THE ORIGIN OF THE VASCULAR PLANTS. 285 



spike with only two rows of sporangia, in the larger species 

 of Botrychium it is a compound panicle with very 

 numerous, but very much smaller sporangia. This increase 

 in the number of sporangia is correlated with a correspond- 

 ing division of the sterile segment by the leaf which becomes 

 very ample and much divided. 



The type of sporangium in Botrychium is very much 

 like that of the Osmundaceae, which in turn are closely con- 

 nected with the true leptosporangiate Ferns ; but whether 

 there is any genetic relation between Botrychium and 

 Osmunda is another question, which at present we are not 

 in a position to answer. In these forms, as well as in all 

 the typical Leptosporangiatae, the archesporium is readily 

 traceable to a single cell, and in the latter group the 

 whole sporangium is derived from a single epidermal cell, 

 which is never the case in the Eusporangiatae. In all the 

 Leptosporangiatae the wall of the sporangium is reduced 

 to a single cell in thickness, and is usually long-stalked, 

 especially in the Polypodiaceae, which are probably the 

 highest of the homosporous Ferns. 



The very complete series of forms leading from the 

 Leptosporangiatae to the Eusporangiatae has often enough 

 been commented upon, but until the recent attempts to 

 prove the primitive character of the latter it was taken as 

 a matter of course that the Leptosporangiatae formed the 

 beginning of the series, and not the end. The objections 

 to this view may be stated briefly as follows. First, 

 the type of sporangium of the Leptosporangiatae is very 

 specialised, while that of the Eusporangiatae approaches the 

 condition found among the Bryophytes ; second, the 

 prothallium of the Eusporangiatae approaches much nearer 

 in structure the thallus of the Hepaticae, both in its growth 

 and in the structure of the sexual organs, and the embryo 

 is much longer dependent upon it ; thirdly, the geological 

 record shows conclusively that the Eusporangiatae were the 

 prevailing forms among the Carboniferous and pre-Carboni- 

 ferous Ferns, and the Leptosporangiatae only appeared 

 later, increasing in number and variety in the more 

 recent strata. . 



