5i8 MOSSES AND FERNS chap. 



embryo, are extraordinarily similar, and it is not unreasonable 

 to suppose that this is something more than accidental. 

 Whether the Cycads belong to the same stock, or, as has been 

 frequently suggested, are more nearly allied to the Filicineae, 

 further investigation must decide. 



The Angiosperms are in all probability all members of a 

 common developmental series, but just what is their relation 

 to one another and to the other vascular plants is not so 

 evident. It is usually held that they have been derived from 

 the Gymnosperms through the Gnetaceae, but it has also 

 been suggested that one or both of the divisions may have 

 originated directly from the Pteridophytes. Attention has 

 been called more than once to the close resemblance between 

 the embryos of the Filicinese and those of typical Monocoty- 

 ledons, and this is especially the case in Isoetes, where, in 

 addition, the structure of the mature sporophyte is much like 

 that of the Monocotyledons. It is possible that the surround- 

 ing of the sporangium by the base of the sporophyll may be 

 the first indication of the ovary of the Angiosperms, but as 

 this applies to the microsporangia as well, much stress cannot 

 be laid upon it. It is quite as easy to trace back the embryo- 

 sac of the Angiosperms to the macrospore of Isoetes as to the 

 embryo-sac of the Gymnosperms ; and when the great similarity 

 between the sporophyte of the former and the Monocotyledons 

 is considered, the probability of the origin of the latter from 

 aquatic or semi-aquatic ancestors resembling Isoetes is certainly 

 considerable. 



The essential similarity in the structure of the embryo-sac 

 in all Angiosperms yet examined (except Castim-ina), as well 

 as the structure of the flower, makes it almost inconceivable 

 that the two branches. Monocotyledons and Dicotyledons, could 

 have arisen from different stocks. Strasburger's suggestion 

 that the Dicotyledons were derived directly from the Gymno- 

 sperms, and that the Monocotyledons are a reduced branch of 

 the former, is open to objections both on morphological and 

 palseontological grounds, and we believe that the evidence we 

 have at present points to the Monocotyledons as the more 

 primitive of the two divisions of the Angiosperms, from which 

 later the Dicotyledons branched off. If, as we have assumed, 

 Isoetes has its affinities with the lower eusporangiate Ferns, 

 the Angiosperms would be connected directly with them 



