26 THE SCOURING-RUSHES. 



less than ten named varieties of Equisetmn hiemale have 

 been reported from America. Many of these are mere 

 ecological forms, of interest only to the systematist. 

 Those wishing to study them will find a full account of 

 all in volume XI of the Fern Bulletin. 



'The Great Scouring- Rush. 



In the Southern and Western States the most abun- 

 dant of the scouring-rushes is probably the great scour- 

 ing-rush {Eqitisetuin robustuni), \\hich is supposed to 

 intergrade with the common scouring-rush and is by 

 many considered only a variety of it. It is a strong and 

 robust species, but except for its larger size it cannot be 

 distinguished from Eqnisetnvi liiemale in a 

 cursory examination. Mr. A. A. Eaton, 

 who has carefully studied the genus, is con- 

 vinced that this is only a well-marked race 

 of the scouring-rush, but as it has always 

 appeared in text-books as a separate species 

 it will be so treated here. Mr. Eaton's 

 view is probably the correct one, however. 

 Aside from its larger size, the character- 

 istics usually depended upon for separating 

 Equisetmn robnstum from Equisetum liiemale 

 are the greater number of grooves in the 

 stem, the three-keeled ridges of the sheaths, 

 and the single series of cross-bands of silex on 

 the carina. Equisetum liiemale is supposed always to 

 have obscurely four-keeled ridges in the sheaths, and two 

 rows of silex tubercles on the carinae, but any of the 

 characteristics of the one may be matched by specimens 

 of the other. 



