THE GENUS EQUISETUM IN NORTH AMERICA. 



By Alvah A. Eaton. 



ELEVENTH PAPER. 



E. FLUVIATILE L. 



Rootstock shining, without felt, dark or yellowish brown 

 rarely tuberiferous, with very wide central cavity. Stems erect. 

 1-4 ft. high. 2-3 twelfths of an inch in diameter at base, naked or 

 variously branched, but always (except in var. 1) with several 

 naked internodes at base, and in the sterile plant at apex also. 

 Basal internodes 1-2 inches long, light or yellowish-green, often 

 flesh-colored, shining, usually wanting stomata : central and ter- 

 minal internodes light green, slightly 10-30 angled when fresh, 

 more prominently so when dry. the angles rounded on the back 

 and smooth: stomata abundant, irregularly filling the broad 

 grooves, smaller and more elongate than in B. lit ovale. Sheaths 

 appressed, concolorous with the stem, nearly as broad as long, 

 the leaves little elevated, not separated by a commisural groove 

 save in the largest specimens ; teeth narrowly lanceolate, rigidly 

 erect, seldom coherent, with rounded sinus and a very narrow 

 white hyaline border, those of basal sheaths usually black or 

 brown, the upper becoming green at base and black at tips. The 

 ribs and leaves increase in number from base upward, the upper 

 internodes usually showing one-third more than the basal. 

 Branches of two kinds; one scattered, 6-8 or more angled, mostly 

 from lower internodes, erect equaling the main stem in height, 

 more properly called secondary stems : the other scattered or in 

 more or less regular verticils, 4-5 angled, 3-8 inches long, at first 

 nearly erect, becoming horizontal below and arching till the tips 

 become erect, their basal internodes always shorter than the 

 stem sheath, their angles low-winged, rough, the grooves with 

 fine cross walls of silex or often sparingly dotted with it ; sheaths 

 rigid, the angles green in the middle, joined at the commissures 

 by a white hyaline border ; teeth green or black tipped, very 

 sharp, erect or spreading. 



ANATOMY. 



The centrum occupies four-fifths of the diameter of the stem, 

 the largest of any species of Bquisetutn; vallecular cavities absent 

 save in the lower internodes of the very largest stems, the carinal 



