FERNS. 



[ 260 ] 



FERNS, 



the character and arrangement of the fibro- 

 Fig. 226. Fig. 227. 



Osmunda regalis. 



Fig. 226. Upper part of a frond l-6th the nat. size. 

 Fig, 227. A fertile pinnule bearing thecae without pa- 

 renchyma. Magnified 10 diams. 



vascular bundles (see TISSUES, VEGETA- 

 BLE), which afford the best examples of that 

 form of elementary tissue called the SCALA- 

 RIFORM DUCTS. The creeping rhizomes 

 are often clothed more or less thickly (as are 

 also the leaf-stalks) with brown membranous 

 scales, called RAMENTA, and these often 

 afford elegant microscopic objects, from the 

 peculiar arrangements of the cells. The 

 leaves are generally very greatly developed, 

 and the green blade is of more or less com- 

 plex structure in different genera. In the 

 Hymenophylla, or Filmy Ferns, the leaf is a 

 mere membrane of a single layer of cells, 

 through which ramify scalariform ducts, to 

 form the veins, consequently there are no 

 stomates there ; but in the larger forms, as 

 in the leaf of Pteris for example, the leaf has 

 an upper and lower epidermis with stomates, 

 with loose cellular tissue (mesophyllum), be- 

 tween and through which ramify the fibro- 

 vascular veins ; the epidermal cells often have 

 elegantly zigzaged or waving side-walls, 

 which produce a pleasing appearance in the 

 sections of the structure obtained in slices 

 shaved off horizontally from the surface of 

 the leaf. 



The mode of ramification of the veins or 

 nerves of the leaves is important in system- 

 atic Filicology, and may be observed for 

 such purposes by immersing the dried leaflets 

 in turpentine or oil, or mounting them in 

 Canada balsam. The collections of sporanges 



or capsules on the back of the leaves some- 

 times occur on all of these ; in other cases 

 there are barren leaves and fertile leaves, the 

 latter of which are generally somewhat mo- 

 dified in form, deprived of a certain portion 

 of the green expanded structure, and reduced 

 occasionally to a mere ramification of veins 

 or ribs supporting the sporanges (fig. 227). 



The groups of sporanges are called sori ; 

 they diifer much in form and arrangement, 

 and are either naked (Poly podium), or co- 

 vered by a special membranous structure, 

 more or less continuous with the epidermis 

 of the lower surface of the leaf, called an 

 indusium (fig. 228); sometimes this indusium 



Fig. 228. 



Nephrodium. 



Pinnule with indusiate sori. 

 Magnified 10 diameters. 



is so constructed as to form a kind of cup 

 (figs. 131 & 154), which again exhibits a 

 great variety of modifications. (See SORI 

 and INDUSIUM.) 



The sporanges or thecte are usually 

 collected in great numbers in the sori, and 

 consist of minute stalked sacs or cases, com- 

 posed of simple cellular membrane, the 

 cells of which are either all alike (Opmo- 

 GLOSSUM), or a row of them running almost 

 round the sac are modified by the thickening 

 of their walls, so as to form an elastic band 



