5i6 MOSSES AND FERNS chap. 



The reasons for regarding the eusporangiate Ferns as the 

 lowest of the FiHcineae have been already given at length, 

 but may be summarised as follows : ( i ) The structure of the 

 gametophyte and sexual organs corresponds more nearly to 

 that of the Liverworts than do those of the Leptosporangiatae, 

 and the prothallium is larger and longer lived than in 

 the latter ; (2) the embryo remains much longer dependent 

 upon the gametophyte, and the latter may live for a long time 

 after the sporophyte becomes independent ; (3) the differenti- 

 ation of the organs and tissues of the embryo takes place 

 later than in the Leptosporangiates, and the tissues of the 

 mature sporophyte are also simpler than in most of the latter ; 

 (4) the sporangia of the Eusporangiatae, especially Ophio- 

 glossum, are of a much less specialised type than in the 

 typical leptosporangiate Ferns, and approximate more nearly 

 the condition found in Anthoceros ; (5) the small number of 

 species of the Eusporangiatae, but the wide divergence of type 

 shown, especially by the two, groups of the Ophioglossaceae and 

 Marattiaceae, indicate that these are remnants of formerly more 

 predominant forms. Finally, the strong evidence of the 

 geological record that the Eusporangiatae were the prevailing 

 types in the earlier formations, and have been supplanted by 

 the more specialised Leptosporangiatae in more recent times, 

 is reasonably conclusive. 



The homosporous Leptosporangiatae constitute a pretty 

 continuous series, beginning with the Osmundaceae, by which 

 they join directly to the Eusporangiatae, and ending with 

 the Polypodiaceae. From this stock the two heterosporous 

 families, the Marsiliaceae and Salviniaceae, have branched off 

 independently of one another. 



The systematic position of Isoetes is very difficult to settle, 

 but on the whole its affinities appear to be with the lower 

 Eusporangiatae. 



The development of heterospory in the different groups 

 of the Pteridophytes is of especial interest, from its bearing 

 upon the question of the origin of the Spermaphytes. That 

 heterospory arose in a number of widely remote groups is 

 unquestionable. While among the living Pteridophytes it is 

 confined to the Ferns and Lycopods, the very perfect fossil 

 remains of Calamostadiys show that heterospory was also 

 developed in the Equisetineae, although apparently the differ- 



