264 
HISTORY OF BRITISH FERNS. 
the species ; probably, also, the particles themselves are in 
this species much finer and less prominent. Sometimes 
the stems are quite unbranched, sometimes furnished with 
irregular whorls of branches along all their central portion ; 
and between these two extremes there occurs also every 
conceivable degree of branching, from the single shoot 
produced here and there, through every gradation of 
imperfect whorls up to whorls of short branches almost 
complete. The branches, which are simple, nearly erect, 
and never acquire much length, are smooth like the stem. 
There is no material difference between the barren and 
fertile stems, except the presence of the fructification in 
the one case and not in the other; they are therefore said 
to be similar in structure. 
The surface of the stem is marked with from sixteen to 
twenty very slight ridges, and the sheaths, which are short, 
rather closely fitted to the stem, and of the same colour in 
the lower part, terminate in an equal number of dark- 
coloured awl-shaped teeth, which sometimes have a pale 
membranous margin. The branches are four to eight¬ 
angled. 
Owing to the shallowness of the ridges and furrows* the 
section of the stem shows a nearly smooth exterior outline, 
