XIV 



ISOETACE^ S6i 



The development of the spores and the early stages of the 

 female gametophyte certainly resemble those of Selaginella, 

 and form the strongest argument for assuming a relationship 

 between the two genera. The embryo, however, is very much 

 more like that of the eusporangiate Ferns, resembling, perhaps, 

 most nearly that of Botrychium, and in connection with the 

 structure of the mature gametophyte and sexual organs, makes 

 it not improbable that there is a real, but extremely remote rela- 

 tionship between Isoetes and the Eusporangiatse. 



As to the affinities of Isoetes with the Spermatophytes, it 

 more nearly resembles them in the formation of the female 

 prothallium than any other Pteridophyte except Selaginella, and 

 the reduction of the antheridium is even greater than there. 

 The embryo resembles very much that of a typical Monocotyle- 

 don, and the histology of the fully-developed sporophyte, the 

 leaves with their sheathing bases surrounding the short bulb- 

 like stem, and the structure of the roots, all suggest a possible 

 relation to the Monocotyledons directly rather than through the 

 Gymnosperms. 



There is, however, a great interval between the flower of 

 the simplest Angiosperm and the sporophylls of Isoetes, and 

 more evidence must be produced on the side of the former 

 before it can be asserted that this relationship is anything more 

 than apparent. 



30 



