EQUISETUMS AND CALAMITES. 83 
tripe The necessary strength and firmness i nd 
obtained by the needa ae . peculiar, long, fibre-like, 
rtical portio: 
stems “hey are a and chiely devsloped: under the ridges. 
epidermis consists of a single row of 
_ Senseally manta dekaned, and containing silica in the walls, the 
mata forming i in the furro ws between the ridges. _ The outer cortical 
oe Mettenius), more or less dark brown in colour in the under- 
ones. Those thickened elongated 
cells are not found in the fertile stems of EF. arvense and i a maximum. 
layer of cells running continuously round the stem in a regular circle, 
the sheath of the fibro-vascular bundles. In other cases each separate 
fibro-vaseular bundle is surrounded by a special sheath of its own. 
The cells of the sheath are often more or less thickened, and in 
rhizomes the walls are generally brownish or yellow. When each 
bundle is surrounded by a separate sheath, then the tissue of the cor- 
a series of thickened cells exists on the inner side of the fibro- 
vascular bundles in the rhizome, the walls of the cells being deeply 
coloured. 
undle. 
ea’ 
part (xylem, Naegeli) consists of only two forms of Set. The 
annular, spiral, or reticulated cells which by fusion me the 
vessels, and elongate chymato lls contai ¢ 
which sometimes form “the bounding cells of the lacuna, at othte times 
a 2 
