218 HISTORY OF BRITISH FERNS. 



toothed sheaths : each joint in fact terminates above in one 

 of these sheaths, into which the base of the next joint fits. 

 The sheaths seem to represent abortive leaves. The fruc- 

 tification consists of terminal cone-like heads, made up of 

 peltate, usually hexagonal scales, to the lower face of which 

 the spore-cases are attached in a series around the margin. 



The stems consist chiefly of cellular matter, but towards 

 the circumference there is a layer of woody fibre, from 

 which a series of plates of a similar nature project towards 

 the centre. The centre, as already mentioned, presents a 

 hollow cavity ; and between the outer and inner cuticle of 

 the cylinder-like stem, occur one or more circles of hollow 

 tubes, or air-cavities, differing in size and position, and in 

 fact affording, by their comparative size, number, and ar- 

 rangement, excellent auxiliary marks for the recognition of 

 the species. Around these cavities, especially towards the 

 exterior surface of the stem, occur numerous spiral vessels of 

 small size. 



The cuticle abounds in siliceous particles secreted in the 

 form of more or less prominent little warts, which impart to 

 the surface a greater or less degree of roughness in proportion 

 to their prominence. In some species this deposit of siliceous 

 matter is so great, that it is said, the whole of the vegetable 



