within one or more outer layers of diploid tissue. These outer layers of tissue constitute an integument that becomes a seed coat after the fertilization and development of an embryo takes place (see Fig. 22.8).
Introduction to See d Plants: Gymnosperms 423
are in ferns and their relatives. Unlike the gametophytes discussed so far, they don't grow independently but develop within sporophyte structures.
Four phyla of living gymnosperms are recognized. Phylum Pinophyta includes about 575 species of pines, firs, spruces, hemlocks, cedars, redwoods, and other coniferous woody plants. Fossils of some conifers extend back 290 million years to the late Carboniferous period. Phylum Ginkgophyta has a single living representative, Ginkgo, which has fan-shaped leaves and seeds enclosed in a fleshy covering. The superficially palmlike cycads are assigned to Phylum Cycadophyta. Phylum Gnetophyta includes three genera of gnetophytes that have wood with vessels—a structural element unknown in other gymnosperms.
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