FER.XS. 355 



The primary bundles which form ihe cylindrical network already mentioned are 

 mostly ribbon-shaped, broad, and, in the case of Tree-ferns, commonly have their 

 margins curved outwards. 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 (as in 

 F/en's aquilina), or may run separately side by side. The leaf-stalk always cor- 

 responds 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 Ptcris aguilhia 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. 



The fibro-vascular bundles of Ferns are, like those of all Vascular Cryptogams, 

 closed ; they consist of a mass of xylem, completely enveloped by a layer of phloem. 

 Besides a few narrow spiral vessels, lying in the foci of the elliptical transverse 

 section, the xylem consists of vessels with bordered pits which usually resemble 

 transverse clefts (scalariform vessels), their ends being mostly obliquely trun- 

 cated, 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 enclosed by a 

 distinct sheath of narrower cells (vascular bundle-sheath) ; the latter often, but not 

 always, have the walls which face the bundle strongly thickened and of a dark 

 reddish-brown colour. 



The Fimdamcntal Tissue of the stem and of the leaf-stalks consists, in some 

 species (as Polypoditim aureian and vulgare, and Aspidhun Fih'x-mas), entirely of 

 thin-walled parenchyma ; in otherg (as Gleichenia, species of Pteris, and Tree-ferns), 

 string-like, ribbon-shaped, or filiform portions of the fundamental tissue become dif- 

 ferentiated, the cells of which undergo great thickening, and become brown-walled, 

 hard, and prosenchymatous. JVIettenius aptly terms them sclerenchyma. In the stem 

 of Pteris aquiliiui (Fig. 263, A) two thick bands of sclerenchyma of this descrip- 

 tion (/>;-) lie between the inner and outer fibro-vascular bundles, and fine threads 

 of sclerenchyma appear on the transverse section of the colourless parenchyma as 

 dark points. In other cases (as in Polypodium vaccijiiifolium and in Tree-ferns), 

 dark layers of sclerenchyma, the nature of which was in these cases first correctly 

 recognised by H. von Mohl, form sheaths round the fibro-vascular bundles. The 

 outer layer of the fundamental tissue of thicker stems and leaf-stalks lying beneath 

 the epidermis, is often dark brown and sclerenchymatous, forming a hard firm sheath, 

 as again, for instance, in Pteris aquilina (Fig. 263, A, r) and Tree-ferns. In order 

 to facilitate, in spite of this firm coat, the communication of the outer air with the 

 inner parenchyma which is rich in assimilated food-materials, it is, in Pteris aquilina, 

 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 pulverulent tissue. 



A a 2 



