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 margin. 
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 
