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Vascular plants with naked seeds are placed in five or more phyla
(divisions): seed ferns (Pteridospermophyta), cycads (Cycadophyta),
ginkgos (Ginkgophyta), conifers (Coniferophyta), and Gnetophyta. Seeds are easily dispersed structures developed from a zygote and enclosing an embryonic sporophyte. Gymnosperms: Vascular plants with naked seeds, borne naked on the surface of reduced, scale-like leaves and not enclosed within ovules (as in angiosperms). All seed plants possess megaphylls, leaves with branched veins whose bases interrupt the vascular bundle to form a leaf gap. Seed ferns (Pteridospermophyta): Extinct plants with large, fern-like leaves, but reproducing by seeds; occasionally growing to the height of small trees. Devonian to Jurassic in age, dominant during the Carboniferous (when they formed great coal swamps). Believed close to the ancestry of other seed plants. Cycads (Cycadophyta): Short, thick-stemmed plants with a crown of large fern-like or palm-like leaves. Seeds borne together in a structure resembling a large pine cone. Flourished during the Mesozoic era; only a few tropical and subtropical genera persist today. Ginkgos (Ginkgophyta): A mostly Mesozoic group with one living species (Ginkgo biloba, an ornamental tree with fan-shaped leaves). Conifers (Coniferophyta): The most familiar and economically important gymnosperms, including pines, spruces, firs, etc. Leaves are typically scale-like or needle-like, with reduced surface area. Seeds are borne in cone-like aggregates. Gnetophyta: This group includes only three living genera (Gnetum, Ephedra, Welwitschia), which differ greatly. All share a partially enclosed type of seed that approaches the angiosperm condition but was probably derived independently. |
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