60 Equisetaceae Equisetum 



Stems usually not branched above the ground unless the plants are injured, or 

 the branches few, irregular and sporadic; stomata in regular rows; teeth of 

 the sheaths or their bristle-tips usually soon deciduous, but several species 

 with persistent teeth or the teeth forming pagodalike caps; cones with or 

 without a point. 

 Teeth of the sheath persistent or only their bristle-tips deciduous, white- 

 margined, not sharply differentiated from the sheath; sheath segments and 

 lower part of teeth distinctly quadricarinate; stems 5-10-grooved, erect in 

 tufts, evergreen; cones apiculate. 

 Ridges of internodes prominently biangulate (2 ridges to a sheath tooth), with 

 a double row of rounded tubercles. 

 Sheaths cylindric, tight, often crusty, partly or completely black; stems 

 rather large to medium, sometimes rather slender. . . .2. E. trachyodon. 

 Sheaths campanulate, usually discoloring tardily; stems mostly very slender 



and small 3. E. variegatum. 



Ridges of internodes not biangulate, with a single row of tubercles or cross 



bands of silica 4. E. Nelsoni. 



Teeth of the sheath soon deciduous, sharply differentiated from the sheath ; main 

 stem usually tall, 10-many-grooved, with a large central cavity. 

 Sheaths cylindrical, short, appressed, or only slightly dilated when young, at 

 first green, but soon turning black or gray, commonly gray with black 

 bands above and below, often split in age; stems usually very rough, ever- 

 green; sheath segments of the main stem tricarinate; ridges of the inter- 

 nodes with one row of tubercles; cones apiculate 5. E. prealtum. 



Sheaths more or less funnel-shaped, elongate, green, the limb normally with a 

 narrow black band, sometimes the lower sheaths with bands of gray or 

 black below; stems evergreen or annual; cones with or without a point. 



Cones tipped with a rigid point 6. E. laevigatum. 



Cones rounded or the tip merely acute ; limb of the long green sheath dilated 



upwards I.E. kansanum. 



Stems usually much branched with several to many whorls of branches, rarely 

 with only few sporadic branches; stomata in broad bands or scattered in the 

 grooves of the internodes or only on the sheaths; teeth of the sheaths per- 

 sistent; cones not apiculate. 

 Branches hollow, usually simple, terete, both fertile and sterile stems green; 

 plants of wet soil or growing in water, sometimes without or with only 

 sporadic branches; sheaths of the main stem usually appressed, 15-20- 

 toothed; stems usually many-grooved, with a very large central cavity and 



thin wall 8. E. fluviatile. 



Branches solid, simple or compound, mostly sharply 3- or 4-angled; fei'tile stems 

 brown and at first without branches, soon withering or developing green 

 branches when mature; usually in moderately moist or dry situations. 

 Teeth of the branches with subulate tips; branches usually 4-angled (some- 

 times 3-angled) ; fertile stems withering after the spores are shed 



1. E. arvense. 



Teeth of the branches not subulate-tipped, deltoid, merely acute or long-acute, 

 usually white-membranous; branches generally 3-angled, very slender, 

 fertile stems developing green branches after the spores are shed. (See 

 excluded species no. 14, p. 1021.) E. pratense. 



1. Equisetum arvense L. Field Horsetail. Map 43. Infrequent to 

 frequent throughout the state. Where it is found it usually forms large 

 colonies, especially in its preferred habitat along railroad embankments. 

 It prefers a moist, sandy soil, usually lean in organic matter, but it is also 

 found in moist places on the borders of bogs and along streams. It grows 

 in both shade and sun and its appearance is so erratic and it is so wide- 

 spread that I am not able to tell what controls its distribution. Once I saw 



