The abundance of petrified coniferous wood in rocks of various ages has led many botanists to investigate the structure of modern genera with a view to determining how far anatomical characters may be used as evidence of generic distinctions. There are a few well-marked types of wood which serve as convenient standards of comparison, but these cannot be used except in a few cases to distinguish individual genera. The genusPinusserves as an illustration of wood of a distinct type characterized by the absence of xylem-parenchyma, except such as is associated with the numerous resin-canals that occur abundantly in the wood, cortex and medullary rays; the medullary rays are composed of parenchyma and of horizontal tracheids with irregular ingrowths from their walls. In a radial section of a pine stem each ray is seen to consist in the median part of a few rows of parenchymatous cells with large oval simple pits in their walls, accompanied above and below by horizontal tracheids with bordered pits. The pits in the radial walls of the ordinary xylem-tracheids occur in a single row or in a double row, of which the pits are not in contact, and those of the two rows are placed on the same level. The medullary rays usually consist of a single tier of cells, but in thePinustype of wood broader medullary rays also occur and are traversed by horizontal resin-canals. In the wood ofCypressus,Cedrus,Abiesand several other genera, parenchymatous cells occur in association with the xylem-tracheids and take the place of the resin-canals of other types. In the Araucarian type of wood (AraucariaandAgathis) the bordered pits, which occur in two or three rows on the radial walls of the tracheids, are in mutual contact and polygonal in shape, the pits of the different rows are alternate and not on the same level; in this type of wood the annual rings are often much less distinct than inCupressus,Pinusand other genera. InTaxus,Torreya(California and the Far East) andCephalotaxusthe absence of resin-canals and the presence of spiral thickening-bands on the tracheids constitute well-marked characteristics. An examination of the wood of branches, stems and roots of the same species or individual usually reveals a fairly wide variation in some of the characters, such as the abundance and size of the medullary rays, the size and arrangement of pits, the presence of wood-parenchyma—characters to which undue importance has often been attached in systematic anatomical work. The phloem consists of sieve-tubes, with pitted areas on the lateral as well as on the inclined terminal walls, phloem-parenchyma and, in some genera, fibres. In theAbietineaethe phloem consists of parenchyma and sieve-tubes only, but in most other forms tangential rows of fibres occur in regular alternation with the parenchyma and sieve-tubes. The characteristic companion-cells of Angiosperms are represented by phloem-parenchyma cells with albuminous contents; other parenchymatous elements of the bast contain starch or crystals of calcium oxalate. When tracheids occur in the medullary rays of the xylem these are replaced in the phloem-region by irregular parenchymatous cells known as albuminous cells. Resin-canals, which occur abundantly in the xylem, phloem or cortex, are not found in the wood of the yew.Cephalotaxus(Taxeae) is also peculiar in having resin-canals in the pith (cf.Ginkgo). One form ofCephalotaxusis characterized by the presence of short tracheids in the pith, in shape like ordinary parenchyma, but in the possession of bordered pits and lignified walls agreeing with ordinary xylem-tracheids; it is probable that these short tracheids serve as reservoirs for storing rather than for conducting water. The vascular bundle entering the stem from a leaf with a single vein passes by a more or less direct course into thecentral cylinder of the stem, and does not assume the girdle-like form characteristic of the cycadean leaf-trace. In species of which the leaves have more than one vein (e.g.Araucaria imbricata, &c.) the leaf-trace leaves the stele of the stem as a single bundle which splits up into several strands in its course through the cortex. In the wood of some conifers,e.g.Araucaria, the leaf-traces persist for a considerable time, perhaps indefinitely, and may be seen in tangential sections of the wood of old stems. The leaf-trace in the Coniferales is simple in its course through the stem, differing in this respect from the double leaf-trace ofGinkgo. A detailed account of the anatomical characters of conifers has been published by Professor D. P. Penhallow of Montreal and Dr. Gothan of Berlin which will be found useful for diagnostic purposes. The characters of leaves most useful for diagnostic purposes are the position of the stomata, the presence and arrangement of resin-canals, the structure of the mesophyll and vascular bundles. The presence of hypodermal fibres is another feature worthy of note, but the occurrence of these elements is too closely connected with external conditions to be of much systematic value. A pine needle grown in continuous light differs from one grown under ordinary conditions in the absence of hypodermal fibres, in the absence of the characteristic infoldings of the mesophyll cell-walls, in the smaller size of the resin-canals, &c. The endodermis inPinus,Piceaand many other genera is usually a well-defined layer of cells enclosing the vascular bundles, and separated from them by a tissue consisting in part of ordinary parenchyma and to some extent of isodiametric tracheids; but this tissue, usually spoken of as the pericycle, is in direct continuity with other stem-tissues as well as the pericycle. The occurrence of short tracheids in close proximity to the veins is a characteristic of coniferous leaves; these elements assume two distinct forms—(1) the short isodiametric tracheids (transfusion-tracheids) closely associated with the veins; (2) longer tracheids extending across the mesophyll at right angles to the veins, and no doubt functioning as representatives of lateral veins. It has been suggested that transfusion-tracheids represent, in part at least, the centripetal xylem, which forms a distinctive feature of cycadean leaf-bundles; these short tracheids form conspicuous groups laterally attached to the veins inCunninghamia, abundantly represented in a similar position in the leaves ofSequoia, and scattered through the so-called pericycle inPinus,Picea, &c. It is of interest to note the occurrence of precisely similar elements in the mesophyll ofLepidodendronleaves. An anatomical peculiarity in the veins ofPinusand several other genera is the continuity of the medullary rays, which extend as continuous plates from one end of the leaf to the other. The mesophyll ofPinusandCedrusis characterized by its homogeneous character and by the presence of infoldings of the cell-walls. In many leaves,e.g.Abies,Tsuga,Larix, &c., the mesophyll is heterogeneous, consisting of palisade and spongy parenchyma. In the leaves ofAraucaria imbricata, in which palisade-tissue occurs in both the upper and lower part of the mesophyll, the resin-canals are placed between the veins; in some species ofPodocarpus(sect.Nageia) a canal occurs below each vein; inTsuga,Torreya,Cephalotaxus,Sequoia, &c., a single canal occurs below the midrib; inLarix,Abies, &c., two canals run through the leaf parallel to the margins. The stomata are frequently arranged in rows, their position being marked by two white bands of wax on the leaf-surface.The chief home of the Coniferales is in the northern hemisphere, where certain species occasionally extend into the Arctic circle and penetrate beyond the northern limit of dicotyledonous trees. Wide areas are often exclusively occupied byDistribution.conifers, which give the landscape a sombre aspect, suggesting a comparison with the forest vegetation of the Coal period. South of the tree-limit a belt of conifers stretches across north Europe, Siberia and Canada. In northern Europe this belt is characterized by such species asPicea excelsa(spruce), which extends south to the mountains of the Mediterranean region;Pinus sylvestris(Scottish fir), reaching from the far north to western Spain, Persia and Asia Minor;Juniperus communis, &c. In north SiberiaPinus Cembra(Cembra or Arolla Pine) has a wide range; alsoAbies sibirica(Siberian silver fir),Larix sibiricaandJuniperus Sabina(savin). In the North American areaPicea alba,P. nigra,Larix americana,Abies balsamea(balsam fir),Tsuga canadensis(hemlock spruce),Pinus Strobus(Weymouth pine),Thuja occidentalis(white cedar),Taxus canadensisare characteristic species. In the Mediterranean region occurCupressus sempervirens,Pinus Pinea(stone pine), species of juniper,Cedrus atlantica,C. Libani,Callitris quadrivalvis,Pinus montana, &c. Several conifers of economic importance are abundant on the Atlantic side of North America—Juniperus virginiana(red cedar, used in the manufacture of lead pencils, and extending as far south as Florida),Taxodium distichum(swamp cypress),Pinus rigida(pitch pine),P. mitis(yellow pine),P. taeda,P. palustris, &c. On the west side of the American continent conifers play a still more striking rôle; among them areChamaecyparis nutkaensis,Picea sitchensis,Libocedrus decurrens,Pseudotsuga Douglasii(Douglas fir),Sequoia sempervirens,S. gigantea(the only two surviving species of this generic type are now confined to a few localities in California, but were formerly widely spread in Europe and elsewhere),Pinus Coulteri,P. Lambertiana, &c. Farther south, a few representatives of such genera asAbies,Cupressus,Pinusand juniper are found in the Mexican Highlands, tropical America and the West Indies. In the far East conifers are richly represented; among them occurPinus densiflora,Cryptomeria japonica,Cephalotaxus, species ofAbies,Larix,Thujopsis,Sciadopitys verticillata,Pseudolarix Kaempferi, &c. In the Himalaya occurCedrus deodara,Taxus, species ofCupressus,Pinus excelsa,Abies Webbiana, &c. The continent of Africa is singularly poor in conifers.Cedrus atlantica, a variety ofAbies Pinsapo,Juniperus thurifera,Callitris quadrivalvis, occur in the north-west region, which may be regarded as the southern limit of the Mediterranean region. The greater part of Africa north of the equator is without any representatives of the conifers;Juniperus proceraflourishes in Somaliland and on the mountains of Abyssinia; a species ofPodocarpusoccurs on the Cameroon mountains, andP. milanjianais widely distributed in east tropical Africa.Widdringtonia Whytei, a species closely allied toW. juniperoidesof the Cedarberg mountains of Cape Colony, is recorded from Nyassaland and from N.E. Rhodesia; while a third species,W. cupressoides, occurs in Cape Colony.Podocarpus elongataandP. Thunbergii(yellow wood) form the principal timber trees in the belt of forest which stretches from the coast mountains of Cape Colony to the north-east of the Transvaal.Libocedrus tetragona,Fitzroya patagonica,Araucaria brasiliensis,A. imbricata,Saxegothaeaand others are met with in the Andes and other regions in South America.AthrotaxisandMicrocachrysare characteristic Australian types.Phyllocladusoccurs also in New Zealand, and species ofDacrydium,Araucaria,AgathisandPodocarpusare represented in Australia, New Zealand and the Malay regions.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—(1)Ephedroideae, genusEphedra; (2)Welwitschioideae, genusWelwitschia; (3)Gnetoideae, genusGnetum. 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 ofGnetum, 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 genusEphedra, with its prothallus and archegonia, which are similar to those of other Gymnosperms, may be safely regarded as the most primitive of the Gnetales. InWelwitschiaalso the megaspore is filled with prothallus-tissue, but single egg-cells take the place of archegonia. In certain species ofGnetumdescribed by Karsten the megaspore contains a peripheral layer of protoplasm, in which scattered nuclei represent the female reproductive cells; inGnetum Gnemona similar state of things exists in the upper half of the megaspore, while the lower half agrees with the megaspore ofWelwitschiain 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 ofGnetum 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 Angiospermic 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 ofCasuarina; 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, which at the time of pollination secretes a drop of liquid, serves the same purpose as the style and stigma of an angiosperm. The megasporeis 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 inPinus, 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 ofEphedra,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. InEphedra 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 ofEphedraconsists 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.Fig. 17.—Gnetum Gnemon.(After Lotsy.)A, Female Flower. a, Imperfect Archegonia.n, Nucellus. e, Partially developed Megaspore.pc, Pollen-chamber. F, Fertile half.i, Integument. S, Sterile half.p′, Inner Perianth. pt, Pollen-tube.p″, Outer Perianth. z, Zygote.B, C, Megaspore. z′, Prothallus.a, Imperfect Archegonia.e, Partially developed Megaspore.F, Fertile half.S, Sterile half.pt, Pollen-tube.z, Zygote.z′, Prothallus.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 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,i), prolonged upwards as a narrow tube and succeeded by an inner and an outer perianth (fig. 17, A,p′ andp″). 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 orthotropous 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, B and C,e) being usually found in close association with the surviving and fully-grown megaspore. InGnetum 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 ofEphedra; 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, B 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,zandz′); 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 ofGnetumforms 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 ofGnetumare characterized by the production of several concentric cylinders of secondary wood and bast, the additional cambium-rings being products of the pericycle, as inCycasandMacrozamia. The structure of the wood agrees in the main with that ofEphedra.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 nameWelwitschia, instituted by Hooker in 1864 in honour of Welwitsch, the discoverer of the plant, is superseded by that ofTumboa, 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 strap-shaped 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 dichotomously-branched 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 ofGnetum. The megaspore ofWelwitschiais 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 best-known anatomical characteristics of the genus is the occurrence of numerous spindle-shaped or branched fibres with enormously-thickened 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 ofWelwitschia, 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 natürlichen Pflanzenfamilien(Leipzig, 1889 and 1897); Strasburger,Die Coniferen und Gnetaceen(Jena, 1872);Die Angiospermen und die Gymnospermen(Jena, 1879);Histologische Beiträge, 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, “Beiträge zur Anatomie der Cycadeen,”Abh. k. sächs.Ges. Wiss.(1860); Treub, “Recherches sur les Cycadées,”Ann. Bot. Jard. Buitenzorg, ii. (1884); Solms-Laubach, “Die Sprossfolge der Stangeria, &c.,”Bot. Zeit.xlviii. (1896); 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. Bot.xiv. (1900); Webber, “Development of the Antherozoids of Zamia,” Bot. Gaz. (1897); Ikeno, “Untersuchungen über die Entwickelung, &c., bei Cycas revoluta,”Journ. Coll. Sci. Japan, xii. (1898); Wieland, “American Fossil Cycads,” Carnegie Institution Publication (1906); Stopes, “Beiträge zur Kenntnis der Fortpflanzungsorgane der Cycadeen,”Flora(1904); Caldwell, “Microcycas Calocoma,”Bot. Gaz.xliv., 1907 (also papers on this and other Cycads in theBot. Gaz., 1907-1909); Matte,Recherches sur l’appareil libéro-ligneux des Cycadacées(Caen, 1904).Ginkgoales: Hirase, “Études sur la fécondation, &c., de Ginkgo biloba,”Journ. Coll. Sci. Japan, xii. (1898); Seward and Gowan, “Ginkgo biloba,”Ann. Bot.xiv. (1900) (with bibliography); Ikeno, “Contribution à l’étude de la fécondation chez le Ginkgo biloba,”Ann. Sci. 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), &c.; Penhallow, “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 Californica,”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 Anatomie lebender und fossiler Gymnospermen-Hölzer,”K. Preuss. Geol. Landes.(Berlin, 1905) (for more recent papers, seeAnn. Bot., New Phytologist, andBot. Gazette, 1906-1909).Gnetales: Hooker, “On Welwitschia mirabilis.”Trans. Linn. Soc.xxiv. (1864); Bower, “Germination, &., in Gnetum,”Journ. Mic. Sci.xxii. (1882); ibid. (1881); Jaccard, “Recherches embryologiques surl’Ephedra helvetica,”Diss. Inaug. Lausanne(1894); Karsten, “Zur Entwickelungsgeschichte der Gattung Gnetum,”Cohn’s Beiträge, vi. (1893); Lotsy, “Contributions to the Life-History of the genus Gnetum,”Ann. 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).
The abundance of petrified coniferous wood in rocks of various ages has led many botanists to investigate the structure of modern genera with a view to determining how far anatomical characters may be used as evidence of generic distinctions. There are a few well-marked types of wood which serve as convenient standards of comparison, but these cannot be used except in a few cases to distinguish individual genera. The genusPinusserves as an illustration of wood of a distinct type characterized by the absence of xylem-parenchyma, except such as is associated with the numerous resin-canals that occur abundantly in the wood, cortex and medullary rays; the medullary rays are composed of parenchyma and of horizontal tracheids with irregular ingrowths from their walls. In a radial section of a pine stem each ray is seen to consist in the median part of a few rows of parenchymatous cells with large oval simple pits in their walls, accompanied above and below by horizontal tracheids with bordered pits. The pits in the radial walls of the ordinary xylem-tracheids occur in a single row or in a double row, of which the pits are not in contact, and those of the two rows are placed on the same level. The medullary rays usually consist of a single tier of cells, but in thePinustype of wood broader medullary rays also occur and are traversed by horizontal resin-canals. In the wood ofCypressus,Cedrus,Abiesand several other genera, parenchymatous cells occur in association with the xylem-tracheids and take the place of the resin-canals of other types. In the Araucarian type of wood (AraucariaandAgathis) the bordered pits, which occur in two or three rows on the radial walls of the tracheids, are in mutual contact and polygonal in shape, the pits of the different rows are alternate and not on the same level; in this type of wood the annual rings are often much less distinct than inCupressus,Pinusand other genera. InTaxus,Torreya(California and the Far East) andCephalotaxusthe absence of resin-canals and the presence of spiral thickening-bands on the tracheids constitute well-marked characteristics. An examination of the wood of branches, stems and roots of the same species or individual usually reveals a fairly wide variation in some of the characters, such as the abundance and size of the medullary rays, the size and arrangement of pits, the presence of wood-parenchyma—characters to which undue importance has often been attached in systematic anatomical work. The phloem consists of sieve-tubes, with pitted areas on the lateral as well as on the inclined terminal walls, phloem-parenchyma and, in some genera, fibres. In theAbietineaethe phloem consists of parenchyma and sieve-tubes only, but in most other forms tangential rows of fibres occur in regular alternation with the parenchyma and sieve-tubes. The characteristic companion-cells of Angiosperms are represented by phloem-parenchyma cells with albuminous contents; other parenchymatous elements of the bast contain starch or crystals of calcium oxalate. When tracheids occur in the medullary rays of the xylem these are replaced in the phloem-region by irregular parenchymatous cells known as albuminous cells. Resin-canals, which occur abundantly in the xylem, phloem or cortex, are not found in the wood of the yew.Cephalotaxus(Taxeae) is also peculiar in having resin-canals in the pith (cf.Ginkgo). One form ofCephalotaxusis characterized by the presence of short tracheids in the pith, in shape like ordinary parenchyma, but in the possession of bordered pits and lignified walls agreeing with ordinary xylem-tracheids; it is probable that these short tracheids serve as reservoirs for storing rather than for conducting water. The vascular bundle entering the stem from a leaf with a single vein passes by a more or less direct course into thecentral cylinder of the stem, and does not assume the girdle-like form characteristic of the cycadean leaf-trace. In species of which the leaves have more than one vein (e.g.Araucaria imbricata, &c.) the leaf-trace leaves the stele of the stem as a single bundle which splits up into several strands in its course through the cortex. In the wood of some conifers,e.g.Araucaria, the leaf-traces persist for a considerable time, perhaps indefinitely, and may be seen in tangential sections of the wood of old stems. The leaf-trace in the Coniferales is simple in its course through the stem, differing in this respect from the double leaf-trace ofGinkgo. A detailed account of the anatomical characters of conifers has been published by Professor D. P. Penhallow of Montreal and Dr. Gothan of Berlin which will be found useful for diagnostic purposes. The characters of leaves most useful for diagnostic purposes are the position of the stomata, the presence and arrangement of resin-canals, the structure of the mesophyll and vascular bundles. The presence of hypodermal fibres is another feature worthy of note, but the occurrence of these elements is too closely connected with external conditions to be of much systematic value. A pine needle grown in continuous light differs from one grown under ordinary conditions in the absence of hypodermal fibres, in the absence of the characteristic infoldings of the mesophyll cell-walls, in the smaller size of the resin-canals, &c. The endodermis inPinus,Piceaand many other genera is usually a well-defined layer of cells enclosing the vascular bundles, and separated from them by a tissue consisting in part of ordinary parenchyma and to some extent of isodiametric tracheids; but this tissue, usually spoken of as the pericycle, is in direct continuity with other stem-tissues as well as the pericycle. The occurrence of short tracheids in close proximity to the veins is a characteristic of coniferous leaves; these elements assume two distinct forms—(1) the short isodiametric tracheids (transfusion-tracheids) closely associated with the veins; (2) longer tracheids extending across the mesophyll at right angles to the veins, and no doubt functioning as representatives of lateral veins. It has been suggested that transfusion-tracheids represent, in part at least, the centripetal xylem, which forms a distinctive feature of cycadean leaf-bundles; these short tracheids form conspicuous groups laterally attached to the veins inCunninghamia, abundantly represented in a similar position in the leaves ofSequoia, and scattered through the so-called pericycle inPinus,Picea, &c. It is of interest to note the occurrence of precisely similar elements in the mesophyll ofLepidodendronleaves. An anatomical peculiarity in the veins ofPinusand several other genera is the continuity of the medullary rays, which extend as continuous plates from one end of the leaf to the other. The mesophyll ofPinusandCedrusis characterized by its homogeneous character and by the presence of infoldings of the cell-walls. In many leaves,e.g.Abies,Tsuga,Larix, &c., the mesophyll is heterogeneous, consisting of palisade and spongy parenchyma. In the leaves ofAraucaria imbricata, in which palisade-tissue occurs in both the upper and lower part of the mesophyll, the resin-canals are placed between the veins; in some species ofPodocarpus(sect.Nageia) a canal occurs below each vein; inTsuga,Torreya,Cephalotaxus,Sequoia, &c., a single canal occurs below the midrib; inLarix,Abies, &c., two canals run through the leaf parallel to the margins. The stomata are frequently arranged in rows, their position being marked by two white bands of wax on the leaf-surface.
The chief home of the Coniferales is in the northern hemisphere, where certain species occasionally extend into the Arctic circle and penetrate beyond the northern limit of dicotyledonous trees. Wide areas are often exclusively occupied byDistribution.conifers, which give the landscape a sombre aspect, suggesting a comparison with the forest vegetation of the Coal period. South of the tree-limit a belt of conifers stretches across north Europe, Siberia and Canada. In northern Europe this belt is characterized by such species asPicea excelsa(spruce), which extends south to the mountains of the Mediterranean region;Pinus sylvestris(Scottish fir), reaching from the far north to western Spain, Persia and Asia Minor;Juniperus communis, &c. In north SiberiaPinus Cembra(Cembra or Arolla Pine) has a wide range; alsoAbies sibirica(Siberian silver fir),Larix sibiricaandJuniperus Sabina(savin). In the North American areaPicea alba,P. nigra,Larix americana,Abies balsamea(balsam fir),Tsuga canadensis(hemlock spruce),Pinus Strobus(Weymouth pine),Thuja occidentalis(white cedar),Taxus canadensisare characteristic species. In the Mediterranean region occurCupressus sempervirens,Pinus Pinea(stone pine), species of juniper,Cedrus atlantica,C. Libani,Callitris quadrivalvis,Pinus montana, &c. Several conifers of economic importance are abundant on the Atlantic side of North America—Juniperus virginiana(red cedar, used in the manufacture of lead pencils, and extending as far south as Florida),Taxodium distichum(swamp cypress),Pinus rigida(pitch pine),P. mitis(yellow pine),P. taeda,P. palustris, &c. On the west side of the American continent conifers play a still more striking rôle; among them areChamaecyparis nutkaensis,Picea sitchensis,Libocedrus decurrens,Pseudotsuga Douglasii(Douglas fir),Sequoia sempervirens,S. gigantea(the only two surviving species of this generic type are now confined to a few localities in California, but were formerly widely spread in Europe and elsewhere),Pinus Coulteri,P. Lambertiana, &c. Farther south, a few representatives of such genera asAbies,Cupressus,Pinusand juniper are found in the Mexican Highlands, tropical America and the West Indies. In the far East conifers are richly represented; among them occurPinus densiflora,Cryptomeria japonica,Cephalotaxus, species ofAbies,Larix,Thujopsis,Sciadopitys verticillata,Pseudolarix Kaempferi, &c. In the Himalaya occurCedrus deodara,Taxus, species ofCupressus,Pinus excelsa,Abies Webbiana, &c. The continent of Africa is singularly poor in conifers.Cedrus atlantica, a variety ofAbies Pinsapo,Juniperus thurifera,Callitris quadrivalvis, occur in the north-west region, which may be regarded as the southern limit of the Mediterranean region. The greater part of Africa north of the equator is without any representatives of the conifers;Juniperus proceraflourishes in Somaliland and on the mountains of Abyssinia; a species ofPodocarpusoccurs on the Cameroon mountains, andP. milanjianais widely distributed in east tropical Africa.Widdringtonia Whytei, a species closely allied toW. juniperoidesof the Cedarberg mountains of Cape Colony, is recorded from Nyassaland and from N.E. Rhodesia; while a third species,W. cupressoides, occurs in Cape Colony.Podocarpus elongataandP. Thunbergii(yellow wood) form the principal timber trees in the belt of forest which stretches from the coast mountains of Cape Colony to the north-east of the Transvaal.Libocedrus tetragona,Fitzroya patagonica,Araucaria brasiliensis,A. imbricata,Saxegothaeaand others are met with in the Andes and other regions in South America.AthrotaxisandMicrocachrysare characteristic Australian types.Phyllocladusoccurs also in New Zealand, and species ofDacrydium,Araucaria,AgathisandPodocarpusare represented in Australia, New Zealand and the Malay regions.
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—(1)Ephedroideae, genusEphedra; (2)Welwitschioideae, genusWelwitschia; (3)Gnetoideae, genusGnetum. 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 ofGnetum, 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 genusEphedra, with its prothallus and archegonia, which are similar to those of other Gymnosperms, may be safely regarded as the most primitive of the Gnetales. InWelwitschiaalso the megaspore is filled with prothallus-tissue, but single egg-cells take the place of archegonia. In certain species ofGnetumdescribed by Karsten the megaspore contains a peripheral layer of protoplasm, in which scattered nuclei represent the female reproductive cells; inGnetum Gnemona similar state of things exists in the upper half of the megaspore, while the lower half agrees with the megaspore ofWelwitschiain 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 ofGnetum 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 Angiospermic 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 ofCasuarina; 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, which at the time of pollination secretes a drop of liquid, serves the same purpose as the style and stigma of an angiosperm. The megasporeis 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 inPinus, 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 ofEphedra,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. InEphedra 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 ofEphedraconsists 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.
A, Female Flower. a, Imperfect Archegonia.
n, Nucellus. e, Partially developed Megaspore.
pc, Pollen-chamber. F, Fertile half.
i, Integument. S, Sterile half.
p′, Inner Perianth. pt, Pollen-tube.
p″, Outer Perianth. z, Zygote.
B, C, Megaspore. z′, Prothallus.
a, Imperfect Archegonia.
e, Partially developed Megaspore.
F, Fertile half.
S, Sterile half.
pt, Pollen-tube.
z, Zygote.
z′, Prothallus.
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 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,i), prolonged upwards as a narrow tube and succeeded by an inner and an outer perianth (fig. 17, A,p′ andp″). 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 orthotropous 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, B and C,e) being usually found in close association with the surviving and fully-grown megaspore. InGnetum 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 ofEphedra; 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, B 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,zandz′); 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 ofGnetumforms 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 ofGnetumare characterized by the production of several concentric cylinders of secondary wood and bast, the additional cambium-rings being products of the pericycle, as inCycasandMacrozamia. The structure of the wood agrees in the main with that ofEphedra.
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 nameWelwitschia, instituted by Hooker in 1864 in honour of Welwitsch, the discoverer of the plant, is superseded by that ofTumboa, 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 strap-shaped 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 dichotomously-branched 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 ofGnetum. The megaspore ofWelwitschiais 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 best-known anatomical characteristics of the genus is the occurrence of numerous spindle-shaped or branched fibres with enormously-thickened 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 ofWelwitschia, 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 natürlichen Pflanzenfamilien(Leipzig, 1889 and 1897); Strasburger,Die Coniferen und Gnetaceen(Jena, 1872);Die Angiospermen und die Gymnospermen(Jena, 1879);Histologische Beiträge, 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, “Beiträge zur Anatomie der Cycadeen,”Abh. k. sächs.Ges. Wiss.(1860); Treub, “Recherches sur les Cycadées,”Ann. Bot. Jard. Buitenzorg, ii. (1884); Solms-Laubach, “Die Sprossfolge der Stangeria, &c.,”Bot. Zeit.xlviii. (1896); 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. Bot.xiv. (1900); Webber, “Development of the Antherozoids of Zamia,” Bot. Gaz. (1897); Ikeno, “Untersuchungen über die Entwickelung, &c., bei Cycas revoluta,”Journ. Coll. Sci. Japan, xii. (1898); Wieland, “American Fossil Cycads,” Carnegie Institution Publication (1906); Stopes, “Beiträge zur Kenntnis der Fortpflanzungsorgane der Cycadeen,”Flora(1904); Caldwell, “Microcycas Calocoma,”Bot. Gaz.xliv., 1907 (also papers on this and other Cycads in theBot. Gaz., 1907-1909); Matte,Recherches sur l’appareil libéro-ligneux des Cycadacées(Caen, 1904).Ginkgoales: Hirase, “Études sur la fécondation, &c., de Ginkgo biloba,”Journ. Coll. Sci. Japan, xii. (1898); Seward and Gowan, “Ginkgo biloba,”Ann. Bot.xiv. (1900) (with bibliography); Ikeno, “Contribution à l’étude de la fécondation chez le Ginkgo biloba,”Ann. Sci. 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), &c.; Penhallow, “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 Californica,”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 Anatomie lebender und fossiler Gymnospermen-Hölzer,”K. Preuss. Geol. Landes.(Berlin, 1905) (for more recent papers, seeAnn. Bot., New Phytologist, andBot. Gazette, 1906-1909).Gnetales: Hooker, “On Welwitschia mirabilis.”Trans. Linn. Soc.xxiv. (1864); Bower, “Germination, &., in Gnetum,”Journ. Mic. Sci.xxii. (1882); ibid. (1881); Jaccard, “Recherches embryologiques surl’Ephedra helvetica,”Diss. Inaug. Lausanne(1894); Karsten, “Zur Entwickelungsgeschichte der Gattung Gnetum,”Cohn’s Beiträge, vi. (1893); Lotsy, “Contributions to the Life-History of the genus Gnetum,”Ann. 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).