EQUISETACEiE. 
22 
stem has been divided into six portions, in order to exhibit the 
whole at a single view ; its diameter and length, together with 
the distances between the sheaths, have been faithfully copied. 
The sheaths in the specimen figured are fourteen in number, 
the internodes decreasing in length towards either end. Both 
the internodes and sheaths are striated, the former more strongly 
so : the striae are usually about twenty in luxuriant stems, but 
this number is liable to considerable variation, and appears to 
depend entirely on the size of the stem, always decreasing to- 
wards its attenuated apex. The stems are hollow, and jointed 
or divided by a strong transverse septum at each of the sheaths : 
the striae of the sheaths correspond in number with those of the 
internodes, and they terminate in an equal number of acute and 
elongate, but membranous and deciduous teeth. Under certain 
but unascertained conditions these teeth become setiform and 
persistent, but in general all trace of them is early lost, the up- 
per margin of the sheath exhibiting a regular series of rounded 
divisions, uniform in number with the striaB of the stem. The 
sheath of the catkin is, however, an exception, its teeth being 
invariably persistent when mature : the basal portion of each 
sheath is black, the central part whitish, and the upper part 
again black, but all the sheaths are at first greyish-green, after- 
wards entirely black, and finally of two colours as described. 
Sir Humphrey Davy detected in the stem of this plant an ex- 
traordinary quantity of silex ; it is this substance that commu- 
nicates to the stem the rough and file-like character from which 
it derives its value as an article of commerce. The silex appears 
in the form of minute crystals, arranged with beautiful and per- 
fect regularity. Under the microscope we find these crystals 
forming longitudinal series of elevations on the stem : in the 
furrows between these are numerous cup-shaped depressions, 
at the bottom of each of which is placed a stoma. In the 
volume on Optics in Lardner’s ‘ Cabinet Cyclopedia,’ Dr. Brew- 
ster has recorded that he found each particle of silex to possess 
an axis of double refraction. We are told by botanists that the 
quantity of silex is so great, and the particles so closely set, 
that the whole of the vegetable matter may be removed by ma- 
ceration without destroying the form of the plant. 
