VI PREFACE. 



is impossible to avoid some confusion in characters, for outliers of 

 the types cannot but overlap. It might be well said that these outliers 

 are connecting links and that groups so connected should be combined, 

 but this would make specific division of the genus and varietal division 

 of the species almost impossible. The groups must, therefore, be sepa- 

 rated along more or less arbitrary lines. But such arbitrary separation 

 does not prevent natural groups, if nature be broadly interpreted. 



The chief value of the work in hand lies in its discussion of varieties. 

 In the descriptions the aim has been to give as tersely as possible an idea 

 of all of the characters of the plums described. With very few exceptions 

 the technical descriptions of varieties are original and were made by those 

 who have taken active part in the preparation of this book. Nearly all 

 of the varieties having full descriptions grow on the Station grounds but 

 whenever possible specimens of each variety from different localities have 

 been compared with those growing here. 



A special effort has been made to give as exactly as possible the regions 

 in which the species and varieties of plums grow. Such an effort is made 

 under the belief that this knowledge is of great value in the study of the 

 factors which govern the distribution of wild and domesticated plants. 

 If the boundaries of the regions in which a few scores of varieties of the 

 several fruits grow can be accurately established valuable generalizations 

 can be drawn regarding life zones and plant distribution. 



The reader should know what considerations have governed the 

 selection of varieties for color-plates and full descriptions. These are: 



(1) The known value of the variety for the commercial or amateur grower. 



(2) The probable value of new varieties. (3) To furnish data for the plum- 

 breeder; to show combinations of species or varieties, or new characters, 

 or the range in variation. (4) Some sorts have been described because 

 of historical value to better show what the trend of plum evolution 

 has been. (5) To indicate the relationships of species and varieties. The 

 varieties are divided into three groups according to their importance as 

 gauged from the standpoints given above. 



In botanical nomenclature the code adopted by the American botan- 

 ists in Philadelphia in 1904 and modified by the International Botanical 

 Congress at Vienna in 1905, has been used. For horticultural names, 

 lacking a better code, the revised rules of the American Pomological Society 

 have been followed, though in a few cases we have not seen fit to follow 

 the rules of this society, as the changes required by their strict observance 



