300 MOSSES AND FERNS chap. 



closely that of the other eusporangiate Filicineai, and the 

 spermatozoids are multiciliate, which is never the case in any 

 of the Lycopodineae, but is universal in the Ferns. The 

 tissues of the sporophyte, especially the vascular bundles, are 

 collateral, and are most like those of Ophioglossum, and the 

 dichotomy of the roots, which was formerly taken as a sign 

 of its relationship with the Lycopods, is now known to occur 

 also in OpJiioglossmn. The sporangium, too, may perhaps as 

 well be compared to the spike of Ophioglossum as to the single 

 sporangium of Lycopodiiwi or Lepidodendron. It would be 

 rash to assert positively that the trabeculse correspond to 

 the partitions between the sporangia of Ophioglossum, and 

 that the sporangium is really compound, but this is not 

 inconceivable. The position and origin of the large spor- 

 angium of Isoetes are certainly not very unlike those of the 

 sporangiophore of Ophioglossum. 



The early stages in the development of the female pro- 

 thallium certainly resemble those of Selaginella, so far as the 

 " free-cell formation " is concerned ; but there is no reason why 

 this may not have arisen independently in the two groups, 

 just as heterospory arose quite independently in all the 

 classes of the Pteridophytes. At present, then, the weight 

 of evidence seems to indicate that Isoetes bears the same 

 relation, but in a much more remote degree, to the lower 

 members of the eusporangiate Filicinese that Selaginella does 

 to Lycopodium. 



As to the affinities of Isoetes with the Spermaphytes, 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 

 Monocotyledon, 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. 



