leaves, giving to the cylindrical forms the appearance of tree ferns. The remarkable feature of Bennettitales, however, is the cone (strobilus), whose structure is unique among Gymnosperms. These cones, instead of being solitary and terminal, in the 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, subtends the more or less extended axis-bearing ovules, and both stamens and ovules are encased by enveloping bracts. This bisporangiate character, and the relation of stamens to ovules in the cone, are so suggestive of such an Angiosperm flower as that of magnolia that some botanists 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 originated during the Mesozoic.
The Cycads (Cyadales) constitute a second group of Mesozoic 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 Cycadofilicales: that the Bennettitales attained a dominant place in the Mesozoic flora; and that the Cycadales, much less conspicuous during the Mesozoic, persisted 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 Cycadofilicales were continued; so that the living Cycads, although relatively modern from the standpoint of history, are structurally the most primitive of living Gymnosperms, because they most resemble the historically ancient Cycadofilicales.
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 present 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 the late Paleozoic to the present time. They are really a Mesozoic type, and their single representative in the present flora has probably continued to exist simply because it is a tree kept in cultivation in the temple grounds of China
