258 
HISTORY 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 Equisetum. In that of 
E. armnse 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. 
