PTERIDOPHYTES 
(food-conducting cells), 
such an arrangement be- 
ing called the concentric 
arrangement. This most 
primitive vascular system 
of the stem, therefore, 
consists of a single, solid 
cylinder with concentric 
xylem and phloem (com- 
monly spoken of as a 
concentric cylinder), to 
which type the name 
proto stele has been given. 
In mature stems of Lyco- 
podium, the solid xylem 
strand may divide into 
branches which run 
through the stele as 
several strands sepa- 
rated by pithlike tissue 
FIGS. 268270. Sporangium of Lyco- 
podium: 268, section showing a young 
sporophyll bearing a superficial initial cell 
(one of a transverse row) on its adaxial 
face near the base; 269, further develop- 
ment of the initial; 270, division of initial 
into primary wall cell (outer) and primary 
sporogenous cell (inner). After BOWER. 
FIG. 267. Cross section of the central region of an 
adult stem of Lycopodium, showing the inner region of 
the cortex surrounding the central stele, in which the 
branches of the xylem mass are irregularly distributed; 
section of four leaf traces are seen in the cortex, each of 
them showing the simple concentric arrangement (xylem 
surrounded by phloem). 
(fig. 267). From the vascular cyl- 
inder strands pass out through the 
cortex, where they are called leaf 
traces (fig. 267), and enter the 
leaves, where they become continu- 
ous with the veins. 
Sporangium. The large spo- 
rangium is borne upon the upper 
(adaxial) surface of the sporophyll, 
near the base. The sporangium 
initial is superficial (fig. 268), and 
is a transverse row of six to twelve 
cells; in some cases it consists of 
two or three such rows. Each of 
these initial cells divides by a peri- 
clinal wall (parallel with the sur- 
face), resulting in an outer and an 
inner transverse row of cells (figs. 
