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 botanists 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 would have brought much confusion. Only those who have to work with a great number of varieties of fruit can know the chaotic conditions of our pomological nomenclature. One of the aims of the work in hand is to set straight in some degree the great confusion in plum names. All synonyms of varieties have been given so far as they could be determined but it did not seem worth while to give all of the references to be found even in standard plum literature. Fewer of these are listed for the leading varieties than in the books on apples or grapes which have preceded, only such being given as have been found of use by the writers or thought of possible use to future plum students. On the other hand some references have been given for all varieties, a task not attempted in _The Grapes of New York_.