The Phylogeny of Angiosperms 



seems to be most unlikely with the exclusion of Gnetum. At the same time, it might 

 be claimed that Dicotyledons represent an independent line from the Gymnosperm 

 stock, that advanced in the same direction and much farther than did the Gnetum line ; 

 but all the essential morphology of the Gymnosperms is less favorable to such an 

 origin than is that of the heterosporous Pteridophytes. 



The Lycopodiales certainly deserve serious consideration in this connection. The 

 structures of Selaginella are about as suggestive of Dicotyledons as those of Isoetes 

 are suggestive of Monocotyledons, the embryo being as distinctly dicotyledonous as 

 that of Isoetes is monocotyledonous, and our study of Selaginella has shown the 

 strikingly seed-like character of the megasporangium. But a seed may be attained 

 by any heterosporous line, and Selaginella and even its ancient Lycopodium stock have 

 too many peculiarities to be considered seriously as ancestral types of Angiosperms. 



The only other alternative is that mentioned in connection with the origin of the 

 Monocotyledons, namely the derivation of the Proangiosperms as an independent 

 heterosporous line from the abundant ancient eusporangiate Filicales, and this view is 

 supported by anatomical testimony. 



It is becoming increasingly evident that the great marattiaceous plexus of the 

 Palaeozoic probably gave rise to several heterosporous lines, one or more of which have 

 certainly been responsible for the Gymnosperms, and others have led to the Angiosperms. 

 As heterospory may lead to seed-formation in any line, it is inconceivable that only 

 one or at most two of the numerous heterosporous lines have attained seed-production. 

 It is more probable that the Angiosperms have arisen from the Marattia-like ferns in 

 several independent lines ; that the group known as Angiosperms is determined by its 

 attainment of seed-production rather than its monophyletic origin ; but that in a 

 certain sense it has a common phylogeny and hence numerous characters in common. 



A summary of these conclusions may be stated as follows : 



1. The Monocotyledons and Dicotyledons comprise at least two independent 

 angiospermous lines, and do not represent a single phylum. 



2. No Angiosperm phylum has been derived from the Gymnosperms or from 

 living heterosporous Pteridophytes. 



3. All Angiosperm phyla have been derived as independent heterosporous lines 

 from the ancient eusporangiate Filicales, which also gave rise to* the Gymnosperms. 



4. Several Angiosperm phyla probably arose independently from the marattia- 

 ceous plexus of the PalsBozoic. 



5. If Angiosperms have a monophyletic origin, which seems very unlikely, it 

 seems clear that the Monocotyledons have been derived, from the more primitive 

 Dicotyledons. 



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