244 HISTORY OF BRITISH FERNS. 



membranous toothed sheaths : each joint, in fact, termi- 

 nates above in one of these sheaths, into which the base 

 of the next joint fits. The sheaths seem to represent 

 abortive leaves. The fructification 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 marHn. 



The stems consist chiefly of cellular matter, coated 

 externally by a layer of hard woody tubes, from which 

 plates of a similar nature project towards the central 

 cavity. Between the outer and inner cuticle of this hollow 

 cylinder-like stem, occur one or more circles of tubes, or 

 air-cavities, differing in size and position ; these afford, by 

 their comparative size, number, and arrangement, excellent 

 auxiliary marks for the recognition of the species. Nume- 

 rous stomates exist in the hollows of the fluted surface of 

 the stems, the depressed part of each channel having two 

 longitudinal series of these minute openings. The cuticle 

 abounds in siliceous particles secreted in the form of 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 substance may be destroyed 



