258 HISTOKY OF BRITISH FERNS. 



minute siliceous warty particles. The sheaths, which fit 

 somewhat closely to the stem, are furrowed like it, and 

 terminate in an equal number of acute wedge-shaped 

 dark-coloured teeth, which are often margined by a narrow 

 brown membrane. Immediately below these sheaths spring 

 out, from other short sheaths with obtuse brown segments, 

 the whorls of branches, which are of variable number and 

 length ; they are four-ribbed, and their sheaths are four- 

 toothed, the teeth being long and acute, of one colour, with 

 a single rib extending to the extreme point of each tooth. 

 The branches are four-angled. 



The section of the stem often affords a good mark of 

 recognition among the species of Eqinsetum. In that of 

 E. arxense it is seen that the interior cavity occupies only 

 about one-third of the diameter. The exterior surface is 

 varied by about a dozen blunt ridges, having corresponding 

 shallow depressions ; within this, occupying about the 

 centre of the ring, and alternating with the ridges, are a 

 series of large roundish-oblong or obovate . cavities, the 

 narrow end of which is turned inwards ; alternating again 

 with them, and consequently opposite to the external 

 ridges, occurs an annular series of small circular cavities, 

 which are placed near the inner surface of the tube. 



