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 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 

 . ., . , 1,1 FIG. 296. Selaginella: 



tnat sporangia are not always produced by sp0 rophyte 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 r - 

 sporangium. On the basis of their origin, P y 

 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 



