Gymnosperms

Gymnosperms, the lower of the two divisions of flowering plants or spermophytes (q.v.), comprising a single class, the Gymnospermia, the living representatives of which, some 400 species referred to about 40 genera, are grouped in three natural orders, the Cycadeae, Coniferre (including Taxinecv) and Gnetaceae. Though including plants differing widely in mode of growth, these three orders have many characters in common, and exhibit intermediate conditions between the Pteridophytes and Angiosperms. On the one hand, they approach the Lycopodinee, especially Isoetes and Selaginella, in some respects, and the Equisetinee in others, so that we can only derive them from some common ancestor of the pteridophytes, whilst on the other hand they approach dicotyledons rather that monocotyledons. They are all woody plants with exogenous stems, which seldom branch in the cycads, but do so freely in conifers. They increase in diameter by a ring of cambium, but the medullary rays are invisible, being often only one cell broad, and tracheids, or elongated cells with bordered pits, take the place of vessels in the secondary wood, i.e. in the rings of wood formed by the cambium. An apical meristem of numerous small cells replaces the large apical cell of most of the higher cryptogamia. The leaves are generally simple with one vein, and do not all produce axillary buds. The flowers are always unisexual, seldom have even the rudiment of a perianth, and have generally an elongated axis, which with its sporophylls is known as a cone. The pollen-sacs or microsporangia are produced separately on the under surface of the staminal leaves o; male sporophylls, and the pollen grains show their affinity to the microspores of lower groups by undergoing division into two or more cells, one or more of which (the included cells) form a rudimentary male prothallium, whilst the pollen-tube is given off by another. Carpels are sometimes absent, as in the yew, and when present do not form an ovary, style, or stigma, whence the ovules are naked, from which character the group derives its name. The ovules may be terminal or (in Gingko) lateral axial structures, or may be marginal lobes of an open carpellary leaf, as in Cgcas, or may be in the axils of the carpels, as in cypress and juniper, or on peculiar placental structures, as in the Abietineae. The embryo sac or microspore is at some distance below the apex of the tercine, has a thickened wall or exospore, and is filled before fertilisation with an archisperm. on the upper surface of which several archegonia, formerly known as "corpuscula," are formed. There'being no stigma or style, the pollen-grain, earned by wind, falls directly into the micropyle; but its i pollen-tube, after penetrating a little way into the tercine, commonly pauses for a considerable time, often for months, before growing on to the top of the embryo sac. Several embryos are frequently developed from one seed, either from the fertilisation of the oosphere or central cell of more than one arehegonium, or from a division of the suspensor (q.v.) which results from the fertilisation of one, each division bearing an embryo (polyembryony). Coniferous wood occurs fossil in Carboniferous rocks, and to the same period belong the Cordaiteae, a group possibly intermediate between cycads and conifers. But we may expect further light on the affinities of the now isolated groups of the Gymnospermia from future palteontological discoveries. [Cycads, Conifers, Welwitschia.]