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ence, rather broader than those of E. hyemalc. From this cause 

 stems of E. Mackaii and variegatum can at once be distingaished, at 

 least in their recent state, by holding them between the eye and a 

 light; E. Mackaii appears almost quite opaque, the furrows being 

 scarcely seen, while E. variegatum and E. hyemale under the same 

 circumstances appear semitransparent. 



No stem of E. Mackaii that I have seen deserves a particular de- 

 scription. The longest I measured was two feet five and a half inches 

 in length, the next longest two feet four inches. The number of arti- 

 culations is about the same as in E. hyemale, the terminal one, when 

 in fruit, being usually much longer than in the latter plant. The 

 standard number of striae I take to be eleven. 



Of E. variegatum I have found no stem longer than about twenty 

 inches, exclusive of the smallest portion of the root ; the usual length 

 is from three to ten. On one of the longest stems I have counted up- 

 wards of thirty articulations, the shortest has no fewer than eight ; 

 the terminal articulation, when bearing a catkin, is longer in propor- 

 tion to the size of the plant than that of E. hyemale. It is likewise 

 as stout as any on the same stem, often the stoutest of all. The stan- 

 dard number of striae is eight. 



The structure of the cuticle is undoubtedly the same in each species 

 or variety ; the largest plant having the narrowest stria;. In all, the 

 furrows are more minutely grained than the ridges. It appears to me, 

 however, that the structure of the cuticle in all the Equiseta is more 

 or less the same, though the siliceous tubercles may not be disposed 

 with the same regularity. 



Sheaths. — These in E. hyemale have only two thirds of their length 

 coloured differently from the internodes ; in E. Mackaii two thirds or 

 one half; in E. variegatum never above one half: the lower third or 

 half being concolorous. In this I think they differ from some of the 

 other Equiseta, whose sheaths are wholly coloured in a different man- 

 ner from the stems. The colour changes according to the growth of 

 the plant, as above described. 



After a good deal of searching, and examination of many specimens, 

 I have not found the semblance of teeth upon the sheaths of E. hye- 

 male, except on one or two of the younger stems and branches, and 

 generally on the upper and lower sheaths. When present they are 

 wholly black, subulate, and flexuose ; they are extremely fugacious, 

 and, from their rare occurrence, appear to me to be quite accidental. 

 The sheath which embraces the catkin has always rather large, coarse, 

 black, flexuose, ])ersistent teeth, unlike the delicate membranaceous 



