EQUISETUM. 225 



position, sometimes becoming again branched. The main 

 stem has from ten to sixteen distinct shallow furrows, with 

 corresponding ridges, and is, as well as the branches, studded 

 over with 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 recog- 

 nition among the species of Equisetum. In that of E. arvense 

 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 conse- 



Q 



