PTERIDOPHYTES 133 



noteworthy feature of the sporophyte body is the occurrence of a single 

 chloroplast in the actively dividing cells (meristem). It will be re- 

 membered that this 1 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 



7 FIG. 296. Selaginella: 



that sporangia are not always produced by sp0 rophyte showing strobili 

 sporophylls, arid in such cases the name sporo- and the gradation from 

 phyll is justified only by its relation to the o ^ leaves to spor - 

 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 Selaginella, showing the pro- extensive abortion of 

 tostele (a single, solid, concentric vascular cylinder). mother Cells occurs, SO 



