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 Blechnum 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. Osmundaeeas. 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. Schiza3ace89 ^. 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 Lygodium 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 ground-tissue as 

 a medulla. 



^ [Prand, Die Schizseaceen, 1881.] 



