Gnetales
GNETALES. These are trees or shrubs with simple leaves. The flowers are dioecious, rarely monoecious, provided with one or two perianths. The wood is characterized by the presence of vessels in addition to tracheids. There are no resin-canals. The three existing genera, usually spoken of as members of the Gnetales, differ from one another more than is consistent with their inclusion in a single family ; we may therefore better express their diverse characters by regarding them as types of three separate families (i ) Ephedroideae, Smus Ephedra; (2) Welwitschioideae, genus Welw ttschia ; (3) netoideae, genus Gnetum. Our knowledge of the Gnetales leaves much to be desired, but such facts as we possess would seem to indicate that this group is of special importance as foreshadowing, more than any other Gymnosperms, the Angiospermous type. In the more heterogeneous structure of the wood and in the possession of true vessels the Gnetales agree closely with the higher flowering plants. It is of interest to note that the leaves of Gnetum, while typically Dicotyledonous in appearance, possess a Gymnospermous character in the continuous and plate-like medullary rays of their vascular bundles. The presence of a perianth is a feature suggestive of an approach to the floral structure of Angiosperms ; the prolongation of the integument furnishes the flowers with a substitute for a stigma and style. The genus Ephedra, with its prothallus and archegonia, which are similar to those of other Gymnosperms, may be safely regarded as the most primitive of the Gnetales. I n Welwitschia also the megaspore is filled with prothallus-tissue, but single egg-cells take the place of archegonia. In certain species of Gnetum described by Karsten the megaspore contains a peripheral layer of protoplasm, in which scattered nuclei represent the female reproductive cells; in Gnetum Gnemon a similar state of things exists in the upper half of the megaspore, while the lower half agrees with the megaspore of Welwitschia in being full of prothallus-tissue, which serves merely as a reservoir of food. Lotsy has described the occurrence of special cells at the apex of the prothallus of Gnetum Gnemon, which he regards as imperfect archegonia (fig. 17, C, a) ; he suggests they may represent vestigial structures pointing back to some ancestral form beyond the limits of the present group. The Gnetales probably had a separate origin from the other Gymnosperms; they carry us nearer to the Angiosperms, but we have as yet no satisfactory evidence that they represent a stage in the direct line of Angipspermic evolution. It is not improbable that the three genera of this ancient phylum survive as types of a blindly-ending branch of the Gymnosperms; but be that as it may, it is in the Gnetales more than in any other Gymnosperms that we find features which help us to obtain a dim prospect of the lines along which the Angiosperms may have been evolved.
Ephedra. This genus is the only member of the Gnetales represented in Europe. Its species, which are characteristic of warm temperate latitudes, are usually much-branched shrubs. The finer branches are green, and bear a close resemblance to the stems of Equisetum and to the slender twigs of Casuarina; the surface of the long internodes is marked by fine longitudinal ribs, and at the nodes are borne pairs of inconspicuous scale-leaves. The flowers are small, and borne on axillary shoots. A single male flower consists of an axis enclosed at the base by an inconspicuous perianth formed of two concrescent leaves and terminating in two, or as many as eight, shortly stalked or sessile anthers. The female flower is enveloped in a closely fitting sac-like investment, which must be regarded as a perianth ; within this is an orthotropous ovule surrounded by a single integument prolonged upwards as a beak-like micropyle. The flower may be described as a bud bearing a pair of leaves which become fused and constitute a perianth, the apex of the shoot forming an ovule. In function the perianth may be compared with a unilocular ovary containing a single ovule; the projecting integument, wh'ich at the time of pollination secretes a drop of liquid, serves the same purpose as the style and stigma of an angiosperm. The megaspore is filled with tissue as in typical Gymnosperms, and from some of the superficial cells 3 to 5 archegonia are developed, characterized by long multicellular necks. The archegonia are separated from one another, as in Pinus, by some of the prothallus-tissue, and the cells next the egg-cells (tapetal layer) contribute food-material to their development. After fertilization, some of the uppermost bracts below each flower become red and fleshy ; the perianth develops into a woody shell, while the integument remains membranous. In some species of Ephedra, e.g. E. altissima, the fertilized eggs grow into tubular proembryos, from the tip of each of which embryos begin to be developed, but one only comes to maturity. In Ephedra helvetica, as described by Jaccard, no proembryo or suspensor is formed; but the most vigorous fertilized egg, after undergoing several divisions, becomes attached to a tissue, termed the columella, which serves the purpose of a primary suspensor ; the columella appears to be formed by the lignification of certain cells in the central region of the embryo-sac. At a later stage some of the cells in the upper (micropylar) end of the embryo divide and undergo considerable elongation, serving the purpose of a secondary suspensor. The secondary wood of Ephedra consists of tracheids, vessels and parenchyma; the vessels are characterized by their wide lumen and by the large simple or slightly-bordered pits on their oblique end-walls.
Gnetum. This genus is represented by several species, most of which are climbing plants, both in tropical America and in warm regions of the Old World. The leaves, which are borne in pairs at the tumid nodes, are oval in form and have a Dicotyledonous type of venation. The male and female inflorescences have the form of simple or paniculate spikes. The spike of an inflorescence bears whorls of flowers at each node in the axils of concrescent bracts accompanied by numerous sterile hairs (paraphyses) ; in a male inflorescence numerous flowers occur at each node, while in a female inflorescence the number of flowers at each node is much smaller. A male flower consists of a single angular perianth, through the open apex of which the flower-axis projects as a slender column terminating in two anthers. The female flowers, which are more complex in FIG. 17. Gnetum Gnemon. (After Lotsy.)
A, Female Flower. o, Imperfect Archegonia.
n, Nucellus. e. Partially developed Megaspore.
$c, Pollen-chamber. F, Fertile half.
i, Integument. 5, Sterile half.
p', Inner Perianth. pt, Pollen-tube.
p". Outer Perianth. z, Zygote.
B, C, Megaspore. z', Prothallus.
-structure, are of two types, complete and incomplete; the latter occur in association with male flowers in a male inflorescence. A complete female flower consists of a nucellus (fig. 17, A, n) , surrounded by a single integument (fig. 17, A, *), prolonged upwards as a narrow tube and succeeded by an inner and an outer perianth (fig. 17, A, p' and p"). The whole flower may be looked upon as an adventitious bud bearing two pairs of leaves; each pair becomes concrescent and forms a perianth, the apex of the shoot being converted into an prthotropous ovule. The incomplete female flowers are characterized by the almost complete suppression of the inner perianth. Several embryo-sacs (megaspores) are present in the nucellus of a young ovule, but one only attains full size, the smaller and partially developed megaspores (fig. 17,8 and C, e) being usually found in close association with the surviving and fully-grown megaspore. In Gnetum Gnemon, as described by Lotsy, a mature embryo-sac contains in the upper part a large central vacuole and a peripheral layer of protoplasm, including several nuclei, which take the place of the archegonia of Ephedra; the lower part of the embryo-sac, separated from the upper by a constriction, is full of parenchyma. The upper part of the megaspore may be spoken of as the fertile half (fig. 17, B and C, F), and the lower part, which serves only as food-reservoir for the growing embryo, may be termed the sterile half (fig. 17,6 and C, S). (Coulter, Bot. Gazette, xlvi., 1908, regards this tissue as belonging to the nucellus.) At the time of pollination the long tubular integument secretes a drop of fluid at its apex, which holds the pollen-grains, brought by the wind, or possibly to some extent by insect agency, and by evaporation these are drawn on to the top of the nucellus, where partial disorganization of the cells has given rise to an irregular pollen-chamber (fig. 17, A, pc). The pollen-tube, containing two generative and one vegetative nucleus, pierces the wall of the megaspore and then becomes swollen (fig. 17, B and C, pt) ; finally the two generative nuclei pass out of the tube and fuse with two of the nuclei in the fertile half of the megaspore. As the result of fertilization, the fertilized nuclei of the megaspore become surrounded by a cell-wall, and constitute zygotes, which may attach themselves either to the wall of the megaspore or to the end of a pollen-tube (fig. 17, C,z and z') ; they then grow into long tubes or proembryos, which make their way towards the prothallus (C, z'), and eventually embryos are formed from the ends of the proembryo tubes. One embryo only comes to maturity. The embryo of Gnetum forms an out-growth from the hypocotyl, which serves as a feeder and draws nourishment from the prothallus. The fleshy outer portion of the seed is formed from the outer perianth, the woody shell being derived from the inner perianth. The climbing species of Gnetum are characterized by the production of several concentric cylinders of secondary wood and bast, the additional cambium-rings being products of the pericycle, as in Cycas and Macrozamia. The structure of the wood agrees in the main with that of Ephedra.
Welwitschia (Tumboa). This is by far the most remarkable member of the Gnetales, both as regards habit and the form of its flowers. In a supplement to the systematic work of Engler and Prantl the well-known name Welwitschia, instituted by Hooker in 1864 in honour of Welwitsch, the discoverer of the plant, is superseded by that of Tumboa, originally suggested by Welwitsch. The genus is confined to certain localities in Damaraland and adjoining territory on the west coast of tropical South Africa. A well-grown plant projects less than a foot above the surface of the ground; the stem, which may have a circumference of more than 12 ft., terminates in a depressed crown resembling a circular table with a median groove across the centre and prominent broad ridges concentric with the margin. The thick tuberous stem becomes rapidly narrower, and passes gradually downwards into a tap-root. A pair of small strapshaped leaves succeed the two cotyledons of the seedling, and persist as the only leaves during the life of the plant ; they retain the power of growth in their basal portion, which is sunk in a narrow groove near the edge of the crown, and the tough lamina, 6 ft. in length, becomes split into narrow strap-shaped or thong-like strips which trail on the ground. Numerous circular pits occur on the concentric ridges of the depressed and wrinkled crown, marking the position of former inflorescences borne in the leaf-axil at different stages in the growth of the plant. An inflorescence has the form of a dichotomouslybranched cyme bearing small erect cones; those containing the female flowers attain the size of a fir-cone, and are scarlet in colour. Each cone consists of an axis, on which numerous broad and thin bracts are arranged in regular rows ; in the axil of each bract occurs a single flower; a male flower is enclosed by two opposite pairs of leaves, forming a perianth surrounding a central sterile ovule encircled by a ring of stamens united below, but free distally as short filaments, each of which terminates in a trilocular anther. The integument of the sterile ovule is prolonged above the nucellus as a spirally-twisted tube expanded at its apex into a flat stigma-like organ. A complete and functional female flower consists of a single ovule with two integuments, the inner of which is prolonged into a narrow tubular micropyle, like that in the flower of Gnetum. The megaspore of Welwitschia is filled with a prothallus-tissue before fertilization, and some of the prothallus-cells function as egg-cells; these grow upwards as long tubes into the apical region of the nucellus, where they come into contact with the pollen-tubes. After the egg-cells have been fertilized by the non-motile male cells they grow into tubular proembryos, producing terminal embryos. The stem is traversed by numerous collateral bundles, which have a limited growth, and are constantly replaced by new bundles developed from strands of secondary meristem. One of the bestknown anatomical characteristics of the genus is the occurrence of numerous spindle-shaped or branched fibres with enormouslythickened walls studded with crystals of calcium oxalate. Additional information has been published by Professor Pearson of Cape Town based on material collected in Damaraland in 1904 and 1906-1907. In 1906 he gave an account of the early stages of development of the male and female organs and, among other interesting statements in regard to the general biology of Welwitschia, he expressed the opinion that, as Hooker suspected, the ovules are pollinated by insect-agency. In a later paper Pearson considerably extended our knowledge of the reproduction and gametophyte of this genus.
AUTHORITIES. General: Bentham and Hooker, Genera Plantarum (London, 1862-1883); Engler and Prantl, Die naturlichen Pflanzenfamilien (Leipzig, 1889 and 1897); Strasburger, Die Coniferen und Gnetaceen (Jena, 1872); Die Angiospermen und die Gymnospermen (Jena, 1879); Histologische Beitrdge, iv. (Jena, 1892); Coulter and Chamberlain, Morphology of Spermatophytes (New York, 1901); Rendle, The Classification of Flowering Plants, vol. i. ( Cambridge, 1904); "The Origin of Gymnosperms" (A discussion at the Linnean Society; New Phytologist, vol. v., 1906). Cycadales: Mettenius, " Beitrage zur Anatomic der Cycadeen," Abh. k. sachs.
Ges. Wiss. (1860); Treub, " Recherches sur les Cycadees," Ann, Bot. Jard. Builenzorg, ii. (1884) ; Solms-Laubach, " Die Sprossfolge der Stangeria, etc.," Bot. Zeit. xlviii. (1806) ; Worsdell, " Anatomy of Macrozamia," Ann. Bot. x. (1896) (also papers by the same author, Ann.Bot., 1898, Trans. Linn. Soc. v., 1900) ; Scott, " The Anatomical Characters presented by the Peduncle of Cycadaceae," Ann. Bot. xi. (1897) ; Lang, " Studies in the Development and Morphology of Cycadean Sporangia, No. I./' Ann. Bot. xi. (1897) ; No. II., Ann.
xii. (1898); Wieland, " American Fossil Cycads," Carnegie Institu tion Publication (1906); Stopes, "Beitrage zur Kenntnis der Fortpflanzungsorgane der Cycadeen," Flora (1904); Caldwell, " Microcycas Calocoma," Bot. Gaz. xliv., 1907 (also papers on this and other Cycads in the Bot. Gaz., 1907-1909); Matte, Recherches sur I'appareil libero-ligneux des Cycadacees (Caen, 1904). Ginkgoales; Hirase, "Etudes sur la fecondation, etc., de Ginkgo biloba," Journ. Coll. Sci. Japan, xii. (1898); Seward and Gowan, " Ginkgo biloba," Ann. Bot. xiv. (1900) (with bibliography) ; Ikeno, " Contribution a 1'dtude de la fdcondation chez le Ginkgo biloba," Ann. Set. Nat. xiii. (1901); Sprecher, Le Ginkgo biloba (Geneva, 1907). Coniferales: " Report of the Conifer Conference " (1891) Journ. R. Hort. Soc. xiv. (1892); Beissner, Handbuch der Nadelholzkunde (Berlin, 1891); Masters, "Comparative Morphology of the Coniferae," Journ. Linn. Soc. xxvii. (1891); ibid. (1896), etc. ; Penhallo w, " The Generic Characters of the North American Taxaceae and Coniferae," Proc. and Trans. R. Soc. Canada, ii. (1896) ; Blackman, " Fertilization in Pinus sylvestris," Phil. Trans. (1898) (with bibliography) ; Worsdell, " Structure of the Female Flowers in Conifers, Ann. Bot. xiv. (1900) (with bibliography); ibid. (1899); Veitch, Manual of the Coniferae (London, 1900); Penhallow, " Anatomy of North American Coniferales," American Naturalist (1904); Engler and Pilger, Das Pflanzenreich, Taxaceae (1903); Seward and Ford, " The Araucarieae, recent and extinct," Phil. Trans. R. Soc. (1906) (with bibliography) ; Lawson, " Sequoia sempervirens," Annals of Botany (1904); Robertson, " Torreya California," New Phytologist (1904); Coker, " Gametophyte and Embryo of Taxodium," Bot. Gazette (1903); E. C. Jeffrey, "The Comparative Anatomy and Phylogeny of the Coniferales, part i. The Genus Sequoia," Mem. Boston Nat. Hist. Soc. v. No. 10 (1903) ; Gothan, " Zur Anatomic lebender und fossiler GymnospermenH6lzer,"X. Preuss. Geol. Landes. (Berlin, 1905) (for more recent papers, seeAnn.Bot.,NewPhytologist,andBot.Gazette, 1906-1909). Gnetales: Hooker, " On Welwitschia mirabilis," Trans. Linn. Soc. xxiy. (1864) ; Bower, " Germination, etc., in Gnetum," Journ. Mic. Sci. xxii. (1882) ; ibid. (1881); Jaccard, " Recherches embryologiques sur I'Ephedra helvetica," piss. Inaue. Lausanne (1894); Karsten, " Zur Entwickelungsgeschichte der Gattung Gnetum," Cohn's Beitrage, vi. (1803); Lotsy, " Contributions totheLife-Historyof thegenusGnetum.'Mnn. Bot. Jard. Buitenzorg, xvi. (1899); Land, " Ephedra trifurca," Bot. Gazette (1904); Pearson, " Some observations on Welwitschia mirabilis," Phil. Trans. R. Soc. (1906) ; Pearson, " Further Observations on Welwitschia," Phil. Trans. R. Soc. vol. 200 (1909). (A. C. SE.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
