PTERIDOPHYTES 
re- 
noteworthy feature of the sporophyte body is the occurrence of a single 
chloroplast in the actively dividing cells (meristeni). It will be 
membered that this same feature appears in 
the gametophyte body of Anthoceros (p. 106). 
The vascular cylinder of the stem is generally 
of the primitive type, being a protostele (p. 125) 
(fig. 297) ; but in some cases the cylinder is 
hollow (a siphonostele), containing pith, a type 
of cylinder derived from the protostele. 
Sporangia. The sporangia, as in all Lyco- 
podiales, are solitary and adaxial with reference 
to the sporophyll, and derived from a transverse 
row of initial cells; but in Selaginella these 
initials occur on the stem just above the origin 
of the sporophyll (figs. 298, 299). This means 
. . . . FIG. 296. Selaginella: 
that sporangia are not always produced by S p 0r0 phyte showing strobili 
sporophylls, and in such cases the name sporo- and the gradation from 
phyll is justified only by its relation to the foliage leaves to s P ro " 
-~. i . F phylls. 
sporangium. On the basis of their origin, 
sporangia often are distinguished as foliar (on the sporophyll) and 
cauline (on the stem). 
Heterospory. The notable feature of Selaginella, however, is that 
all of the sporangia in a strobilus do not mature alike, resulting in hetero- 
spory. They all develop alike, and as described under Lycopodium 
(p. 125), as far as the 
mother cell stage (fig. 
300), after which a 
great difference ap- 
pears. In some of 
the sporangia (usually 
the larger number) all 
or nearly all of the 
mother cells function, 
resulting in the pro- 
duction of numerous 
spores (fig. 301). In 
the other sporangia an 
FIG. 297. Section of stem of SelagineUa, showing the pro- extensive abortion of 
tostele (a single, solid, concentric vascular cylinder). mother cells occurs, SO 
