EQUISETUM. 
279 
The scales are about twenty in number, and the spore-cases 
are whitish. 
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. mriegatum, 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. mriegatum. 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. mriegatum. 
This plant grows in water at Mucruss, in the immediate 
