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13. Simple life
| 13.31 |
Level IV life: Seed bearing trees (gymnosperms)
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| Gymnosperm (Greek gumnos,"naked"; sperma,"seed"), common name for any seed-bearing vascular plant without flowers. There are several types: the cycad, ginkgo, conifer, yew, and gnetophyte. Gymnosperms are woody plants, either shrubs, trees, or, rarely, vines (some gnetophytes). They differ from the other great division of seed plants, the flowering plants (seeAngiosperm), in that the seeds are not enclosed in carpels but rather are borne upon seed scales arranged in cones. The gymnosperms include the most ancient of the living seed plants; they appear to have arisen from fern-like ancestors in the Devonian period (about 408.5 million to 362.5 million years ago). Cycads retain the most primitive characteristics of the extant seed plants. Gnetophytes are considered from morphological and molecular evidence to share a common ancestry with the flowering plants. Living gymnosperms are distributed throughout the world, with a majority, particularly the conifers, in temperate and sub arctic regions. Cycads and gnetophytes are mainly tropical to subtropical. There are about 70 genera with 750 species of living gymnosperms, far fewer than many families of flowering plants. | ||||||||||||
| 13.31.1 | Structure | |||||||||||
| Gymnosperms are divided into four categories. There are estimated to be fewer than 1,000 species that relate to these categories: | ||||||||||||
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| A tree differs from a shrub in that it usually produces a single, well-defined main stem, or trunk, and from a herbaceous plant in that the stem is composed almost entirely of woody tissue. Trees of some smaller species sometimes develop with more than one stem, like a shrub, but most larger species of tree never produce shrubby forms. Some species, when they reach maturity, are only a few metres tall, with trunks as slender as 15 cm (6 in) in circumference; the largest species may reach heights of more than 112 m (367 ft), with trunks that have a diameter of more than 6 m (20 ft). | ||||||||||||
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| 13.31.2 | Evolution | |||||||||||
| Trees have existed since the Devonian period (408.5 million to 362.5 million years ago) of the Paleozoic era. The oldest trees known to palaeontologists are those of the genus Cordaites, which originated in the early Devonian period and became extinct by the end of the Paleozoic era (245 million years ago). The oldest known surviving order of trees, the broad leaved, gymnospermous Ginkgoales, is now represented by a single species, the maidenhair tree, Ginkgo biloba (see Ginkgo). Coniferous trees have existed since the middle of the Carboniferous period (about 285 million years ago). Angiospermous trees first appeared in the early Cretaceous period (145.6 to 97 million years ago) of the Mesozoic era, and by the beginning of the Pliocene epoch (5.2 million years ago) of the Cenozoic era virtually all tree genera now in existence were growing. The majority of fossil tree leaves found in Pliocene rocks are indistinguishable from leaves of present-day trees. See also Paleontology. | ||||||||||||
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