Gymnosperm

Gymnosperms are seed plants that form neither flowers nor fruits. Their ovules, and the seeds that grow from them, sit exposed on the surface of modified leaves, usually the scales of a cone. Living gymnosperms fall into four groups: conifers such as pines, firs, spruces, redwoods, cedars and yews; cycads, which look rather like palms; one surviving ginkgo species, Ginkgo biloba; and the gnetophytes, an odd trio of genera that includes Ephedra and Welwitschia. Together they number only about a thousand species.

In a pine, small male cones shed vast amounts of pollen, carried mainly by wind, and larger female cones hold two ovules on each scale. A pollen grain caught near an ovule grows a pollen tube slowly, and fertilization may follow more than a year after pollination. The seed then matures on the open scale. As in all seed plants, the plant itself is the sporophyte, and the gametophytes are tiny and develop within the cones. Most gymnosperms move water through tracheids, a simpler kind of conducting cell than the vessels of most flowering plants.

Gymnosperms dominated land vegetation for much of the age of dinosaurs and still dominate cold northern forests. They supply most softwood timber and paper pulp. About four in ten species are threatened with extinction, a far higher share than for plants overall.