THE HISTORY OF GYMNOSPERMS 201 



leaves, giving to the cylindrical forms the appearance of tree ferns. 

 The remarkable feature of Bennettitales, however, is the cone (stro- 

 bilus), whose structure is unique among Gymnosperms. These cones, 

 instead of being solitary and terminal, in tlie midst of the rosette of 

 leaves, as in most Cycads, are lateral on dwarf branches which arise in 

 profusion from the stem. But this is a small feature as compared with 

 the fact that the cone is " bisporangiate." In other Gymnosperms the 

 ovules (and of course seeds) and stamens are in different cones, and 

 often these cones are on different plants. In Bennettitales, the rosette 

 of stamens, which resemble small fern fronds bearing sporangia, sub- 

 tends the more or less extended axis-bearing ovules, and both stamens 

 and ovules are encased by enveloping bracts. This bisporangiate char- 

 acter, and the relation of stamens to ovules in the cone, are so sugges- 

 tive of such an Angiosperm flower as that of magnolia that some botan- 

 ists would see in Bennettitales the ancestral forms of Angiosperms. It 

 is certainly true that the Bennettitales were abundant and wide-spread 

 during the Mesozoic, and it seems to be true that the Angiosperms orig- 

 inated during tlie Mesozoic. 



The Cycads (C3Tadales) constitute a second group of jMesozoic 

 Gymnosperms, associated with the Bennettitales of common origin, 

 but apparently not a conspicuous part of the vegetation. It is very 

 likely true that Bennettitales and Cycadales represent two independent 

 Mesozoic derivatives from the Paleozoic C'ycadofilicales : that the Ben- 

 nettitales attained a dominant place in the Mesozoic flora ; and that the 

 Cycadales, much less conspicuous during the Mesozoic, ])ersisted until 

 the present day as the only living representatives of the Cycadophytes. 



The Cycadophyte line is characterized by the retention of many of 

 the fern-like features of the Cycadofilicales. In habit, in foliage, in 

 stem structure, in sporangia, in reproductive habits, the features of 

 Cj^cadofilicales were continued; so that the living Cycad«, although 

 relatively modern from the standpoint of history, are structurally the 

 most primitive of living Gymnosperms, because they most resemble the 

 historically ancient C3'cadofilicales. 



The two other groups of Mesozoic Gymnosperms were derived from 

 the Paleozoic Cordaitales, the Paleozoic member of the Coniferophyte 

 branch. They are known as Ginkgoales and Coniferales, the former 

 being nearly or quite extinct to-day, and the latter comprising the pres- 

 ent conspicuous Gymnosperm vegetation of the temperate regions. 



The Ginkgoales were abundant during the Mesozoic, but apparently 

 remained quite constant in characters, so that they can be represented 

 structurally by a single line extending from tlie late Paleozoic to the 

 present time. They are really a Mesozoic type, and their single repre- 

 sentative in the present flora has probably continued to exist simply 

 because it is a tree kept in cultivation in the temple grounds of China 



VOL. LXXX. — 14. 



