FILICINEAE. — IIOMOSPOROUS FILICINEAE. ■ 221 



threads become differentiated from the rest, and their cells become firm and proscn- 

 chymatous with their walls much thickened and of a brown colour. Two such thick 

 bands of sclerenchyma (Fig. 179 A, pr) run in the stem oi Ptcris aquilina between the 

 inner and outer vascular bundles, and slender threads of sclerenchyma appear as dark 

 points on the transverse section of the colourless parenchyma. In other cases, as in 

 Polypodiiim vaccinifoliian and the Tree-ferns, dark layers of sclerenchyma, the nature 

 of which was first correctly conceived by Von Mohl, form thick and very firm sheaths 

 round the vascular bundles, and are the chief cause of the stiffness of the erect stem. 

 In stouter stems and leaf-stalks the outer layer also of the fundamental tissue imme- 

 diately beneath the epidermis is often dark brown and sclerenchyniatous and forms a hard 

 and firm shell, as for instance in Pteris aquiliiia (Fig. 179 A, r) and in the Tree-ferns. 

 In order to ensure communication in spite of this solid coat of mail, between the outer 

 air and the inner parenchyma, which is rich in assimilated material, this hard shell is 



j?Ul 



Fig. 170. Rhizome of Plcris aqttiliim. A end of a short member of a vessel. The oblique scalariform surface 

 of the extremityyj and a portion of the lateral wall in surface view. B a portion of --/ at X. C thin longitudinal section 

 through part of a lateral wall, where the surfaces of two vessels touch one another. D a similar section through the 

 oblique partition waliy and its margin adjoining the lateral wall. At / the pits are open. From De Bary, Vergl. Anat. 

 A magn. 142. B and C 375 times. 



interrupted in Pteris aquilhia on the two sides of the stem, and there the colourless 

 parenchyma comes to the outer surface ; in the Tree-ferns, on the other hand, there are 

 pits in the swollen bases of the leaves, where the sclerenchyma is replaced, according 

 to Mohl, by a loose and pulverulent tissue. 



We may notice here in passing a circumstance which stands alone in histology, that 



