PHYLOGENY OF ANGIOSPERMS 2S."> 



that it may be the nearest living representative of their ancestral 

 forms. Isoetes is a remarkably isolated group among the Pteri- 

 dophytes, with no clear affinities, so that its own connection with 

 the Pteridophyte stock is not evident. The most striking re- 

 semblance to Monocotyledons occurs in the embryo, in which 

 the single cotyledon is terminal and the stem-tip arises later 

 as a lateral structure. The development of the male gameto- 

 phyte resembles Angiosperms more than it does Gymnosperms, 

 while the female gametophyte is equally suggestive. However, 

 these gametophyte characters are shared by Selaginella. The 

 general habit and vegetative structure of Isoetes bear some re- 

 semblance to those of an aquatic Monocotyledon, and the anat- 

 omy of the stem is suggestive of such forms as Yucca and Dra- 

 caena. There can be no question that the resemblances of Isoe- 

 tes to the Monocotyledons are more numerous than those of any 

 other living Pteridophyte. The most telling resemblance is 

 the character of the embryo, but the fact that its axis is trans- 

 verse to that of the suspensor is a serious obstacle. Campbell 

 has called attention to the fact, however, that in the embryos 

 of Lilaea subulata and Zannichellia the apex of the root is not 

 directed toward the suspensor but to one side, so that the axis 

 of the embryo is oblique to that of the suspensor. A possible 

 explanation of these laterally directed roots, however, is sug- 

 gested by Murbeck's recent account of Ruppia (page 196), in 

 which a primary root is formed with the normal orientation, 

 but soon disorganizes, while a lateral root, formed very early, 

 is the first functional one. As between the Gnetum origin 

 of Angiosperms and the Isoetes origin of Monocotyledons the 

 latter view must be preferred. Such a view, of eonrse, does 

 not imply that the present Monocotyledons have been derived 

 from the present Isoetaceae, but that the ancestral forms of 

 the two were probably genetically connected. If this be true, 

 doubtless Isoetes represents a reduced branch of some old stock 

 that gave rise to the more vigorous Monocotyledons. The only 

 possible alternative as to the origin of Monocotyledons, in case 

 they have arisen independently of the Dicotyledons, seems to 

 be to regard them as the end of a heterosporous line that 

 developed independently from the eusporangiate Filicales, 

 whose Pteridophyte members are extinct. Such an hypothes- 

 is only necessary in the event that those based upon known 



