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GYMNOSPERMS

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Originally appearing in Volume V12, Page 754 of the 1911 Encyclopedia Britannica.
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GYMNOSPERMS , in See also:

Botany. The Gymnosperms, with the See also:Angiosperms, constitute the existing See also:groups of See also:seed-bearing See also:plants or Phanerogams: the importance of the seed as a distinguishing feature in the plant See also:kingdom may be emphasized by the use of the designation Spermophyta for these two groups, in contrast to the See also:Pteridophyta and See also:Bryophyta in which true seeds are unknown. See also:Recent discoveries have, however, established the fact that there existed in the Palaeozoic era See also:fern-like plants which produced true seeds of a highly specialized type; this See also:group, for which See also:Oliver and See also:Scott proposed the See also:term Pteridospermae in 1go4, must also be included in the Spermophyta. Another instance of the See also:production of seeds in an See also:extinct plant which further reduces the importance of this See also:character as a distinguishing feature is afforded by the Palaeozoic genus Lepidocarpon described by Scott in 19o1; this lycopodiaceous type possessed an integumented megaspore, to which the designation seed may be legitimately applied (see See also:PALAEOBOTANY: Palaeozoic). As the name Gymnosperm (Gr. yvµv6s, naked, Q7rEpµa, seed) implies, one characteristic of this group is the See also:absence of an ovary or closed chamber containing the ovules. It was the See also:English botanist See also:Robert See also:Brown who first recognized this important distinguishing feature in conifers and cycads in 1825; he established the gymnosperrny of these seed-bearing classes as distinct from the angiospermy of the monocotyledons and See also:dicotyledons. As See also:Sachs says in his See also:history of botany, " no more important See also:discovery was ever made in the domain of See also:comparative See also:morphology and systematic botany." As Coulter and See also:Chamberlain See also:express it, " the habitats of the Gymnosperms to-See also:day indicate that they either are not at See also:home in the more genial conditions affected by Angiosperms, or have not been able to maintain themselves in competition with this group of plants." These naked-seeded plants are of See also:special See also:interest on See also:account of their See also:great antiquity, which far exceeds that of the Angiosperms, and as comprising different types which carry us back to the Palaeozoic era and to the forests of the See also:coal See also:period. The best known and by far the largest See also:division of the Gymnosperms is that of the See also:cone-bearing trees (pines, firs, cedars, larches, &c.), which See also:play a prominent See also:part in the vegetation of the See also:present day, especially in the higher latitudes of the See also:northern hemisphere; certain members of this class are of considerable antiquity, but the conifers as a whole are still vigorous and show but little sign of decadence. The division known as the See also:Cycadophyta is represented by a few living genera of limited See also:geographical range and by a large number of extinct types which in the Mesozoic era (see PALAEOBOTANY: Mesozoic) played a conspicuous part in the vegetation of the See also:world. Among existing Cycadophyta we find surviving types which, in their present See also:isolation, their See also:close resemblance to fossil forms, and in certain morphological features, constitute links with the past that not only connect the present with former periods in the See also:earth's history, but serve as sign-posts pointing the way back along one of the many lines which See also:evolution has followed. It is needless to discuss at length the origin of the Gymnosperms. The two views which find most favour in regard to the Coniferales and Cycadophyta are: (I) that both have been derived from remote filicinean ancestors; (2) that the cycads are the descendants of a fern-like stock, while conifers have been. evolved from lycopodiaceous ancestors.

The See also:

line of descent of recent cycads is comparatively clear in so far as they have undoubted See also:affinity with Palaeozoic plants which combined cycadean and filicinean features; but See also:opinion is much more divided as to the nature of the phylum from which the conifers are derived. The Cordaitales (see PALAEOBOTANY: Palaeozoic) are represented by extinct forms only, which occupied a prominentposition in the Palaeozoic period; these plants exhibit certain features in See also:common with the living Araucarias, and others which invite a comparison with the See also:maidenhair See also:tree (Ginkgo biloba), the solitary survivor of another class of Gymnosperms, the See also:Ginkgoales (see PALAEOBOTANY: Mesozoic). The See also:Gnetales are a class apart, including three living genera, of which we know next to nothing as regards their past history or line of descent. Although there are several morphological features in the three genera of Gnetales which might seem to bring them into line with the Angiosperms, it is usual to regard these resemblances as parallel developments along distinct lines rather than to interpret them as See also:evidence of See also:direct relationship. Gymnospermae.—Trees or shrubs; leaves vary considerably in See also:size and See also:form. See also:Flowers unisexual, except in a few cases (Gnetales) without a perianth. Monoecious or dioecious. Ovules naked, rarely without carpellary leaves, usually See also:borne on carpophylls, which assume various forms. The single megaspore enclosed in the nucellus is filled with See also:tissue (prothallus) before fertilization, and contains two or more archegonia, consisting usually of a large See also:egg-See also:cell and a small See also:neck, rarely of an egg-cell only and no neck (Gnetum and Welwitschia). Microspore spherical or See also:oval, with or without a See also:bladder-like See also:extension of the exine, containing a prothallus of two or more cells, one of which produces two non-motile or motile male cells. Cotyledons two or several. Secondary xylem and phloem produced by a single cambium, or by successive cambial zones; no true vessels (except in the Gnetales) in the See also:wood, and no See also:companion-cells in the phloem.

I. Pteridospermae (see PALAEOBOTANY, PALAEOZOIC). II. Cycadophyta. A. Cycadales (recent and extinct). B. Bennettitales (see PALAEOBOTANY: Mesozoic). IV. Ginkgoales (recent and extinct). V. Coniferales.

A. Taxaceae. B. Pinaceae. There is no doubt that the result of recent See also:

research and of See also:work now in progress will be to modify considerably the grouping of the conifers. The See also:family Araucarieae, represented by See also:Araucaria and Agathis, should perhaps be separated as a special class and a re-arrangement of other genera more in See also:accord with a natural See also:system of See also:classification will soon be possible; but for the present its twofold subdivision may be retained. VI. Gnetales. A. Ephedroideae. B. Gnetoideae.

C. Welwitschioideae (Tumboideae).

End of Article: GYMNOSPERMS

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