John M. Coulteb 



the most primitive Angiosperms have no perianth. Further, the presence of true vessels 

 in the secondary wood is an argument as much in favor of the origin of Angiosperms 

 from certain heterosporous Pteridophytes as from Gnetum. Every indication points 

 to the conclusion that Gnetum is a highly specialized and comparatively recent 

 member of the Grymnosperm phylum, and as such could not have given rise to the 

 Angiosperms. 



If the Gymnosperms are not the ancestral forms of the Angiosperms, the direct 

 derivation of the latter from Pteridophytes becomes a matter of course. Perhaps it 

 was natural to turn at first to the heterosporous Pteridophytes, and among them the 

 only forms that could seem to be within the range of probability are Selaginella 

 and Isoetes. The latter has been persistently associated with the origin of the 

 Monocotyledons, especially in connection with the former idea that Monocotyledons 

 are the primitive Angiosperm stock. Any supporter of this view now would be almost 

 forced to maintain the polyphyletic origin of Angiosperms. The most striking 

 resemblance of Isoetes to the Monocotyledons occurs in the embryo, in which the 

 single cotyledon is terminal and the stem-tip lateral. A thorough investigation of 

 Isoetes, however, has developed so many difficulties in the way of accepting it as related 

 in any way to the Monocotyledons that the theory must be regarded as untenable. 



The only possible alternative as to the origin of Monocotyledons, in case they 

 have arisen independently of the Dicotyledons, is to regard them as the end of a 

 heterosporous line that developed independently from the eusporangiate Filicales, 

 whose Pteridophyte members are extinct. Since several independent heterosporous 

 lines are already recognized, it is not at all necessary to connect any seed-plants with 

 living heterosporous Pteridophytes. 



More important, however, seems to be the determination of the origin of 

 the Dicotyledons, whether they represent an independent phylum or the primitive 

 angiospermous stock. The fact that they emerged from the so-called "Proangiosperms,'" 

 which were largely displayed in the earlier part of the lower Cretaceous, seems to be 

 fairly well established. The question, therefore, has to do with the origin of the 

 Proangiosperms. They do not seem to warrant the belief that they represent a common 

 stock from which both Monocotyledons and Dicotyledons have been derived, for the 

 Monocotyledons are believed to have existed in unmistakable forms before the large 

 assemblage of Proangiosperms gave rise to unmistakable Dicotyledons. Still less con- 

 ceivable is it that Proangiosperms represent the transition forms from Monocotyledons 

 to Dicotyledons, for nothing in their known structure seems to suggest such a view. 

 That they were derived from Gnetum-like forms is discredited by the fact that there is 

 no sure record of the existence of Gnetum at such an early period, and to have given 

 rise to such an 'assemblage of forms it must have been a conspicuous group. 



If we turn to the earlier groups that were sufficiently prominent and at all 

 suggestive of having given rise to the Proangiosperms, we encounter the Coniferales, 

 Cycadales, Lycopodiales, and Filicales. The Gymnosperm origin of Dicotyledons 



195 



f 



