82 Braun and Engelmann's North American Equiseta. 



Equiseta speiropora) ; in the second division they are disposed 

 regularly in two ranges on the sides of the grooves (E. sticho- 

 pora). These ranges consist either of one series or row of sto- 

 mata each (as in all the northern species), or of two or more rows 

 (as in many tropical ones). The stomata are only found in the 

 grooves, never in the secondary farrows, even when these are of 

 equal size and depth with the former. 



The sheaths consist, of united verticillate leaves, free only on 

 the points, which constitute the teeth ; and these teeth correspond 

 in number and position with the carina? of the stem ; they are 

 either persistent or deciduous. The leaves have either a single 

 medial carina, or this carina is divided by a furrow, so as to ap- 

 pear double ; or sometimes the margins of the leaves are elevated 

 and form two lateral carinas ; thus the leaves may present from 

 one to four carinas. The furrow which sometimes divides the 

 principal carina is the carinal furrow ; and another which is fre- 

 quently found on the connecting line or the commissure of two 

 leaves is the commissural furrow. 



The section of the stem exhibits the following structure. In 

 the centre is a larger or smaller air-cavity or hollow space, lacuna, 

 (wanting only in E. scirpoides) ; and around. this a circle of 

 generally much smaller air-cavities which correspond with the 

 grooves or valleculas, and which we therefore call vallecular air 

 cavities. These are wanting in only one species, viz. E. limo- 

 sum. Exterior to these again is a circle of alternating and still 

 smaller air-cavities just under, or corresponding with the carinas, 

 {carinal air- cavities,) but they are sometimes very minute or 

 nearly obliterated. 



In some Equiseta there is no distinction of fertile and sterile 

 stems : such are E. homophyadica. Some of these have annual 

 stems, (E. jestivalia, Summer-Equiseta,) such as E. palustre, 

 and E . limosum : others have perennial stems, not perishing in 

 winter, (E. hyemalia, Winter- Equiseta,) such as E. hyemale, 

 E. scirpoides, etc. In a second division, the fertile stems are dif- 

 ferent from the sterile ones ; the latter only being herbaceous, 

 branching and persistent through the season, while the fertile are 

 discolored and simple : such are E. heterophyadica. In some of 

 these the fertile stems are deciduous after fructification (E. ver- 

 nalia, Vernal Equiseta,) as in E. arvense and E. eburneum: 

 others, after fructification, produce verticillate herbaceous branch- 



