232 



PLANT MORPHOLOGY 



are either similar or only slightly differentiated. Where the foliage leaves 

 are of two kinds, the sporophylls are smaller than the large leaves (Fig. 

 189.4). As in Lycopodium, the sporangia are solitary, adaxial, unilocular, 

 and eusporangiate in development (Fig. 1895, C). Each is probably 

 derived from a transverse row of initials. These generally appear on the 



Fig. 188. Stem structure of Selaginella flabellata. A, diagram of cross section, showing 

 central flattened protostele surrounded by cortex, X 10. B, enlarged view of portion of 

 stele, showing exarch xylem surrounded by a wide zone of phloem and an air space traversed 

 by trabeculae, X 200. 



stem just above the place where the young sporophyll arises from it (Fig. 

 190A-C). Although cauline in origin, the young sporangium is soon car- 

 ried out on the sporophyll and then looks as if it had originated there. 

 The wall of the young sporangium consists of a single layer of cells but it 

 soon becomes two-layered. In contrast to the other lycopods, the tape- 

 tum is not derived from the wall tissue but from the outermost layer of 

 sporogenous tissue. As in Lycopodium, it does not break down until the 

 spores are formed. The innermost wall layer also disorganizes at this 

 time, and so the mature sporangium has only a single layer of wall cells. 

 Each sporangium is borne on a short stalk. Dehiscence takes place by 

 means of a vertical slit. 



