GYMNOSPERMS. All living seed-bearing plants are divided into two groups, the Gymnosperms and the Angiosperms (q.v.), and it is certain that these two groups are only distantly related, if indeed they are directly related at all. The primary distinction between the two lies in the fact that, at the time of pollination, the ovule (or rudimentary seed) is freely exposed, and the pollen deposited on it (or very rarely near it) in Gymnosperms (indica ting plants with naked seeds), but in Angiosperms is wholly en closed in an ovary, on a specialized part of which (the stigma) the pollen is deposited. There are other important differences, both in anatomy and in reproductive structures. In particular the pro thallus (or embryo-sac) is a much larger and more massive struc ture in Gymnosperms than in Angiosperms, and (with the excep tion of two tropical genera) the female organ characteristic of the group is a large archegonium similar in structure and development to the much smaller archegonium found in all mosses and ferns and their allies, but of which no trace is found in Angiosperms. As regards anatomical features no companion cells are found in the phloem of Gymnosperms, nor, except in one division, are true vessels met with in the wood.
Although no other living plants besides Gymnosperms and true flowering plants (Angiosperms) have seeds, yet fossil representa tives of the club mosses are known which possess such structures. These plants are, however, so clearly and definitely related to the club mosses (Lycopodineac) on other grounds, and not to the Gymnosperms, that it is not necessary to discuss them further. (See PTERIDOPHPTA and PALAEOBOTANY : Palaeozoic.) The Gymnosperms are of special interest on account of their great antiquity, which far exceeds that of the true flowering plants, and as comprising different types which carry us back to the Palaeozoic era and to the forests of the coal period. It is not surprising in a group of such antiquity to find that some divisions are wholly extinct. There are, in all, seven of these divisions of which three are extinct, one is represented by a single living spe cies, the maidenhair tree, common enough in cultivation but almost extinct in the wild state, and three are flourishing living divisions, though only one of these, the Coniferales (pines, cedars, larches, firs, yews, etc.) is a really large and important division of existing plants.
The seven divisions of the Gymnosperms show many indications of fairly close relationship and this appears to indicate that the whole group had a common origin, though this view is not by any means certain and is not universally accepted. If we accept the view that all the Gymnosperms had a common origin then there can scarcely be any doubt that the group from which they were evolved was the Filicales or true ferns, the resemblance between the latter and the most primitive division of Gymnosperms, the Pteridospermae or Cycadofilicales, being very striking. (See PALAEOBOTANY: Palaeozoic.) Although the reproductive structures of the Gymnosperms are often described as "flowers," it seems better to avoid this term, as implying a resemblance, which scarcely exists, to the "flowers" of Angiosperms. The term "cone" will, therefore, be used through out this article in preference to "flower," except in the case of the highest division, the Gnetales, where the resemblance to Angiosperms is more obvious.
The seven divisions of Gymnosperms are as follows:— I. PTERIDOSPERMAE or CYCADOFILICALES. Wholly extinct. (See PALAEOBOTANY : Palaeozoic.) II. CYCADALES. A living division, also represented in the Meso zoic.
V. CORDAITALES. Wholly extinct. (See PALAEOBOTANY : Palaeo zoic.) VI. CONIFERALES. By far the largest and most important divi sion of living Gymnosperms.
VII. GNETALES. Includes only three genera, with no known Vii. GNETALES. Includes only three genera, with no known fossil representatives, but with some characters very suggestive of Angiosperms.
No further reference will be made here to the extinct divisions of Gymnosperms.
The living Gymnosperms agree in the following characters: Woody plants. Cones unisexual, monoecious or dioecious. Perianth not present except in Gnetales. Ovules naked, usually borne on leaf-like structures. The single megaspore enclosed in the lus becomes filled with tissue (prothallus) before fertilization (partially only or not at all in Gnetum) ; the microspore ops at least four nuclei (usually four to six) of which two are the male cells, very large and actively motile in Cycads and Ginkgo, much smaller and only motile in so far as they are passively ried by the pollen tube, in "iferales and Gnetales.
Some account will now be given in turn of each of the four living divisions.