320 as Mie ae Ph sistolog sical | Bie 
pair of vascular bundles g. In addition, there Ofte Gene 
also the so-called ‘ essential’ air-cavities (Fig. 444 B, L’) in. 
the vascular bundles. In some species the epidermis, which 
consists of a single layer of cells and is more or less strongly 
silicified, has stomata (the ‘ Equiseta phaneropora’) ; while 
in others (‘Equiseta cryptopora’), it is penetrated by crevices 
beneath which are the truestomata. The stomata (Fig. 444 
B, st) are always placed in a single or in several rows between 
the elevated ridges of the stem. ‘The cortex consists of thin- 
walled or of only moderately thickened parenchyma. It is 
separated on the inside from the vascular bundles by a — 
sheath which sometimes encloses all the bundles together 
| (Fig. 444 B, s) ; while at other times each separate bundle is. 
surrounded by a sheath (Fig. 444 A, g). The vascular 
bundles ascend in a vertical direction, and parallel to one 
another, through the internodes, and form annular coils in 
the nodes. The ‘essential’ air-cavity occupies the largest 
part of the woody portion of each vascular bundle ; one or 
more vascular cells project into it on each side; and its 
outer part is formed of elongated parenchymatous cells 
containing starch. ‘Two other groups of vascular cells are 
found on the cortical side of the bundle. The bast-portion 
contains three elements; parenchyma, bast-fibres, and 
sieve-tubes ; 1t hes between the four groups of vascular cells 
and the vascular bundle-sheath. In the root is an axial 
bundle of vascular cells surrounded by elongated parenchy- 
. matous cells, with which sieve-tubes and bast-fibres are inter- 
mixed. ‘The sporangia are capsules placed on the under side 
of scales belonging to the fructification (Fig. 443 11). The 
fructification or receptacle is often placed on.special shoots 
which are distinguished by their external form and by their 
brown colour from the sterile green stems (Fig. 443 1.). 
The spores are provided with two hygroscopic bands or 
elaters (Fig. 443 111.), only loosely attached to them [formed _ 
by the splitting into narrow strips and the partial detaching 
of the icin hed or outermost of the three coats of the De ae 
