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(Pinophyta), the most ancient class or group of seed plants, which occupy a position between the Pteridophyta and the Angiospermae (flowering plants). Gymnospermae are distinguished from Pteridophyta by the presence of ovules and from Angiospermae by the lack of carpels. (In Gymnospermae the pollen falls directly on the micropyle of the ovule.)
The Gymnospermae, including all contemporary species, have strobiles. Shortened and restricted in length, these reproductive shoots bear sporophylls. Only the most ancient classes of Lyginopterides, or “seed ferns,” lack strobiles. All Gymnospermae are trees or shrubs that often reach gigantic size, as in some conifers. Some Gymnospermae are heavily branched and bear many small or tiny, often scaled, leaves (Cordaites, ginkgoes, and conifers), while others have few or no branches but relatively large, usually pinnate leaves (Lyginopterides, Bennettites, Cycadales, and Wel-witschia). The leaves differ greatly in number, size, and anatomical structure. Most Gymnospermae have only tracheidal water-conducting elements of the xylem. (They are ladder-like or, more often, linear.) Only the Welwitschia, Gnetum, and Ephedra have vessels. The stoma also vary, a fact that often makes it possible to determine the genera from nothing more than a leaf fragment.
Like other seed plants, all Gymnospermae are heterospor-ous. Their microsporophylls and megasporophylls differ greatly in form, size, and structure. Some are large and pinnate, while others are integral and small. The conifers have megasporophylls, which, as a result of reduction and coalescence, are practically indistinguishable in the fused structure (that is, seed scales). In most of the oldest and most primitive seed ferns, strobiles did not develop, but both microsporophylls and megasporophylls were situated freely on ordinary, long shoots. In the remaining Gymnospermae the sporophylls are gathered into more or less compact strobiles. In some of the Bennettites, the strobiles are bisexual. However, in most of the species the strobiles are unisexual (dioecious)—that is, the strobiles consist either of microsporophylls (male strobiles) or megasporophylls (female strobiles) of varying structure.
Gymnospermae existed during the Upper Devonian period. Representatives of most orders of Gymnospermae have been traced to the Carboniferous and Permian periods, and Gymnospermae flourished in the Mezozoic era. Today there are only about 600 species, most of which are conifers. There are about 100 species of Cycadales, about 45 species of Ephedra, and about 40 species of Gnetum.
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Takhtadzhian, A. L. Vysshie rasteniia, vol. 1. Moscow-Leningrad, 1956.
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Chamberlain, C. J. Gymnospermus, Structure, and Evolution. Chicago, 1935.
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A. L. TAKHTADZHIAN