The thickness, smoothness, color and manner of exfoliation of the outer bark and the color of the inner bark have considerable value in determining species but are little used in determining horticultural groups. It is well recognized that all plums have lighter colored bark in the South than in the North. The branches are very characteristic in several species. The length, thickness and rigidity of the branch and the length of its internodes should be considered, while the direction of the branch, whether straight or zigzag, are very valuable determining characters and relatively stable ones, seeming to change for most part only through long ranges of climatic conditions. So, too, the arming of a branch with spines or spurs and the structure of such organs are important. The color, smoothness, amount of pubescence, direction, length, thickness and the appearance of the lenticels, the presence of excrescences on the branchlets of the first and second year’s growth and the branching angle, are all worthy of consideration though quite too much has been made of these characters, especially of pubescence, in determining species, for they are all extremely variable. [Illustration: 1. _P. HORTULANA MINERI_ 2. _P. AMERICANA_ 3. _P. CERASIFERA_ 4. _P. DOMESTICA_ 5. _P. INSITITIA_ 6. _P. HORTULANA_ 7. _P. MUNSONIANA_ 8. _P. NIGRA_ 9. _P. TRIFLORA_] The size, shape and color of leaf-buds and of their outer and inner scales and the margins of the scales differ in different species. Possibly the most evident, and therefore readiest means of identifying species, at least, is by the leaves. It is true that leaves are very variable but always within limits, and either individually or collectively in giving the general aspect to a tree they are characteristic. Modifications of leaves most often occur in very young plants, those growing in bright sunshine or deep shade and on sprouts or suckers, but none of these are usually sufficient to mislead as to species. Leaf-size and leaf-form are the first characters to be noted in determining a plum but these are closely followed in value by leaf-color, leaf-surface, leaf-thickness and leaf-margin. Leaf-size is variable, depending much upon the conditions noted above but leaf-form varies but little in the several species. So, too, the color of leaves is very constant throughout a species, for both surfaces, though impossible to describe accurately in words and very difficult to reproduce in color-printing. There is a marked difference in autumnal tints not only of species but of varieties but these are not very constant in any one location and must vary greatly under different environments. The thickness of the leaves of the several species is a distinctive character. Species of plums have very different leaf-surfaces as regards reticulation, rugoseness, pubescence and coriaceousness, all of these characters being quite constant, though it is to be noted that roughness