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, terniinate in an equal number of dark- 

 coloured awl-shaped teeth, which sometimes have a pale 

 membranous margin. The branches are four to eiirht- 

 angled. 



Owing to the shallowness of the ridges and furrows, the 

 section of the stem shows a nearly smooth exterior outline, 



