FERNS. 



[ 320 ] 



FERNS. 



Filmy Ferns, the leaf is ordinarily a mere 

 membrane of a single layer of cells, through 

 which ramify scalarifrom ducts, to form the 

 veins consequently there are no stomata ; 

 but in the other orders, in Pteris for ex- 

 ample, the leaf has an upper and lower epi- 

 dermis with stomata, with loose cellular 

 tissue (mesophyllum), between and through 

 which ramify the fibro- vascular veins : the 

 epidermal cells often have elegantly zig- 

 zagged or waving side-walls, which pro- 

 duce 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 leaf- 

 lets in turpentine or oil, or mounting them 

 in Canada balsam. The collections of spo- 

 rangia or capsules on the back of the leaves 

 sometimes occur on all of these ; in other 



Fig. 224. 



Nephrodium. 



Pinnule with indusiate sori. 

 Magnified 10 diameters. 



there are barren leaves and fertile 

 leaves, the latter of which are generally 

 somewhat modified in form, deprived of a 

 certain portion of the green expanded struc- 



ture, and reduced occasionally to a mere 

 ramification of veins or ribs supporting the 

 sporangia (fig. 223). 



The groups of sporangia are called sori ; 

 they differ much in form and arrangement, 

 and are either naked (Polypodium) , or co- 

 vered by a special membranous structure, 

 more or less continuous with the epidermis 

 of the lower surface of the leaf, called an in- 

 dusium or involucre (fig. 224) ; sometimes 

 this indusium is so constructed as to form a 

 kind of cup (figs. 127 & 151), which, again, 

 exhibits a great variety of modifications. 

 (See SORI and INDUSIUM.) 



The theccB or sporangia are usually col- 

 lected in great numbers in the sori ; and con- 

 sist of minute stalked sacs or cases, com- 

 posed of simple cellular membrane, the cells 

 of which are either all alike (OPHIOGLOS- 

 SUM), or a row of them running almost 

 round the sac are modified by the thick- 

 ening of their walls, so as to form an elastic 

 band (annulus), which causes the bursting 

 of the sac when ripe. In the Polypodiaceae 

 the annulus starts from the stalk of the 

 capsule (fig. 225) j in Hymenophyttum and 



Fig. 225. 



Polypodiuin verrutosum. 

 Stalked thecae with annuli. Magnified 25 diameters. 



Fig. 226. Fig. 227. 



Ceratopteris thalictroides. 

 Fig. 226. Theca. Magn. 50 diarns. 

 Fig. 227. Do. bursting. Do. 



