EQUISETUM. 247 



Genus XXIV. EQUISETUM, or HORSETAIL. 



The jointed tubular siliceous stems, and the terminal 

 cones of fructification consisting of spore-cases attached 

 to peltate scales, are marks by which the Equisetums 

 may always be readily distinguished from other plants. 

 The species are, however, not so easily recognized among 

 themselves, owing to the great sameness which occurs 

 in certain groups of them. The chief features relied 

 on for their discrimination, are the similarity or otherwise 

 of the fertile and barren stems, the number of ridges or 

 striae which occur on the exterior surface of these stems, 

 and the structure of the sheaths which surround the joints. 

 By means of the peculiarities which these parts present, 

 the species may be certainly identified, and after a little 

 experience has been had, several of them may be at once 

 known by means of those primd-facie appearances, which 

 become associated with the plants in the mind of the 

 attentive student. One peculiarity of the Equisetums is, 

 that they have no leaves, these organs being represented 

 by the tubular sheaths which are produced at every joint. 



