Page:Popular Science Monthly Volume 80.djvu/201

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O great group of plants seems to have left so continuous and full a record of itself, through so long a stretch of time, as have the Gymnosperms. Further work may uncover equally extensive records of certain other vascular groups, but our knowledge of the history of Gymnosperms is at present more complete than that of any other group of plants.

Several things have contributed to the completeness of this knowledge of Gymnosperms. They have always been abundant in the flora of every period that has left a plant record, and they are still abundant. This has given continuity and wealth of material throughout the whole history of vegetation, so far as that history is known. Those who picture an historical succession of the great plant groups are not representing the records in sight, for our earliest records show a vegetation as varied and complex as that of to-day, so far as the great groups are concerned. Although Angiosperms are probably relatively modern, seed-plants were represented early in the Paleozoic by Gymnosperms. This means that the evolution of the plant kingdom, in all its essential outlines, had been attained at least as early as any known records of vegetation.

Another fact which has contributed to the completeness of our knowledge of the group is what may be called the renascence of paleobotany as a morphological subject. Not only did this involve the comparative study from sections, of the essential structures, but also it enormously extended the range of structures used in indicating relationships, by including the vascular system in the evolutionary scheme. The incorporation of vascular anatomy into modern morphology was significant not merely because it supplied another line of attack, but also because it deals with the most completely recorded structure of vascular plants and really gives continuity to the paleobotanical record.

The Gymnosperms were represented during the Paleozoic by two great groups, Cycadofilicales and Cordaitales. They were not merely members of the Paleozoic flora, but they were conspicuous members, together constituting the seed-plant vegetation. Neither of these groups has been traced with certainty into the Mesozoic, so that even though a few lingering forms may be found at a later period, they are essentially restricted to the Paleozoic, and our knowledge of them has been derived