Such wide variation in so important an organ suggests generic difference. But here we are met by the association of the different forms in species evidently closely allied. The two Foxtail Pines are so similar in most characters that they have been considered, with good reason, to be specifically identical; yet the seed-wing of P. Balfouriana is adnate, that of P. aristata articulate. P. Ayacahuite produces not only the characteristic wing of the Strobi, adnate, long and effective, but also, in the northern variety, a seed with a rudimentary wing, the exact counterpart of the seed of P. flexilis. In both sections of the genus are found the effective adnate wing (Strobi and Longifoliae) and the inefficient articulate wing (Gerardianae and Pineae). A little examination of all forms of the seed will show that they blend gradually one into another. The color of the wing is occasionally peculiar, as in the group Longifoliae. There is usually no constancy in this character, for the wing may be uniform in color or variously striated in seeds of the same species. The length and breadth of the seed-wing, being dependent on the varying sizes of the cone-scale, differ in the same cone. They are also inconstant in different cones of the same species, and of this inconstancy the seed of P. ayacahuite furnishes the most notable example. [Illustration: PLATE VI. CONE-TISSUES AND SEEDS] THE WOOD. Plate VII. With the exception of the medullary rays, a very small proportion of the whole, the wood of Pinus, as seen in cross-section (fig. 82), is a homogeneous tissue of wood-tracheids with interspersed resin-ducts. In tangential section the medullary rays appear in two forms, linear, without a resin-duct, and fusiform, with a central resin-duct. In radial section the cells of the linear rays are of two kinds, ray-tracheids, forming the upper and lower limits of the ray, characterized by small bordered pits, and ray-cells, between the tracheids, characterized by simple pits. The walls of the ray-tracheids may be smooth or dentate; the pits of the ray-cells may be large or small. These conditions admit of four combinations, all of which appear in the medullary rays of Pinus, and of which a schematic representation is given in Plate VII. These combinations are Ray-tracheids with smooth walls. Soft Pines. Ray-cells with large pits Subsection Cembra fig. 80. Ray-cells with small pits Subsection Paracembra fig. 81. Ray-tracheids with dentate walls. Hard Pines. Ray-cells with large pits Group Lariciones fig. 83. Ray-cells with small pits Other Hard Pines fig. 84.