Discovery Science: Earth Biology – Plants and fungi – Gymnosperms

Discovery Science: Earth Biology – Plants and fungi – Seed Plants

Seed-bearing plants (spermatophytes) include all plants whose structures can be divided into roots, stems, and leaves.

Gymnosperms, in which the ovules are not enclosed within an ovary, differ from angiosperms (Magnoliaphyta; formerly AngiospermaeJ, or flowering plants, in which the ovules lie within a ripened ovary, or fruit.

Earth Science: Biology – Plants and fungi – Gymnosperms

Gymnosperms are a group of spermatophyte seed-bearing plants. This group includes only about 650 recent species, most of which belong to the class of conifers (Pinopsida).

Conifers are mainly evergreen trees (along with some shrubs), almost all with needle- or scale-shaped leaves (for protection against drying out in winter, among other functions). The male microsporangia and female gametophytes typically develop in separate cone-shaped structures.

Conifers are nearly always pollinated by the wind; since no ovary is formed, the pollen grains fall directly onto the micropyle, the small channel that allows the pollen tube to enter the ovule for fertilization. Conifers are widespread in temperate and subarctic regions, where they are sometimes the dominant vegetation.

An example is the taiga or boreal forest, which covers an area of some 5.4 million square miles (14 million km 2) in the Northern Hemisphere. Some species from this group can become extremely large and ancient, notably the giant sequoias (Sequoiadendrum giganteum), with extremely impressive specimens in the northwestern United States.

Among the largest is “General Sher- man”; estimated to be some 2,500 years old, it has reached a height of more than 272 feet (83 m), a diameter of about 36 feet (1 1 m) at the base, and a total mass of 1,500 tons (1,361 mT). The true Methuselah of trees, however, is the bristlecone pine (Pinus longaeva), found in the mountains of eastern California.

Only about 33 feet (10 m) tall, these trees have highly gnarled, bare-looking trunks. Only a few branches with green needles show that they are still alive. Experts estimate that some of these pines sprouted over 4,600 years ago, a time when humans just began a settled agricultural lifestyle.

Other divisions among the gymno-sperms-which cannot be considered a group with a unified heritage—are the Ginkgophyta, with only one living species, quite recent in its development (Ginkgo biloba); the Cycadophyta, similar in appearance to palms; and the Gnetophyta, which include the unusual Welwitschia (Welwitschia mirabilis).