Among species with early bark-formation are two forms of bark: 1, cumulative, sufficiently persistent to acquire thickness and the familiar dark gray and fuscous-brown shades of bark long exposed to the weather; 2, deciduous, constantly falling away in thin scales and exposing fresh red inner surfaces. The latter are commonly known as Red Pines, as distinguished from Black Pines with dark cumulative bark. Deciduous bark changes after some years to cumulative bark, and the upper trunk only of mature trees is red. Red Pines, although usually recognizable by their bark, are by no means constant in this character. Oecological or pathological influences may check the fall of the bark-scales, and then the distinction between the upper and lower parts of the trunk becomes lost. [Illustration: PLATE VII. THE WOOD] SUMMARY The various characters that have been considered in the previous pages may be classified under different heads, some of them applicable to the whole genus, others to larger or smaller groups of species. GENERIC CHARACTERS Several characters, quite distinct from those of other genera, are common to all the species. 1. The primary leaf--appearing as a scale or bract throughout the life of the tree. 2. The bud--its constant position at the nodes. 3. The internode--its three distinct divisions. 4. The secondary leaves--in cylindrical fascicles with a basal sheath. 5. The pistillate flower--its constant nodal position and its verticillate clusters. 6. The staminate flower--its constant basal position on the internode and its compact clusters. 7. The cone--its clearly defined annual growths. Pinus is also peculiar in the dimorphism of shoots and leaves and in their constant interrelations with the diclinous flowers. Evolutionary processes develop features peculiar to Pinus alone (the oblique cone, etc.), but confined to a limited number of species. SECTIONAL CHARACTERS There are several characters that actually or potentially divide the genus into two distinct sections, popularly known as Soft and Hard Pines. 1. The fibro-vascular bundle of the leaf, single or double. 2. The base of the bract subtending the leaf-fascicle, non-decurrent or decurrent 3. The phyllotaxis of the cone, simple or complex. 4. The flower-bud, its less or greater development. Some characters indicate the same distinction but are subject each to a few exceptions. 5. The fascicle-sheath, deciduous or persistent. 6. The walls of the ray-tracheids, smooth or dentate. 7. The connective of the pollen-sacs, large or small. 8. The formation of bark, late or early. SUBSECTIONAL CHARACTERS An exact subdivision of the Soft Pines is possible on the following characters. 1. The umbo of the cone-scales, terminal or dorsal. 2. The scales of the conelet, mutic or armed. 3. The pits of the ray-cells, large or small. EVOLUTIONAL CHARACTERS