326 
HICH AND CASH : Al FlNlTIEy OF LEPIDODENDRON. 
erect steins, e.g. L. sektgo, the xylem is more symmetrical with 
respect to the axis, and takes the form of wedge-shaped masses which 
radiate from the centre and widen outwards. 
Tlie phloem of tlie cylinder forms a sort of matrix in whicli the 
xylem is imbedded. It consists of the nsual elements, cells and 
sieve-tubes, or their homologues, the latter of which have wider 
lamina than the former. The phloem is surrounded by a few layers 
of cells, which form the phloem sheath, but this, according to 
Hegelmaier,'" seems to correspond with the phloem-sheatli of ferns, 
and probably belongs to the cortex. 
Two or three layers of cells at the innermost periphery of the 
cortex are differentiated from the rest, and form the ' bundle sheath.' 
The cells are tangentially compressed, and tlieir walls are somewhat 
suberized. 
As already stated, the cortex is relatively thick, and in several 
species is readily separable into two layers, though these generally 
pass gradually the one into the other. These correspond to the 
hypoderma and inner parenchyma of De Bary, and vary in character 
in different species, as is indicated below. 
At the periphery of the stem, the hypoderma passes over into 
the tissues of the leaves, wliich for tlie most part are not deciduous, 
and are attached by a broad base. 
Stem of Selagix ell a. 
The stems of plants included in this genus present few varieties 
of structure, save those found in the vascular tissues and in the 
number of the vascular bundles. 
In S. Martensil, there is a single central vascular bundle of the 
concentric type, and composed of normal xylem and phloem. 
The xylem in cross section is in the foru) of a flattened ellipse, 
and consists of two opposite marginal groups of spiral vessels con- 
nected by scalariform tracheides. It is developed centripetaUy from 
the two opposite points, and does not include a pith. 
The phloem completely invests the xylem, and is composed of 
parenchymatous cells and sieve-tubes, or their homologues. Enclosing 
the phloem is the phloem-sheath, composed of one, two, or three 
* Sachs' Text Book of Botany, 2nd Eng-. ed., p. 468. 
