INTRODUCTION. 27 



to make direct comparisons between the different forms without resorting to computa- 

 tions. The only features not drawn to scale are the thumb sketches sometimes inserted 

 to indicate habit. These were prepared for some species after studies had been made 

 by the artist in the field; most of them, however, were constructed from photographs 

 of living plants taken for the purpose. 



AH of the drawings, with the exception of the thumb sketches just mentioned, are 

 believed to be accurate as to shape and size. The smaller details were drawn as they 

 appear under the binocular dissecting microscope, and measurements were made with 

 the aid of a micrometer disk. In all cases a subsequent examination was made to be 

 certain that the dimensions adopted represented the average of the material in hand. 



Phylogenetic charts. — Since one of the main objects of the present monographs is to 

 present a classification based upon phylogeny, it has been thought desirable to give a 

 graphic presentation of the relationships between the numerous forms. A number of 

 charts has been prepared to accompany the discussions of phylogeny. There is no 

 thought of finality in the diagrams as here presented. Their purpose is to express 

 the results of evolution as now understood, and it is fully appreciated that the evidence 

 for certain phyletic lines is still far from conclusive. However, the importance of natural 

 groupings and their expression in a readily inteUigible form is so great that the charts 

 are here presented as a contribution to the general subject of phylogeny. Moreover, in 

 some groups the relationships are so well understood that only shght modifications will 

 be necessary as the result of further studies. 



Many of the charts are to be looked upon as expressions of degree of relationship 

 rather than as illustrating definite lines of evolution. This is because it is sometimes 

 easier to make out the relationships of biologic forms than it is to demonstrate the Unes 

 along which they have evolved. This is particularly true as regards the direction of 

 evolution. For example, in many of the charts the relationships of certain groups are 

 pretty well understood, but it is at present impossible to determine which one represents 

 the original stock. Therefore, it may be necessary in some cases to start with one of 

 the forms here placed fairly well up in the scheme, proceed thence towards the lower ones, 

 and then around to other branches leading to the most highly developed of all. 



Even though the exact lines of evolution can not always be indicated with certainty, 

 the bringing together of related forms into increasingly larger and larger natural groups 

 should be undertaken wherever possible, for this is the basis of taxonomy. The graphic 

 representation of such relationships, if carried to all groups of plants, would doubtless 

 be of much value to geneticists, breeders, and others who need to know the related forms 

 of plants with which they are working. Furthermore, if properly prepared, phylo- 

 genetic charts may replace, to some extent at least, the analytical keys now in vogue. 

 Largely with this use in mind, the differentiating characters as well as the names of the 

 groups themselves have been inserted on the charts here presented. 



Citation of authors, synonyms, and specimens. — The almost universal custom of fol- 

 lowing the specific, subspecific, and varietal name by that of the author who first used 

 it is followed throughout this monograph. It is hoped, however, that as names and 

 combinations of names become established, this practice will be deemed unnecessary, 

 since its object is accuracy in the application of terms rather than credit to the author. 

 When new combinations are made for the first time in this work, the first author of the 

 final term is cited in parenthesis. There is no important reason why this should be 

 followed by the name of the author of the new combination, but as a concession to 

 custom this is done when such combination has been made prior to the issuance of this 

 volume. It therefore follows that the present authors are responsible for all those com- 

 binations in which the parenthesis is not followed by the name of some other worker. 

 In the case of a few trinomials the combination itself is not new, but the rank has been 



