98 ELEMENTS OF PLANT ANATOMY. 
and periblem, the inner to the plerome of the phanerogamic 
stem. 
As the simple structure of the moss stem has already been 
explained, we may pass at once to the consideration of the stem 
of the vascular cryptogams or Pteridophytes. A few of these 
have a cluster of initial cells instead of a simple one at the stem 
apex, and the plants possessing such a structure may be con- 
sidered a link between the vascular cryptogams and phanero- 
gams. The stem contains compound vascular bundles which 
are generally concentric, and in all cases closed. The ground 
system is sharply distinct from the epidermal layer, which is 
supplied with stomata. In all the young plants of this class 
the bundle system of the stem may be considered a sympodium 
built up of leaf-trace bundles, a single one of which runs into 
the stem from each leaf. In most cases the first bundle starts 
in the foot of the embryonic plant, where it ends free. From 
the outer extremity it bends out into the first leaf or cotyledon, 
and at the point where it bends outward another new bundle 
starts, which runs along in the stem a short distance and then 
bends outward into the second leaf. In this way arises an 
axillary bundle which may be considered as made up entirely 
of leaf-traces. 
This is the simplest possible arrangement of the stem and 
leaf bundles, and we have seen it already intimated in the stem 
and leaf of Polytrichum, where the place of the real bundle is 
supphed by long fibrous cells. In a number of Pteridophytes 
this simple structure remains throughout the entire life of the 
plant. In others, for example those of the Lycopodiaceae and 
some of the Selaginellaceae, the manner of development of the 
young plant is such that the axillary bundle of the grown stem 
is rather to be considered as belonging solely to the stem, while 
its corners are formed by the leaf-traces. 
Another large class of Pteridophytes begin the development 
of their bundle system as above described, continuing this 
