33 2 The Botanical Gazette. [December, 



larly in Equisetum and many Polypodiaceae. This tendency 

 has been attended by a greater and greater reduction of the 

 prothallium, which finally, in such forms as Isoetcs, has lost 

 all power of independent existence, and serves simply to nour- 

 ish the sporophyte until it can live alone. In these two, the 

 sex of the prothallium is already indicated by the two sorts 

 of spores. This carried a step further resulted in the macro- 

 spore being retained permanently within the sporangium 

 which did not separate from the sporophyte until the prothal- 

 lium was developed. 



At first, as is still the case in Ginkgo* and some cycads, 

 fertilization was not effected until after the sporangium (seed) 

 became detached ; but in the higher forms, fertilization and 

 the formation of the embryo were completed before the seed 

 ripened, as is seen in the Abietineae, for example. 



Inasmuch as heterospory was developed independently in 

 several widely separated groups, the question naturally arises 

 whether the formation of seeds may not also have taken place 

 in more than one line, and that all of the spermaphytes have 

 not necessarily arisen from the same stock. 



The great gap between gymnosperms and angiosperms it is 

 at present impossible to bridge over, and the possibility of a 

 separate origin of the latter directly from some group of 

 pteridophytes is by no means improbable. The writer's re- 

 cent investigations upon the embryo of fsoetes 2 have shown 

 that it much more nearly resembles that of a typical monocot- 

 yledon than it does the gymnosperms, and as the prothallium 

 is hardly more differentiated than in the latter, it is about as 

 easy to imagine the monocotyledons to be derived directly 

 from forms like Isoctcs as from the gymnosperms. 



As might be expected, there is much difference of opinion 

 concerning the inter-relationships of the spermaphvtes. The 

 view ordinarily accepted is that of Strasburger, 3 who derived 

 the gymnosperms from forms intermediate between ferns 

 and lycopods, but having their nearest affinity among living 

 forms with Selaginella. This common stock then divided into 

 two branches, cycads and conifers, and from the latter through 

 the Gnetaceae, were derived the dicotyledons, from which as 

 a degenera te group the monocotyledons have descended. 



1 Goebel: Outlines, p 338 



