This bast tissue is hygroscopic and, with its greater thickness on the dorsal surface, there is a much greater strain on that side of the scale, tending to force the scales apart when they are ripe and dry, and subsequently closing and opening the cone on rainy and sunny days. The cone, during the second season's growth, is completely closed, its scales adhering together with more or less tenacity. In most species the hygroscopic energy of the scales is sufficient to open the cone under the dry condition of its maturity, but with several species the adhesion is so persistent that some of the cones remain closed for many years. These are the peculiar serotinous cones of the genus. THE SEROTINOUS CONE. As an illustration of the area to which the adhesion is confined, a section may be sawed from a cone of P. attenuata (fig. 71). The axis and the scales that have been severed from their apophyses (b) can be easily pushed out of the annulus (a), which is composed wholly of apophyses so firmly adherent that they will successfully resist a strong effort to break them apart. When immersed in boiling water, however, the ring falls to pieces. An examination of these pieces discovers adhesion only on a narrow ventral border under the apophysis and on a corresponding dorsal border back of the apophysis. The rest of the scale is not adherent, so that the seed is free to fall at the opening of the cone. The serotinous cone is a gradual development, wanting in most species, rare in a few, less or more frequent in others. A similar evolution of the persistent cone, of the oblique cone and of the cone-tissues has been already discussed. All these progressive characters culminate in mutual association in P. radiata and its allies. The result is a highly specialized fruit that should convey taxonomic significance of some kind. With all serotinous species that I have seen, some of the trees open their cones at maturity, others at indefinite intervals. That is to say, the seed of a prolific year is not at the mercy of a single, perhaps unfavorable season. The chances of successful germination are much increased by the intermittent seed-release peculiar to these Pines. Such a method of dissemination must accrue to the advantage of a species. In other words, this intermittent dissemination and the oblique form of cone with its perfected tissues all mark the highest development of the genus. THE SEED. Plate VI. Figs. 72-79.