THE GENUS EQUISETUM WITH REFERENCE TO THE 

 NORTH AMERICAN SPECIES. 



By Alvah A. Eaton. 



SIXTH PAPER. 

 E. Pratexse Ehrh. 



RHIZOME solid at center, sharply 7-8 angled, dull black, 

 without felt, the sheaths and teeth usually whole. Stems 

 dimorphous ; fertile at first, fleshy and wanting chlorophyll, 

 often perishing at this stage; others develop a few whorls of 

 branches at the upper internodes where the spike withers, and 

 persist through the season with the appearance of sterile stems. 

 One form appears to be intermediate, with fewer branches than 

 the sterile and smaller spikes than the normal fertile plants. 

 Sterile stems smoothish below, but becoming very rough above, 

 grooves 8-20, small; keels convex, with cross bands of silicious 

 teeth, in threes; sheaths cup-shaped; leaflets nearly fiat, with in- 

 distinct groove in the keel ; commisural groove narrow, very sharp 

 and deep; teeth with dark center and hyaline borders, mostly 

 united except at the short, sharp point; stomata in two lines in 

 each groove; central cavity >~ the total diameter of the stem. 



Branches in thick verticils, solid, 3 or rarely 4-5 angled; 

 sheaths with ridged leaflets and broad ovate short teeth. The 

 branches are recurved at first, but finally become horizontal or 

 even erect, beyond the middle. They are usually about 4 inches 

 long, or occasionally an inch more, the general aspect being that 

 of arvense neniorosum. The stems are about a foot high, the 

 lower third naked. The basal mternodes are an inch long, and 

 the others gradually decrease to lengths of X inch near the top. 

 They are constricted under the nodes when dry, and densely cov- 

 ered with rough teeth, which may be seen without a lens when 

 held to the light. In many respects this is intermediate between 

 arvense and silvaticiim. 



This is a northern species, common to both hemispheres from 

 43 to the Arctic sea in Europe and Asia, and frcm Behring sea 

 and Alaska to nearly 60" in America. It is very local, especially 

 in the southern part of its range, and more common in Europe 

 than America. It does not range south of Siberia in Asia. It 

 reaches its southernmost limit in America at Gloucester, X. J. 

 From here it ranges northeastward to Gaspe, Ouebec, and north- 



