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CONIFEISALES
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CONIFEISALES .—Trees and shrubs characterized by a copious branching of the See also: stem and frequently by a See also:regular pyramidal See also:form. Leaves See also:simple, small, linear or See also:short and See also:scale-like, usually persisting for more than one See also:year. See also:Flowers monoecious or dioecious, unisexual, without a perianth, often in the form of cones, but never terminal on the See also:main stem. The See also:plants usually included in the Coniferae constitute a less homogeneous class than the Cycadaceae. Some authors use the See also:term Coniferae in a restricted sense as including those See also:External genera which have the See also:female flowers in the form of cones, features. the other genera, characterized by flowers of a different type, being placed in the Taxaceae, and often spoken of as Taxads. In See also:order to avoid confusion in the use of the term Coniferae, we may adopt as a class-designation the name Coniferales, including both the Coniferae—using the term in a restricted sense—and the Taxaceae. The most striking characteristic of the See also:majority of the Coniferales is the regular manner of the monopodial branching and the pyramidal shape. See also:Araucaria imbricata, the See also:Monkey-See also:puzzle See also:tree, A. excelsa, the See also:Norfolk See also:Island See also:pine, many pines and firs, cedars and other genera illustrate the pyramidal form. The See also:mammoth redwood tree of See also:California,See also:Sequoia (Wellingtonia) gigantea, which represents the tallest Gymnosperm, is a See also:good example of the regular tapering main stem and narrow pyramidal form. The cypresses afford instances of tall and narrow trees similar in See also:habit to See also:Lombardy poplars. The See also:common See also:cypress (Cupressus sempervirens), as found See also:wild in the mountains of See also:Crete and See also:Cyprus, is characterized by See also:long and spreading branches, which give it a See also:cedar-like habit. A pendulous or weeping habit is assumed by some conifers, e.g.Picea excelsa See also: var. virgata represents a form in which the main branches attain a considerable See also:horizontal See also:extension, and trail themselves like See also:snakes along the ground. Certain See also:species of Pinus, the yews (Taxus) and some other genera grow as bushes, which in See also:place of a main See also:mast-like stem possess several repeatedly-branched leading shoots. The unfavourable conditions in See also:Arctic regions have produced a See also:dwarf form, in which the main shoots grow See also:close to the ground. Artificially induced dwarfed plants of Pinus, Cupressus, Sciadopitys (See also:umbrella pine) and other genera are commonly cultivated by the See also:Japanese. The dying off of older branches and the vigorous growth of shoots nearer the See also:apex of the stem produce a form of tree illustrated by the See also:
A new genus of conifers, Taiwania, has recently been described from the island of See also:
In Pinus the needles occur in pairs, or in clusters of 3 or 5 at the apex of a small and inconspicuous short shoot of limited growth (See also: spur), which is enclosed at its base by a few scale-leaves, and See also:borne on a branch of unlimited growth in the axil of a scale-See also:leaf. In the Californian Pinus monophylla each spur bears usually one needle, but two are not uncommon; it would seem that rudiments of two needles are always produced, but, as a See also:rule, only one develops into a needle. In Sciadopitys similar spurs occur, each bearing a single needle, which in its grooved surface and in the See also:possession of a See also:double vascular bundle bears traces of an origin from two needle-leaves. A peculiarity of these leaves is the inverse See also:orientation of the vascular See also:tissue; each of the two veins has its phloem next the upper and the xylem towards the See also:lower surface of the leaf; this unusual position of the xylem and phloem may be explained by regarding the needle of Sciadopitys as being composed of a pair of leaves borne on a short axillary shoot and fused by their margins (fig. 15, A). Long and short shoots occur also in Cedrus and Larix, but in these genera the spurs are longer and stouter, and are not See also:shed with the leaves; this See also:kind of short shoot, by accelerated apical growth, often passes into the See also:condition of a long shoot on which the leaves are scattered and separated by comparatively long internodes, instead of being crowded into tufts such as are borne on the ends of the spurs. In the genus Phyllocladus (New Zealand, &c.) there are no See also:green foliage-leaves, but in their place flattened branches (phylloclades) borne in the axils of small scale-leaves. The cotyledons are often two in number, but sometimes (e.g. Pinus) as many as fifteen; these leaves are usually succeeded by foliage-leaves in the form of delicate spreading needles, and these primordial leaves are followed, sooner or later, by the adult type of leaf, except in Retinosporas, which retain the juvenile foliage. In addition to the first foliage-leaves and the adult type of leaf, there are often produced leaves which are intermediate both in shape and structure between the seedling and adult foliage. Dimorphism or heterophylly is fairly common. One of the best known examples is the Chinese See also:juniper (Juniperus chinensis), in which branches with spinous leaves, longer and more spreading than the ordinary adult leaf, are often found associated with the normal type of branch.In some cases, e.g. Sequoia sempervirens, the fertile branches bear leaves which are less spreading than those on the vegetative shoots. Certain species of the southern hemisphere genus Dacrydium afford particularly striking instances of heterophylly, e.g. D. Kirkii of New Zealand, in which some branches bear small and appressed leaves, while in others the leaves are much longer and more spreading. A well-known fossil conifer from Triassic strata—Voltzia heterophylla—also illustrates a marked dissimilarity in the leaves of the same shoot. The variation in leaf-form and the tendency of leaves to arrange themselves in various ways on different branches of the same plant are features which it is important to bear in mind in the See also: identification of fossil conifers. In this connexion we may See also:note the striking resemblance between some of the New Zealand Alpine Veronicas, e.g. See also:Veronica Hectori, V. cupressoides, &c. (also Polycladus cupressinus, a Composite), and some of the cypresses and other conifers with small appressed leaves. The long linear leaves of some species of Podocarpus, in which the lamina is traversed by a single vein, recall the pinnae of Cycas; the branches of some Dacrydiums and other forms closely resemble those of lycopods; these superficial resemblances, both between different genera of conifers and between conifers and other plants, coupled with the usual occurrence of fossil coniferous twigs without cones attached to them, render the determination of See also:extinct types a very unsatisfactory and frequently an impossible task. A typical male See also:flower consists of a central axis bearing numerous spirally-arranged sporophylls (stamens), each of which consists of a slender stalk (filament) terminating distally in a more Flowers. or less prominent knob or triangular scale, and bearing two or more See also:pollen-sacs (microsporangia) on its lower surface.The pollen-grains of some genera (e.g. Pinus) are furnished with See also: bladder-like extensions of the See also:outer See also:wall, which serve as See also:aids to See also:wind-dispersal. The stamens of Araucaria and Agathis are See also:peculiar in bearing several long ,and narrow See also:free pollen-sacs; these may be compared with the sporangiophores of the horsetails (Equisetum) ; in Taxus (See also:yew) the filament is attached to the centre of a large circular distal expansion, which bears several pollen-sacs on its under surface. In the conifers proper the female reproductive See also:organs have the form of cones, which may be styled flowers or inflorescences according to different interpretations of their See also:morphology. In the Taxaceae the flowers have a simpler structure. The female flowers of the Abietineae may be taken as representing a common type. A pine See also:cone reaches maturity in two years; a single year suffices for the full development in Larix and several other genera. The axis of the cone bears numerous spirally disposed flat scales (cone-scales), each of which, if examined in a young cone, is found to be double, and to consist of a lower and an upper portion. The latter is a thin flat scale bearing a median See also:ridge or See also:keel (e.g. Abies), on each See also:side of which is situated an inverted ovule, consisting of a nucellus surrounded by a single integument. As the cone grows in size and becomes woody the lower See also:half of the cone-scale, which we may See also:call the carpellary scale, may remain small, and is so far outgrown by the upper half (seminiferous scale) that it is hardly recognizable in the mature cone. In many species of Abies (e.g.Abies pectinata, &c.) the ripe cone differs from those of Pinus, Picea and Cedrus in the large size of the carpellary scales, which project as conspicuous thin appendages beyond the distal margins of the broader and more woody seminiferous scales; the long carpellary scale is a prominent feature also in the cone of the See also:
In Araucaria the cone-scale is regarded as consisting of a flat carpel, of which the See also: placenta has not grown out into the scale-like structure. The seminiferous scale of Pinus, &c., is also spoken of sometimes as a ligular outgrowth from the carpellary leaf. See also:Robert Brown was the first to give a clear description of the morphology of the Abietineous cone in which carpels bear naked ovules;, he recognized gymnospermy as an important distinguishing feature in conifers as well as in cycads. Another view is to regard the cone as' an inflorescence, each carpellary scale being a bract bearing in its axil a shoot the axis of which has not been See also:developed; the seminiferous scale is believed,to represent either a single leaf or a. fused pair of leaves belonging to the partially suppressed axillary shoot. •' In 1869 See also:van Tieghem laid stress on anatomical See also:evidence as a See also:
In a young cone the seminiferous scale appears as a hump of tissue at the base or in the axil of the carpellary scale, but Celakovsky, a strong supporter of the axillary-bud theory, attaches little or no importance to this kind of evidence, regarding the See also: present manner of development as being merely an example of a short cut adopted in the course of See also:evolution, and replacing the See also:original:See also:production of a branch in the axil of each carpellary scale. Eichler, one of the See also:chief supporters of the simpler view, does not recognize in the inverse orientation of the vascular bundles an See also:argument in support of the axillary-bud theory, but points out that the seminiferous scale, being an outgrowth from the surface of the carpellary scale, would, like outgrowths from an ordinary leaf, naturally have its bundles inversely orientated. In such cone-scales as show little or no external indication of being double in origin, e.g. Araucaria (fig. 15, D) Sequoia, &e., there are always two sets of bundles; the upper set, having the phloem uppermost, as in the seminiferous scale of Abies or Pinus, are regarded as belonging to the outgrowth from the carpellary scale and specially developed to See also:supply the ovules. Monstrous cones are fairly common; these in some instances lend support to the axillary-bud theory, and it has been said that this theory owes its existence to evidence furnished by abnormal cones. It is "difficult to estimate the value of abnormalities as evidence bearing on morphological interpretation ; the chief danger lies perhaps in attaching undue See also:weight to them; but there is also a See also:risk of minimizing their importance. Monstrosities at least demonstrate possible lines of development, but when the abnormal forms of growth in various directions are fairly evenly balanced, trustworthy deductions become difficult. The occurrence of buds in the axils of carpellary scales may, however, simply mean that buds, which are (C and D after Worsdelt) A, Double needleof Sciadopitys (a, a, leaves; b, shoot; Br, bract). B; seminiferous scale as leaf of axillary shoot (b, shoot; Sc, seminiferous scale; Br, bract). C, seminiferous scale as fused pair of leaves (P, ls, 13, first, second and third leaves; b, shoot; Br, bract), D, cone-scale of Araucaria (n, nucellus; i, integument; x, xylem): usually undeveloped in the axils of sporophylls, occasionally afford evidence of their existence. Some monstrous cones lend no support to the axillary-bud theory.In Larix the axis of the cone often continues its growth; similarly in Cephalotaxus the cones are often proliferous. (In rare cases the proliferated portion produces male flowers in the leaf-axils.) In Larix the. carpellary scale may become leafy, and the seminiferous scale may disappear. Androgynous cones may be produced, as in the cone' of Pinus rigida (fig. 16), in which the lower See also: part bears stamens and the upper portion carpellary and seminiferous scales. An interesting case has been figured by Masters, in which scales of a cone of Cupressus Lawsoniana bear ovules on the upper surface and stamens on the lower face. One argument that has been adduced in support of the axillary bud theory is derived from the Palaeozoic type Cordaites, in which each' ovule occurs on an axis borne in the axil of a bract. The whole question is still unsolved, and perhaps insoluble." It may be that the interpretation of the female cone of the Abietineae as an inflorescence, which finds favour with many botanists, cannot be applied to the cones of Agathis" and Araucaria. Without ex-pressing any decided See also:opinion as to. the morphologyof the double cone-scale of the Abietineae, preference may be See also:felt in favour of regarding , the cone-scale of the Araucarieae as a simple carpellary leaf bearing a single ovule. A discussion of this question may be found in a See also:paper on the Araucarieae by See also:Seward and See also:Ford, published in the Transactions of the Royal Society FIG. i6—Abof See also:London (1906). Cordaites is an extinct type normal Cone of which in certain respects resembles Ginkgo, cycads 1' i n u s r a g l d a. and the, Araucarieae, but its agreement with true (After Masters.) conifers is probably too remote to justify our attri- buting much weight to the bearing of the morphology of its female flowers on the interpretation of that of the Coniferae. The greater simplicity of the Eichler theory may See also:prejudice us in its favour;" but, on the other See also:hand, the arguments advanced in favour of the axillary-bud theories are perhaps not sufficiently cogent to See also:lead us to accept an explanation based chiefly, on the uncertain evidence of monstrosities. A pollen-See also:grain when first formed from its See also:mother-See also:cell consists of a single cell ; in this condition it may be carried to the nucellus of the ovule (e.g.Taxus, Cupressus, &c.), or more usually Micro-(Pinus, Larix, &c.) it reaches maturity before the dehis- spores cence of the microsporangium. The See also: nucleus of the and mea, microspore divides and gives rise to a small cell within spores. the large cell, a second small cell is then produced; , this is the structure of the ripe pollen-grain in some conifers (Taxus, &c.). The large cell grows out as a pollen-See also:tube; the second of the two small cells (See also:body-cell) wanders into the tube, followed by the nucleus of the first small cell (stalk-cell). In Taxus the body-cell eventually divides into two, in which the products of See also:division are of unequal size, the larger constituting the male generative cell, •, which fuses with the nucleus of the See also:egg-cell. In Juniperus the products of division of the body-cell are equal, and both See also:function as male generative cells. In the Abietineae cell-formation in the pollen-grain is carried farther. Three small cells occur inside the cavity of the microspore; two of them collapse and the third divides into two, forming a stalk-cell and a larger body-cell. The latter ultimately divides in the apex of the pollen-tube into two non-motile generative cells. Evidence has lately been adduced of the existence of numerous nuclei in the pollen-tubes of the Araucarieae, and it seems probable that in this as in several other respects this family is distinguished from other members of the Coniferales. The precise method of fertilization in the Scots Pine was followed by V. H. Blackman, who also succeeded in showing that the nuclei of the sporophyte See also:generation contain twice as many chromosomes as the nuclei of the gametophyte.Other observers have in See also: recent years demonstrated a similar relation in other genera between the number of chromosomes in the nuclei of the two generations. The ovule is usually surrounded by one integument, which projects beyond the tip of the nucellus as a wide-open lobed See also:funnel, which at the time of See also:pollination folds inwards, and so assists in bringing the pollen-grains on to the nucellus. In some conifers (e.g. Taxus, Cephalotaxus, Dacrydium, &c.) the ordinary integument is partially enclosed by an arillus or second integument. It is held by some botanists (Celakovsky) that the seminiferous scale of the Abietineae is homologous with the arillus or second integument of the Taxaceae, but this view is too strained to gain See also:general See also:acceptance. In Araucaria and Saxegothaea the nucellus itself projects beyond the open micropyle and receives the pollen-grains See also:direct. During the growth of the cell which forms the megaspore the greater part of the nucellus is absorbed, except the apical portion, which persists as a cone above the megaspore; the partial disorganization of some of the cells in the centre of the nucellar cone forms an irregular cavity, which may be compared with the larger pollen-chamber of Ginkgo and the cycads. In each ovule one megaspore comes to maturity, but, exceptionally, two may be present (e.g. Pinus sylvestris). It has been shown by See also:Lawson that in Sequoia sempervirens (See also:Annals of See also:Botany, 1904) and by other workers in the genera that several megaspores may attain a fairly large size in one prothallus. The megaspore becomes filled with tissue (prothallus), and from some of the superficial cells archegonia are produced, usually three to five in number, but in rare cases ten to twenty or even sixty may be present. In the genus Sequoia there may be as many as sixty archegonia (Arnoldi and Lawson) in one megaspore; these occur either separately or in some parts of the prothallus they may form See also:groups as in the Cupressineae; they are scattered through the prothallus instead of being confined to the apical region as in the majority of conifers.Similarly in the Araucarieae and in Widdringtonia the archegonia are numerous and scattered and often sunk in the prothallus tissue. In Libocedrus decurrens (Cupressineae) Lawson describes the archegonia as varying in number from 6 to 24(Annals of Botany xxi.,1907). An archegonium consists of a large See also: oval egg-cell surmounted by a short See also:neck composed of one or more tiers of cells, six to eight cells in each tier. Before fertilization the nucleus of the egg-cell divides and cuts off a ventral See also:canal-cell; this cell may represent a second egg-cell. The egg-cells of the archegonia may be in lateral contact (e.g. Cupressineae) or separated from one another by a few cells of the prothallus, each ovum being immediately surrounded by a layer of cells distinguished by their granular contents and large nuclei. During the development of the egg-cell, See also:food material is transferred from these cells through the pitted wall of the ovum. The tissue at the apex of the megaspore grows slightly above the level of the archegonia, so that the latter come to See also:lie in a shallow depression. In the See also:process of fertilization the two male generative nuclei, accompanied by the pollen-tube nucleus and that of the stalk-cell, pass through an open See also:pit at the apex of the pollen-tube into the See also:protoplasm of the ovum. After fertilization the nucleus of the egg divides, the first stages of karyokinesis being apparent even before See also:complete See also:fusion of the male and female nuclei has occurred. The result of this is the production of four nuclei, which eventually take up a position at the bottom of the ovum and become separated from one another by See also:vertical cell-walls; these nuclei See also:divide again, and finally three tiers of cells are produced, four in each tier. In the Abietineae the cells of the See also:middle tier elongate and push the lowest tier deeper into the endosperm; the cells of the bottom tier may remain in lateral contact and produce together one embryo, or they may separate (Pinus, Juniperus, &c.) and form four potential embryos.The ripe albuminous seed contains a single embryo with two or more cotyledons. The seeds of many conifers are provided with large thin wings, consisting in some genera (e.g. Pinus) of the upper cell-layers of the seminiferous scale, which have become detached and, in some cases, adhere loosely to the seed as a thin membrane ; the loose See also: attachment may be of use to the seeds when they are blown against the branches of trees, in enabling them to fall away from the wing and drop to the ground. The seeds of some genera depend on animals for dispersal, the carpellary scale (Microcachrys) or the outer integument being brightly coloured and attractive. In some Abietineae (e.g. Pinus and Picea)—in which the cone-scales persist for some time after the seeds are ripe—the cones hang down and so facilitate the fall of the seeds; in Cedrus, Araucaria and Abies the scales become detached and fall with the seeds, leaving the See also:bare vertical axis of the cone on the tree. In all cases, except some species of Araucaria (See also:sect. Colymbea) the germination is epigean. The seedling plants of some Conifers (e.g. Araucaria imbricata) are characterized by a See also:carrot-shaped hypocotyl, which doubtless serves as a food-See also:reservoir. The roots of many conifers possess a narrow See also:band of See also:primary xylem-tracheids with a See also:group of narrow See also:spiral protoxylem-elements at each end (diarch). A striking feature in the roots of several genera, excluding the Abietineae, is the occur- See also:Anatomy. rence of thick and somewhat irregular bands of thickening on the cell-walls of the cortical layer next to the endodermis.These bands, which may serve to strengthen the central See also: cylinder, have been compared with the netting surrounding the delicate wall of an inflated See also:balloon. It is not always easy to distinguish a See also:root from a stem; in some cases (e.g. Sequoia) the primary See also:tetrarch structure is easily identified in the centre of an old root, but in other cases the primary elements are very difficult to recognize. The sudden termination of the secondary tracheids against the See also:pith-cells may afford evidence of root-structure as distinct from stem-structure, in which the radial rows of secondary tracheids pass into the irregularly-arranged primary elements next the pith. The See also:annual rings in a root are often less clearly marked than in the stem, and the xylem-elements are frequently larger and thinner. The primary vascular bundles in a` young conifer stem are collateral, and, like those of a Dicotyledon, they are arranged in a circle See also:round a central pith and enclosed by a common endodermis. It is in the nature of the secondary xylem that the Coniferales are most readily distinguished from the See also:Dicotyledons and Cycadaceae; the See also:wood is homogeneous in structure, consisting almost entirely of tracheids with circular or polygonal bordered pits on the radial walls, more particularly in the See also:late summer wood. In many genera xylem-parenchyma is present, but never in See also:great abundance. A few Dicotyledons, e.g. Drimys (Magnoliaceae) closely resemble conifers in the homogeneous See also:character of the wood, but in most cases the presence of large See also:spring vessels, wood-See also:fibres and abundant parenchyma affords an obvious distinguishing feature. The abundance of petrified coniferous wood in rocks of various ages has led many botanists to investigate the structure of See also: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 See also:standards of comparison, but these cannot be used except in a few cases to distinguish individual genera.The genus Pinus serves as an See also: illustration of wood of a distinct type characterized by the See also:absence of xylem-parenchyma, except such as is associated with the numerous See also: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 See also:section of a pine stem each See also: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 See also: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 the Pinus type of wood broader medullary rays also occur and are traversed by horizontal resin-canals. In the wood of Cypressus, Cedrus, Abies and 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 (Araucaria and Agathis) 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 in Cupressus, Pinus and other genera. In Taxus, Torreya (California and the Far See also:East) and Cephalotaxus the 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 See also:work. The phloem consists of See also:sieve-tubes, with pitted areas on the lateral as well as on the inclined terminal walls, phloem-parenchyma and, in some genera, fibres. In the Abietineae the phloem consists of parenchyma and sieve-tubes only, but in most other forms tangential rows of fibres occur in.regular See also:alternation with the parenchyma and sieve-tubes. The characteristic See also:companion-cells of See also:Angiosperms are represented by phloem-parenchyma cells with albuminous contents.; other parenchymatous elements of the bast contain See also:starch or crystals of See also: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 of Cephalotaxus is 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 See also: water. The vascular bundle entering the stern from a leaf with a single vein passes by a more or less direct course into the central cylinder of the stem, and does not assume the See also: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 See also: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 consider-able 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 of Ginkgo. A detailed See also:account of the anatomical characters of conifers has been published by See also:Professor D. P.See also: Penhallow of See also:Montreal and Dr. Gothan of See also: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 See also: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 in Pinus, Picea and 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—0) 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 in Cunninghamia, abundantly represented in a similar position in the leaves of Sequoia, and scattered through the so-called pericycle in Pinus, Picea, &c. It is of See also:interest to note the occurrence of precisely similar elements in the mesophyll of Lepidodendron leaves. An anatomical peculiarity in the veins of Pinus and 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 of Pinus and Cedrus is 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 of Araucaria 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 of Podocarpus (sect. Nageia) a canal occurs below each vein; in Tsuga, Torreya, Cephalotaxus, Sequoia, &c., a single canal occurs below the midrib; in Larix, 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 See also:
Several conifers of economic importance are abundant on the See also: Atlantic side of North Ainerica—Juniperus virginiana (red cedar, used in the manufacture of lead pencils, and extending as far south as See also:Florida), Taxodium distichum (swamp cypress), Pinus rigida (See also:pitch pine), P. mitis (yellow pine), P. taeda,P. palustris, &c. On the See also:west side of the American See also:continent conifers See also:play a still more striking role; among them are Chamaecyparis 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 Coulteii, P. Lamberliana, &c. Farther south, a few representatives of such genera as Abies, Cupressus, Pinus and juniper are found in the Mexican See also:Highlands, tropical See also:America, and the West Indies. Inthe far East conifers are richly represented; among them occur Pinus densiflora,Cryptomeria japonica, Cephalotaxus, species of Abies, Larix, Thujopsis, Sciadopitys verticillata, Pseudolarix Kaempferi, &c. In the See also:Himalaya occur Cedrus deodara, Taxus, species of Cupressus, Pinus excelsa, Abies Webbiana, &c. The continent of See also:Africa is singularly poor in conifers. Cedrus atlantica, a variety of Abies 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 pat of Africa north of the See also:equator is without any representatives of the conifers; Juniperus procera flourishes in See also:Somaliland and on the mountains of See also:Abyssinia; a species of Podocarpus occurs on the Cameroon mountains, and P. milanjiana is widely distributed in east tropical Africa. Widdringtonia Whytei, a species closely allied to W. juniperoides of the Cedar-See also:berg mountains of Cape See also:Colony,is recorded from Nyassaland and from N.E. See also:Rhodesia; while a third species, W. cupressoides, occurs in Cape Colony. Podocarpus elongata and P.Thunbergii (yellow wood) form the See also: principal See also:timber trees in the belt of forest which stretches from the See also:coast mountains of Cape Colony to the north-east of the See also:Transvaal. Libocedrus tetragona, Fitzroya patagonica, Araucaria brasiliensis, A. imbricata, Saxegothaea and others are met with in the See also:Andes and other regions in South America. Athrotaxis and Microcachrys are characteristic Australian types. Phyllocladus occurs also in New Zealand, and species of Dacrydium, Araucaria, Agathis and Podocar pus are represented in See also:Australia, New Zealand and the See also:Malay regions.Additional information and CommentsThere are no comments yet for this article.
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