EQUISETUM. 279 



The scales are about twenty in number, and the spore-cases 

 are ^Yhitish. 



The section of the stem shows a small central cavity, an 

 exterior surface of rather prominent ridges, each channelled 

 so as to form two projecting angles, and a circle of moderate- 

 sized cavities occurring about the centre of the tissues. 



Insensibly merging into the form just described appears 

 to be another, called arenarium, which in its extreme state 

 is smaller and more slender, its stems always procumbent, 

 and not having more than six furrows ; in this form the 

 teeth of the sheaths are said to be wedge-shaped, but we 

 do not detect any differences in respect to the teeth between 

 specimens having the erect and the prostrate habit of 

 growth. 



The variety, Wilsoni, Equisetum Wilsoni of Newman, 

 which is allied to E. variegatum, is at least a permanent 

 variety, and may be still more distinct. It is a stouter and 

 taller plant, three feet high, and growing smoother than 

 the larger form of E. variec/atum. The section of its stem 

 also differs ; the central cavity and the ring of cavities 

 occurring in the cylinder of the stem being much larger, 

 and the latter differing in form from those in E. variegatum. 

 This plant grows in water at Mucruss, in the immediate 



