EQUISETUM. 227 



of a double row of elevated points. Their surface is very 

 rough from the presence of these points, which consist of a 

 coating of crystallized siliceous particles. In this species 

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

 nearly cylindrical ; they are marked by ridges of the same 

 number as those on the stem, but less prominent, and they 

 terminate in a series of teeth equal in number to the ridges, 

 the teeth being black, membranous, and bristle-shaped, soon 

 falling off, and leaving 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 be- 

 come 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 exterior 

 a series of distinct ridges, formed of twin projections, and 

 varying in number, as has been already explained ; opposite 

 to the furrows, between them, and occupying about the cen- 

 tre of the solid cylinder, is a ring of moderate-sized cavities. 

 The central cavity is comparatively large. 



The cones of fructification are comparatively small, and 

 are seated on the apices of a number of the stems ; they are 



