440 
VASCULAR CRYPTOGAMS. 
being mostly obliquely truncated, or fusiform and pointed. Between the vessels lie 
narrow thin-walled cells, which contain starch in winter. The phloem, in addition to 
cells similar to those last named, contains wide sieve-tubes or latticed cells, and at the 
circumference narrow, bast-like, thick-walled fibres. The whole bundle is usually en- 
closed by a distinct sheath of narrower cells (vascular bundle sheath) ; the latter often, 
but not always, is invested by a layer of brown sclerenchymatous cells, the walls of 
which are very much thickened either, as in Platycerium, on that side which is next 
the bundle, or, as in Bkchnum brasiliense, on that side which is most distant from it. 
This layer is easily mistaken for the bundle-sheath itself. A single layer or several 
layers of cells may often be found at the periphery of the phloem lying just inside the 
true bundle-sheath. Russow regards this structure as belonging, like the bundle-sheath, 
to the ground tissue, and he terms it the phloem-sheath. Such a phloem-sheath is repre- 
sented in Fig. 308 as a layer of cells containing starch lying between sg and b. 
The fibro-vascular bundles are single and axial in very slender filiform stems, as in 
those of Hymenophyllaceae, and in the young plants of larger species. When the stems 
of the latter become thicker with increase of growth, a network of anastomosing bundles 
is formed in place of the axial bundle, presenting, in typical cases, a wide-meshed hollow 
cylinder, by which the fundamental tissue of the stem is separated into an outer cortical 
layer and an inner medullary portion (Fig. 302, A and E). Not unfrequently, however, 
isolated bundles also arise in addition ; thus in Pteris aquilina two strong broad cauline 
bundles are formed within the medullary portion (Fig. 302, A, ig), and in Tree-ferns 
a number of filiform bundles are scattered through it which enter into the leaf-stalk 
through the meshes of the primary bundle. The primary bundles which form the cylin- 
drical network already mentioned are mostly ribbon-shaped, broad, and, in the case of 
Tree-ferns, commonly have their margins curved outwards, so that they with their 
thick, firm, brown sheaths of sclerenchyma occupy most of the circumference of the 
stem. From these margins spring the more slender filiform bundles which enter the 
leaf-stalk, and are more numerous in proportion to its thickness. These may also 
coalesce laterally into plates of different forms, or may run separately side by side. The 
leaf-stalk always corresponds to an opening of the meshes of the cylinder of the primary 
bundle. The thick bundles which run through the stem appear to be all cauline. Hof- 
meister found in Pteris aquilina ^ that they exhibit the same distribution on the leafless 
elongated ends of the stem as on its leafy parts, a proof that the distribution does not 
depend on the leaves, as in Phanerogams. The end of the bundle may even be followed 
up to near the apical cell of the stem, in places where the nearest leaf-stalks have not 
yet begun to form bundles. 
(b) Taxonomy. The Ferns may be classified as follows: — 
Family i. OsmundaeesB. In Osmunda the fructification is paniculate, the sporangia 
being borne on the laciniae of leaves the mesophyll of which is not developed. In Todea 
the fertile leaves resemble the sterile ones. The shortly-stalked, unsymmetrically- 
rounded sporangia are furnished on one side of the apex with a group of peculiarly- 
formed cells, and they split open longitudinally on the other side. The stem, which 
is densely covered with roots, throws out lateral shoots resembling itself. 
Family 2. SchizaBacese ^. Except in Mohria, where the sporangia lie on the under 
surface of the leaf near the margin which is incurved over them, the laciniae bearing 
the sporangia are arranged in spikes or panicles. In Scbizcea and Ly^odium the sporangia 
are arranged in two rows upon the under surface of a very much contracted lacinia, 
each sporangium of Lygodium being invested by a sac-like indusium. In Aneimia the 
two lowest branches of the lamina have no mesophyll, and form stalked panicles on the 
^ I found a stem of Pteris aquilina in which the two internal cauline bundles had coalesced 
laterally so as to form a hollow cylinder enclosing one part of the parenchymatous giound-tissue as 
a medulla. 
2 [Prantl, Die Schizseaceen, 1881.] 
