EQUISETUM. 273 



aerial stems are of a deep glaucous green, and all alike in 

 structure^ those wliich bear fructification, differing in no 

 other particular from those ^vhich do not. They grow 

 upright, from two to three feet high, and are scarcely 

 ever branched : -n-hen this does occur, a solitary branch is 

 produced, and this protrudes from below the base of one 

 of the sheaths of the stem ; they are cylindrical, tapering 

 off at the apex, and marked on the thicker parts with 

 from fourteen to twenty ridges, formed of a double row of 

 elevated points, consisting of crystallized siliceous parti- 

 cles ; hence the stems are very rough. In this species the 

 sheaths fit closely around the stems, so that they are nearly 

 cylindrical ; they are marked by the same number of ridges 

 as the stem, but they are less prominent, and terminate in 

 a series of black, membranous, bristle-shaped teeth, which 

 soon fall off, and leave the margin crenated. The sheath 

 immediately below the cone of fructification has, however, 

 its teeth persistent, and it is somewhat funnel-shaped. 

 The sheaths are at first pale green, with a black margin ; 

 from this they change to be entirely black ; and finally 

 they become whitish in the middle, leaving a narrow ring 

 of black at the base and margin. 



In this species a section of the stem shows on the 



T 



