Firs, cedars, spruce, and pines rank among the great vegetation formations of the world. All of these trees belong to one group of gymnosperms—the conifers, or cone-bearers. Gymnosperms derive their name (which means "naked-seeded") from the fact that their ovules and seeds are not protected by ovary or fruit tissue.
In conifers, the same diploid sporophyte plant has both pollen-producing strobili and egg-producing cones. Most conifer ovules—which, upon fertilization, develop into seeds—are borne exposed on the upper surfaces of the modified branches that form the scales of the cone. At maturity, the scales of the cones separate, and the seeds are released into the air to be carried sometimes considerable distances by the wind.
In conifers, the same diploid sporophyte plant has both male pollen-bearing cones (called microstrobili) and female seed-bearing cones called megastrobili. A female cone is a modified stem bearing a tight cluster of scales, which are reduced branches specialized for reproduction. A male cone is a cluster of scales that are modified leaves inserted on an axis.
Female gametophytes are produced in the megastrobili, whereas male gametophytes come from the microstrobili. Megastrobili are much larger than microstrobili.
The megasporangium, in which the female gametophyte will form, is enclosed in a protective layer of sporophytic tissue—the integument—that will eventually develop into the seed coat. The integument, the megasporangium inside it, and the tissue attaching it to the maternal sporophyte constitute the ovule.
The megasporocyte within the megasporangium undergoes meiosis to produce four haploid megaspores, only one of which is functional. The nonfunctional megaspores degenerate, while the functional megaspore divides mitotically, and the resulting cells divide again to produce a multicellular female gametophyte.
Male gametophytes develop in a similar manner. The scales of the microstrobilus contain microspore mother cells. Meiosis gives rise to haploid microspores which, in turn, mature into pollen grains—the male gametophytes.
The production of male gametophytes in the form of pollen grains frees the plant completely from its dependence on liquid water for fertilization. Instead of water, wind assists conifer pollen grains in their travel from the microstrobilus to the female gametophyte inside the megastrobilus.
The tiny pollen grain enters through a small opening in the integument at the tip of the ovule, the micropyle. The pollen grain germinates, forming a pollen tube. When the pollen tube reaches the female gametophyte, it releases two sperm, one of which degenerates after the other unites with an egg.
Following fertilization, the sporophyte embryo begins to develop within the ovule, while the integument matures into the seed coat that protects the embryo.
The conifer seed contains tissues from three generations: the diploid embryo, the haploid gametophytic tissue—which supplies nutrients for the developing embryo—and the seed coat, which develops from tissues of the sporophyte parent (the integument).
The seed eventually falls to the ground and, under suitable environmental conditions, germinates to form a new pine tree.
Conifers have cones, but no motile cells. Like most seed plants, the conifers do not rely on liquid water for sexual reproduction. Instead, wind assists conifer pollen grains in their first stage of travel from the strobilus to the female gametophyte. The haploid gametophytes develop partly or entirely while attached to and nutritionally dependent on the diploid sporophyte.
The seeds of gymnosperms are complex and contain tissues from three generations. The seed coat develops from tissues of the diploid sporophyte parent (the integument). Within the megasporangium is the haploid female gametophytic tissue from the next generation, which contains a supply of nutrients for the developing embryo. In the center of the seed is the third generation, the embryo of the new diploid sporophyte. The seeds of some species may remain viable (capable of growth and development) for many years, germinating when conditions are favorable for the growth of the sporophyte.