INTRODUCTION vii 



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. 



Illustrations. 



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 



*Abrams, LeRoy, "The Deserts and Desert Floras of the West," in Nature and Science 

 on the Pacific Coast. 



