Floras of the Past 1

Abstract

THIS “extract” from the twentieth annual report of the U.S. Geological Survey forms a bulky volume of more than 200 pages and 150 plates. Prof. Lester Ward tells us in the introductory remarks that the aim of the memoir is “to give a succinct account of the progress thus far made in the direction of developing the Mesozoic floras of the United States.” The present instalment deals with the vegetation of the Triassic and Jurassic periods, the Cretaceous floras being reserved for a second part. The numerous scattered references to the Mesozoic botany of the United States, and the conflicting opinions that have been expressed as to the geological age of the plant-bearing beds, enable us to thoroughly appreciate the value of a comprehensive report compiled by one who possesses a wide knowledge of palæobotanical literature. It is, however, not solely with published facts that the volume is concerned, for a large portion of it is devoted to a systematic account of recently discovered species. The Triassic floras are represented by fossils from the Connecticut Valley, Pennsylvania, Maryland, Virginia and other regions, while plants of Jurassic age are described from California, Oregon and Wyoming. One of the chief desiderata from the point of view of palæobotanical research is a careful and critical examination of the records of ancient floras, which may be of use in the consideration of the broad problems connected with plant evolution and distribution. In the treatment of work of this kind it is essential to carefully weed out such material as cannot he determined with sufficient accuracy to furnish trustworthy evidence. This obvious reflection is suggested by a perusal of the numerous determinations and descriptions contained in the volume before us. It is unfortunate that the plants from the Jurassic strata of California (the Oroville flora) are in most cases represented by fragmentary samples, and in several instances these have been referred to genera and species on evidence which cannot be accepted as satisfactory. Systematic work on fossil plants has too frequently been marred by a want of self-control on the part of authors who appear to be led away by a desire to attach names to specimens that are absolutely valueless as botanical records; we are compelled to add that the utility of the descriptive portions of this work is seriously impaired through lack of courage to discard worthless material. More thaneighty specimens of cycadean stems are recorded from the Freezeout Hills of Wyoming-probably of J urassic age. These stems are referred by Prof. Ward to a new genus, Cycadella, which is described as being characterised by the relatively -small size of the trunks and by a dense covering of ramental tissue “exuberantly developed from the leaf-bases and extruded from the armour, massed and matted in the fossil state so as to form a thick outer covering.” The exceptional development of the ramental scales suggests a comparison with the abundance of woolly hairs on the carpophylls of the recent cycadean genus Dioon, and constitutes an interesting feature which may serve as an index of climatal conditions.