316 Dr. G. Ogilvie on the Vascular 
stance, which is deficient in the rhizome, makes its appearance in 
the petioles as sheaths of dark tissue round the vascular fasci- 
culi. In Asplenium lanceolatum, in which there is no dark sheath 
of this kind, the scalariform vessels and cambium-layer of the 
fasciculi become themselves the seat of a deposit of dark sub- 
stance, by which they are not merely tinged brown as in L. Filiz 
mas, but the cavities of the vessels are more or less filled up. 
The induration is confined, so far as I have observed, to the 
fasciculi near the base of the petiole, not extending either up- 
wards towards the frond, or downwards into the rhizome. 
In all the species, indeed, now mentioned, the dark tracts 
stop short just. above the origin of the petioles from the root- 
stock ; but there are others in which they pass some way into 
the substance of the latter, accompanying the vascular bundles 
to their junction with the netted cylinder, and even bordering 
some of the anastomosing fasciculi by whose interlacement the 
cylinder is formed, so that they appear as dark spots in a trans- 
verse section of the rhizome. 
Thus in Scolopendrium vulgare, in which we find in each 
petiole two vascular fasciculi, running into one above in the 
midrib of the frond, these are accompanied, from their origin 
in the rhizome, by dark lines on their outer margins. In the 
petiole itself, a little above the base, other dark lines appear on 
the inner margins of the fasciculi, gradually expanding, as they 
ascend, into two half-sheaths, which become united by their 
convexities as the fasciculi approach, and finally stop choot at 
the point of junction of the latter, while the outer marginal 
lines run far on, along the midrib of the frond. The general 
arrangement of these parts is illustrated in Plate VI. figs. 1, 2, 3. 
In Ceterach officinarum (Scolopendrium Ceterach, Grammitis 
Ceterach). the two fasciculi of the petiole are accompanied, from 
their origin in the netted cylinder, by three dark lines—two on 
the outer margins, and one median, the latter at first somewhat 
on the upper or mner aspect of the petiole, but gradually in- 
sinuating itself between the fasciculi, so as to form two half- 
sheaths united by their convexities, as in the last species. Higher 
up in the petiole, where the fasciculi unite into a single vascular 
cord in the midrib of the frond, the median tract of dark tissue 
comes to lie on its upper surface, the others continuing to run 
along its lateral margins. 
In Lastrea Oreopteris the petiole has two fasciculi of scalariform 
vessels, and on the inner margin of each (that lying next to the 
axis of the stalk) a chain of dark-coloured cells—sometimes 
continuous, sometimes interrupted. The two chains unite be- 
low, like the sides of the letter V, just above the junction of the 
vascular bundles of the petiole with those of the netted cylinder 
