Pine trees are gymnosperms, nonflowering plants that produce exposed seeds not enclosed in an ovary. They are monoecious, bearing gametes of both sexes on the same tree. These gametes are housed in a structure called a strobilus, or cone. Female pinecones are generally found in the upper branches of the tree crown, above the male cone. This reduces the possibility of self-fertilization by the wind-borne male gametes. The female pinecone, or megastrobilus, produces the ovule, or unfertilized seed. Scales (modified leaves) on the female cone open to receive pollen, then close. Fertilization takes place late the following spring. Over a two to three year period after fertilization, the woody female pinecone develops. In some species, the cones open at maturity and the seeds are released. In others the cones remain closed for several years until opened by rotting, by food-seeking animals, or by fire. In some pines the scale bearing the nutlike seed may be expanded to form a wing for airborne dispersal.