An attempt is made in _The Plums of New York_ to review the plum flora of this continent, but the species considered fall far short of being all of the promising indigenous plums; not only are there more to be described, but it is probable that species here described will in some cases be sub-divided. The development of the pomological plum-wealth of North America is but begun. Not nearly as much has been done to develop the possibilities of the European plums in America as in the case of the other tree-fruits. Probably a greater percentage of the varieties of Old World plums commonly cultivated came from across the sea, than of the varieties of any other of the orchard-fruits which have been introduced. Much remains to be done in securing greater adaptability of foreign plums to American conditions. Native and foreign plums are also being hybridized with very great advantage to pomology. _The Plums of New York_ is written largely with the aim of furthering the development of plums in America, the possibilities of which are indicated in the preceding paragraph. With this end in view the first task is to name and discuss briefly the characters of plums whereby species and varieties are distinguished, with a statement, so far as present knowledge permits, of the variability of the different characters. It is absolutely essential that the plum-grower have knowledge, especially if he aspires to improve the fruit by breeding, of the characters of the plants with which he is to work. These are in the main as follows: All species and some horticultural varieties have more or less characteristic trees. Making due allowance for environment—food, moisture and light—many plum groups can be readily distinguished by the general aspect of the plant. Of the gross characters of trees, size is usually most characteristic. A species, for example, is either shrubby or tree-like. Yet under varying environment, size of plant and of the parts of the plant, are probably the first to change. Habit of growth is nearly as important as size and varies but little under changing conditions. A species or variety may be upright, spreading, drooping or round-topped in growth; head open or dense; the tree rapid or slow-growing. Hardiness is a very important diagnostic character, plums being either hardy, half-hardy or tender. Both species and varieties respond in high degree to the test of hardiness, the range for varieties, of course, falling within that of the species. Productiveness, regularity of bearing, susceptibility to diseases and insects, and longevity of tree are all characters having value for species and varieties and with the exception of the first named, are little subject to variation.