iv PREFACE 



The book is a forerunner of one on the vegetation of the Rocky Moun- 

 tains, which has been under way since 1899. The latter is planned not only 

 to outline the structure and development of the vegetation, but it is hoped 

 also that it will be of practical value to the forester and others who touch 

 the habitat and formation in their daily work. Such a book can be of 

 practical use only when the units with which it deals can be recognized 

 with some readiness and certainty. The opportunity for such recognition 

 is materially increased by using relationship to determine the units. This 

 is enhanced by the number of units illustrated, 175 in color and 355 in 

 line. In addition, the illustrations make possible the recognition of a num- 

 ber of common plants by the plant-lover without botanical training. 

 Furthermore, to the botanist the present book will serve to summarize 

 the first results of the quantitative study of the origin of new forms, and to 

 indicate the basis of the work under way in the experimental and statisti- 

 cal study of variation and adaptation. 



The descriptions of several hundred units were written in the field 

 from a large number of individuals, whenever possible under different 

 conditions. It was early found that full descriptions would make the book 

 unwieldy, and they were dropped, in so far as the units were concerned. 

 It is felt that the generic descriptions and the full keys in the larger 

 genera will be sufficient in practically all cases of doubt. For the same 

 reason, no synonyms are given under the units. In fact, the value of 

 synonyms is slight until much fuller experimental and statistical studies 

 have been made. In general, the synonyms given by Nelson have been 

 accepted, usually after the scrutiny of co-types. In other cases, the 

 union of segregates is clearly indicated by the key. While the nomen- 

 clature conforms in general with the American code, no compunction has 

 been felt in correcting improperly formed names, or in using short and 

 significant names in preference to long ones without meaning. In the 

 matter of measurements, the smaller units of the metric system have been 

 used below an inch as a rule, in preference to lines or fractions. 



Constant use has been made of Rydberg's "Flora of Colorado," Nel- 

 son's "New Manual of Rocky Mountain Botany," Coulter's "Manual of 

 Rocky Mountain Botany," Britton and Brown's "Illustrated Flora," and 

 Gray's "New Manual of Botany," as well as the original papers of East- 

 wood, Greene, Nelson, Rydberg and others, for which "rateful acknowledg- 

 ment is made. In determining the relationship of recent segregates, as well as 

 of earlier "species," and in confirming the field and experimental results, 

 Torrey and Gray's "Flora of North America" and Gray's "Synoptical 

 Flora" in particular have been invaluable. The Besseyan system of classifi- 



