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.
The seed of Pinus contains an embryo, with the cotyledons clearly defined, embedded in albumen, which is protected by a bony testa with an external membranous spermoderm, produced, in most species, into an effective wing. While the seed of other genera of the Abietineae shows no striking difference among the species, that of Pinus is remarkably variable, presenting alike the most primitive and the most elaborate forms among the Conifers. These differences are valuable for the segregation of kindred species and for some specific distinctions.
WINGLESS SEEDS.
With wingless seeds the main distinction is found in the spermoderm, which is entire in one species only, P. koraiensis. In P. cembra it is wanting on the ventral surface of the nut, but on the dorsal surface, it is adnate partly to the nut, partly to the cone-scale. The nut of P. albicaulis and that of P. cembroides are quite bare of membranous cover. The spermoderm of P. flexilis is reduced to a marginal border, slightly produced into a rudimentary wing adnate to the nut.
THE ADNATE WING.
In P. strobus, longifolia and their allies and in P. Balfouriana the spermoderm is prolonged into an effective wing-blade from a marginal adnate base like that of P. flexilis. This adnate wing cannot be detached without injury.