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Rh America, or are represented in the eastern region by closely related species. The presence of these boreal plants on the high mountain ranges of the Pacific States, a thousand miles or more south of their normal range, is partly due, no doubt, to the Glacial Period. Great Ba.sin Element. The flora east of the Cascade-Sierra Nevada Divide is chiefly of the Great Basin element, which has developed since the humid condi- tions of Miocene and Pleiocene times and the devastating Glacial Period. The principal source, at least for the Upper Sonoran Zone, has been from the south. Mexican Element. "While the common genera familiar to us all w-ere evolving in the moist temperate climates to the north, and spreading over northern North America, Europe and Asia by means of land connections that have disap- peared, drought-resisting plants were taking form on the great arid plateau of Mexico. Here originated the cacti, yuccas, agaves, and most of the other genera peculiar to the American deserts. At the end of the Glacial Period this Mexican flora pushed northward into the western United States, following increased aridity. It is this ancient Mexican element that gives the unique character to our desert vegetation."* California Element. For a continental area the Pacific slope, especially that Iving within the Upper Sonoran Zone, has an unusual number of endemic genera and species. Reasons for the unique character of the flora are to be found in the climatic conditions, both of the present and the past, and in the isolation brought about by the climatic and physical barriers prohibiting direct communication with the eastern part of the continent. Throughout Tertiary Time, while the present- day floras were developing, the Pacific Ocean, responding much less readily to the climatic changes taking place on the main continental areas, has acted as a great thermostat along the Pacific Coast. The climate of the Pacific States has therefore undergone less change than in other parts of the continent. The climate of today, at least in the immediate coastal region, cannot be very ditTerent in temperature from that of Cretaceous Time. Sequoia, Tuuiion. and other genera still flourish, mere remnants of the great cretaceous forests that spread over the northern hemis- phere, even to the Arctic Circle. This theory of the earlier climatic and floral conditions on the Pacific Coast has been strengthened recently by the discovery of fossil remains of Sequoia seuipcrz'ircns, Quercus chrysolcpis. Arbutus uicu- ciesii, and other living species peculiar to the Pacific region, in Pleiocene rocks of central California, showing that the flora and therefore the climate just prior to the Glacial Period was much like that of the present time.

In the preparation of the illustrations, drawings have been made natural size, either from fresh material or herbarium specimens, and under the direct super- vision of the author or the special contributor. All of the figures are from these original drawings except those of such species as are illustrated in Britton and Brown's 'Tllustrated Flora of the Northern United States and Canada." Privilege to use these has been kindly granted b}' Dr. Britton. and is hereby gratefull- acknowledged. The numerals placed on the figures indicate the amount of reduction : for example, the numeral % indicates that the figure is three-fifths the natural size ; but where enlarged details of essential parts have been added no indication of magnification has been attempted. Mr. W. S. Atkinson has made most of the drawings for the families con- tributed by the author, also those of the Eqnisetaceae, SeJagincUaccac, Juiicaccae, and Sali.v: Mrs. Rose E. Gamble has made those of the Ophioglossaceae and Poly- podiaccac; Dr. Norma Pfeififer those of the Isoctaceac : ]irs. ]Iary W". Gill those of the Poaccae; Miss Mary E. Eaton those of the Cypcraceae : and Mrs. Helen on the Pacific Coast.
 * Abrams, LeRoy, "The Deserts and Desert Floras of the West," in Nature and Science