Page:Popular Science Monthly Volume 80.djvu/203

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THE HISTORY OF GYMNOSPERMS

vegetation consisted of these fern-like plants, and so the coal period was pictured as a time of luxuriant fern vegetation, rivalling our present tropics in that feature. Approximately ten years ago these fern-like plants were observed to bear seeds, and the Cycadofilicales became established as the most fern-like group of Gymnosperms. All of the great Paleozoic "fern" groups were found involved in the seed-bearing habit, until now the residuum of real ferns in the Paleozoic seems to be quite small. In any event, it has been made clear that the Cycadofilicales were derived from ferns; and if so, probably all the other Gymnosperms. It should be understood that the ordinary ferns of to-day are relatively modern, and are quite unlike those very ancient ferns which gave rise to the Cycadofilicales, and which have received the general name Primofilices.

This ancient group of Gymnosperms resembled ferns in every important particular except in the seed-bearing habit. Whereas in ordinary ferns the sporangia are borne in groups or so-called "fruit dots"

(sori) on the fronds, in Cycadofilicales some of the sori were replaced by seeds, which makes a seed the morphological equivalent of a sporangium or a group of sporangia (a sorus). The bearing of seeds necessitated also the presence of structures corresponding to stamens, and producing pollen. These pollen-bearing structures remained like the fern sporangia (in sori), and for a long time confirmed the notion that these fern-like plants were really ferns. To say that a fern-like leaf must belong to a fern might be unsafe, but to say that such a leaf bearing sporangia in sori must belong to a fern seemed absolutely safe. And still many of these "fern sporangia" have turned out to be the pollen sacs of seed plants.

If the bearing of seeds distinguished Cycadofilicales from ferns, the absence of cones distinguished them from other Gymnosperms. The seeds and pollen sacs were borne as freely on the fronds as are the sporangia on the fronds of ferns. In the later groups of Gymnosperms, the seed-bearing leaves and pollen sac-bearing leaves (both kinds called "sporophylls") became distinct from the ordinary foliage leaves, and were finally compactly organized into the cone-like structure (strobilus) characteristic of most Gymnosperms. But among the Cycadofilicales the strobilus stage was not reached.

The Cycadofilicales seem to have given rise to two great branches of Gymnosperms, both of which are represented in the present flora. One of them includes the Cycads, and therefore have been called the Cycadophytes; the other includes the Conifers, and therefore may be called the Coniferophytes. The Coniferophytes differentiated from the Cycadofilicales earlier than our records of vascular vegetation, for the Paleozoic representative (Cordaitales) of Coniferophytes is distinct from the Cycadofilicales as far back as records go. On the other hand.