. A textbook of botany for colleges and universities ... Botany. PTERIDOPHYTES ^33. 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. io6). The vascular cylinder of the stem is generally of the primitive type, being a protostele (p. 125) (fig. 297) ; but in some caste the cylinder is hollow (a siphonostele), containing pith, a type of cylinder derived from the protostele. Sporangia. —The sporangia, as in all Lyco- podiales
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. A textbook of botany for colleges and universities ... Botany. PTERIDOPHYTES ^33. 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. io6). The vascular cylinder of the stem is generally of the primitive type, being a protostele (p. 125) (fig. 297) ; but in some caste 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 , . Ill Fig. 29O. — Selaginella: that sporangia are not always produced by sporophyte showing strobili sporophylls, and in such cases the name sporo- and the gradation from phyll is justified only by its relation to the ^°"''Ee leaves to sporo- sporangium. On the basis of their ongin, 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 5<;/a£(«e//o, showing the pro- e.xtensive abortion of tostele (a single, solid, concentric vascular cylinder). mother cells OCCUrs, SO. Please note that these images are extracted from scanned page images that may have been digitally enhanced fo