214 
BRITISH FERNS. 
manner, and displayed figures of singular beauty. From 
these data Dr. Brewster concludes that the crystalline 
portions of silex and other earths which are found in 
vegetable films are not foreign substances of accidental 
occurrence, but are integral parts of the plant itself, and 
probably perform some important function in the pro- 
cesses of vegetable life.” 
A section of the stem exhibits numerous ridges on the 
exterior surface, varying in number and reaching as high 
as thirty-two ; between the ridges large openings are 
situated in a circle, with an intervening circle of smaller 
apertures as in other species. 
The Bough Horsetail is recommended by Galen for 
its astringency. It is of great use in the arts, to polish 
wood, bone, and metal. Lightfoot tells us that it is 
used by Northumbrian milkmaids to clean their pails. 
The Dutch rushes of commerce are imported from Hol- 
land, where they are grown upon the embankments, 
their branching and interlacing caudices being most 
useful in binding together the loose sandbanks. 
It is often found in the North of England and Scot- 
land, but is unknown in the southern and midland 
counties. 
It is common throughout Europe, and in North Africa 
and America. 
65. Rquisetum trachyodon, Braun. Rough-toothed 
Horsetail. 
Stem putting out one or two branches from joints at the base. 
Branches long, supple, simple, or branched once again, very 
rough, furrowed. Sheaths with from six to thirteen narrow 
teeth. Cones small, pointed. 
The Bough-toothed Horsetail bears much resemblance 
