FILICINE^. 
439 
which is rich in assimilated food-materials, it is, in Ptei'is aquilbm, interrupted along 
two lateral lines, where the colourless parenchyma rises to the surface. In Tree-ferns, 
on the other hand, according to H. von Mohl, depressed cavities appear on the enlarged 
base of the rachis of the leaf, where the sclerenchyma is replaced by a loose and pul- 
verulent tissue. 
It may be mentioned here in addition, as an isolated histological peculiarity, that in 
Aspidium Filix-mas, according to Schacht, roundish stalked glands occur in the funda- 
mental tissue of the stem, which I have also noticed in the green parenchyma of the 
leaves, and on the pedicels of the sporangia of the same Fern (Fig. 304, C, d). 
The lamina of the leaf consists in Hymenophyllaceae only of a single layer of cells, 
as in Mosses ; in all other Ferns it is formed of several layers. Between the upper and 
under epidermis lies a spongy parenchyma containing chlorophyll, the mesophyll, pene- 
trated by the fibro-vascular bundles which form the venation of the leaf. The course 
FIG. yi-^.—Pteris aquiliiia : A transverse section 
of the stem, r its brown sheath (the layer of scleren- 
chyma beneath the epidermis), / the soft colourless 
parenchyma of the fundamental tissue ; inner fibro- 
vascular bundles ; ag upper broad outer bundle ; B 
the separated upper fibro-vascular bundles of the 
stem J/, and of its branches st' and j^", b bundles of 
the leaf-stalk, u u outline of the stem (natural size). 
Fig. 308. — a quarter of the transverse section of a fibro-vascular 
bundle from the stem of Pteris aquilma, with the adjacent parenchyma 
P containing starch, sg the bundle-sheath, b the layer of bast-fibres, 
sp the large sieve-tubes, g g the large vessels of the xylem thickened in a 
scalariform manner, ^ a spiral vessel surrounded by cells containing 
starch (x 300). 
of the veins is very various ; sometimes they run branching dichotomously at acute 
angles, or spreading like a fan upwards and sideways, without anastomosing and without 
forming a mid-rib ; more often the undivided lamina, or a division of the lobed, incised, 
or pinnate leaf, is penetrated by a distinct median vein though but slightly projecting, 
from which spring more slender branches, which themselves again ramify dichotomously 
or apparently monopodially, and run to the margins. The finer veins frequently anasto- 
mose hke those of the leaves of most Dicotyledons, and divide the surface into areolae 
of characteristic appearance. 
The Fibro-'vascular Bundles of Ferns are closed; they consist of a mass of xylem, 
completely enveloped by a layer of phloem. Besides a few narrow spiral vessels, lying 
at certain definite points in the transverse section, the xylem consists of vessels with 
bordered pits which usually resemble transverse clefts (scalariform vessels), their ends 
