Of all structures of the plum the fruit is most variable, yet fruits are sufficiently distinct and constant, especially within species, to make their characters very valuable in classification. Species, whether wild or cultivated, may be distinguished in greater or less degree by the period of ripening of the fruits, though in this regard the cultivated varieties of the several species vary greatly and in the wild state trees of native plums in the same locality, even in the same clump, may vary in ripening as much as from two to four weeks. Species are distinguished by size, shape, color, flesh, flavor and pit among the grosser characters of the structure and by amount of bloom, stem, cavity, apex, suture and skin among the minor characters. The fruit is usually the first part of the plant to respond to changed conditions. Characters derived from seed structures are generally accounted of much value by botanists in determining species. Such is the case with plums. This Station has a collection of stones of over three hundred cultivated varieties of plums and some specimens of nearly all the different species. The stones illustrated in the color-plates in this book show that this structure is quite variable in size, shape, in the ends, surfaces, grooves and ridges, even within a species; nevertheless in describing the several hundred forms of plums for _The Plums of New York_ the stone has been quite as satisfactory, if not the most satisfactory, of any of the organs of this plant for distinguishing the various species and varieties. The reproductive organs of plums afford several characters and would seem to offer means of distinguishing botanical and horticultural groups, but they are so variable in both cultivated and wild plants as to be very misleading. Not only do these organs differ very often in structure but also in ability to perform their functions. Bailey[1] has called attention to the remarkable self-sterility of some varieties of the native species of plums, due to the impotency of the pollen upon flowers of the same variety. C. W. H. Heideman[2] made some very interesting observations on what he considers distinct forms of the flowers of the Americana plums, describing for this species all of the six possible variations of flowers enumerated by Darwin in his _Different Forms of Flowers in Plants of the Same Species_. Heideman thinks that other species of Prunus exhibit similar variations. Waugh[3] made the pollination of plums a subject of careful and extended study and found much variation in the pistils of plants of the same species, insufficient pollen in some plants, pollen impotent on the stigma of the same flower, and considerable difference in the time of maturity of pollen and stigma in some plums, especially the Americana plums. These variations, most important to the plum-grower, are of more or less use in identifying plums.