bundles corresponding to the netted cylinder of other Ferns, 
though, from the way in which the stem is drawn out longitu- 
dinally, the reticulations are represented only by long wide slits 
formed by the irregular anastomosis of the bundles. Inside the 
more external layer of parenchyma containing these bundles are 
two dark bands, of a hard woody nature, running the whole 
length of the rhizome,—one on its upper, and the other on its 
under aspect. The former is flat; the latter, which is broader, 
is turned at the edges, like the sides of a boat; so that the two 
together partially enclose the axis or central portion of the stem, 
consisting of soft white parenchyma, like that on their exterior. 
But the great peculiarity of the rhizome of P. aquilina, in which 
it differs from all our other Ferns, is the occurrence of a second 
vascular system in this central region, consisting of two bands 
or broad fasciculi lying immediately on the inner aspect of the 
woody tracts (Pl. VI. fig. 7). The fasciculi are in the main di- 
stinct from each other, and from those of the outer series, though 
there is an occasional communication by anastomosing fibres. 
In the same way the two woody bands are generally quite di- 
stinct, though here and there they may become fused together 
for a space, on one or both sides, so as in the latter case to form 
a complete ring round the medullary parenchyma and the vas- 
cular fasciculi imbedded in it. This occurs at points where 
several fronds are given off in close proximity—a circum- 
stance which affects the disposition of the tissues of the stem, 
both woody and vascular, in consequence of these branching out 
tc form the corresponding tissues of the petioles. In the petiole 
the main tract of dark tissue forms a plaited band, having a 
T-shaped section,—the perpendicular lamina, which lies towards 
the inner aspect of the petiole, arising mainly from the upper 
band of the rhizome, and the fluted transverse lamina from the 
lower. These central plates, along with subsidiary and more 
superficial tracts, arising in the petiole itself, serve partially to 
ensheath the numerous fasciculi of vessels which are derived 
from both the vascular systems of the rhizome, outer and inner 
(Pl. VI. fig. 8). The brown tint and woody character of the 
plates disappear in the upper part of the petiole, their tissue 
merging into the general parenchyma of the part. The paren- 
chyma of the rhizome, both medullary and cortical, is also 
marked occasionally with interrupted strie of brown tissue, 
like those occurring in the medullary parenchyma of Lastrea 
dilatata, but much more attenuated, and of very considerable 
length. 
in Allosorus crispus (Cryptogramma crispa) there is a perceptible 
tracery of dark tissue round the several fasciculi of the netted 
cylinder; but the principal accumulation is along the axis of the 
