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ANGIOSPERMS
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ANGIOSPERMS . The botanical See also:term " Angiosperm " (ayyeCov, receptacle, and u7r pya, See also:seed) was coined in the See also:form Angiospermae by See also:Paul See also:Hermann in 1690, as the name of that one of his See also:primary divisions of the plant See also:kingdom, which included , flowering See also:plants possessing seeds enclosed in capsules, in contra-distinction to his Gymnospermae, or flowering plants with achenial or schizo-carpic fruits—the whole See also:fruit or each of its pieces being here regarded as a seed and naked. The term and its antonym were maintained by See also:Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Its use with any approach to its See also:modern See also:scope only became possible after See also:Robert See also: The great See also:forest-See also:tree has a shoot, which in the course perhaps of hundreds of years, has See also:developed a wide-spreading See also:system of See also:trunk and branches, bearing on the ultimate twigs or branchlets innumerable leaves, while beneath the See also:soil a widely-branching root-system covers an area of corresponding extent. Between these two extremes is every conceivable gradation, embracing aquatic and terrestrial herbs, creeping, erect or climbing in habit, shrubs and trees, and representing a much greater variety than is to be found in the other subdivision of seed-plants, the Gymnosperms. In See also:internal structure also the variety of See also:tissue-formation far exceeds that found in Gymnosperms (see PLANTS: See also:Anatomy). The vascular bundles of the See also:stem belong to the See also:col-at structure. lateral type, that is to say, the elements of the See also:wood or struct xylem and the bast or phloem stand See also:side by side on the same See also:radius. In the larger of the two great See also:groups into which the Angiosperms are divided, the Dicotyledons, the bundles in the very See also:young stem are arranged in an open See also:ring, separating a central See also:pith from an See also:outer cortex. In each bundle, separating the xylem and phloem, is a layer of See also:meristem or active formative tissue, known as cambium; by the formation of a layer of cambium between the bundles (interfascicular cambium) a See also:complete ring is formed, and a See also:regular periodical increase in thickness results from it by the development of xylem on the inside and phloem on the outside. The soft phloem soon becomes crushed, but the hard wood persists, and forms the great bulk of the stem and branches of the woody perennial. Owing to See also:differences in the See also:character of the elements produced at the beginning and end of the See also:season, the wood is marked out in transverse See also:section into concentric rings, one for each season of growth—the so-called See also:annual rings. In the smaller group, the Monocotyledons, the bundles are more numerous in the young stem and scattered through the ground tissue. Moreover they contain no cambium and the stem once formed increases in See also:diameter only in exceptional cases. As in Gymnosperms, branching is monopodial; See also:dichotomy or the forking of the growing point into two See also:equivalent branches which replace the See also:main stem, is absent both in the See also:case Vegetative of the stem and the root. The leaves show a remark-See also:organs. able variety in form (see LEAF), but are generally small in comparison with the See also:size of the plant; exceptions occur in some Monocotyledons, e.g. in the Aroid See also:family, where in some genera the plant produces one huge, much-branched leaf each season. In rare cases the main See also:axis is unbranched and ends in a See also:flower, as, for instance, in the See also:tulip, where See also:scale-leaves, forming theunderground bulb, green foliage-leaves and coloured floral leaves are See also:borne on one and the same axis. Generally, See also:flowers are formed only on shoots of a higher See also:order, often only on the ultimate branches of a much branched system. A potential See also:branch or bud, either foliage or flower, is formed in the axil of each leaf; sometimes more than one bud arises, as for instance in the See also:walnut, where two or three stand in See also:vertical See also:series above each leaf. Many of the buds remain dormant, or are called to development under exceptional circumstances, such as the destruction of existing branches. For instance, the clipping of a hedge or the lopping of a tree will cause to develop numerous buds which may have been dormant for years. Leaf-buds occasionally arise from the roots, when they are called See also:adventitious; this occurs in many fruit trees, poplars, elms and others. For instance, the young shoots seen springing from the ground around an See also:elm are not seedlings but root-shoots. Frequently, as in many Dicotyledons, the primary root, the See also:original root of the seedling, persists throughout the life of the plant, forming, as often in biennials, a thickened tap-root, as in See also:carrot, or in perennials, a much-branched root system. In many Dicotyledons and most Monocotyledons, the primary root soon perishes, and its See also:place is taken by adventitious roots developed from the stem. The most characteristic feature of the Angiosperm is the flower, which shows remarkable variety in form and elaboration, and supplies the most trustworthy characters for the Flower. distinction of the series and families or natural orders, into which the group is divided. The flower is a shoot (stem bearing leaves) which has a See also:special form associated with the special See also:function of ensuring the fertilization of the See also:egg and the development of fruit containing seed. Except where it is terminal it arises, like the leaf-shoot, in the axil of a leaf, which is then known as a bract. Occasionally, as in See also:violet, a flower arises singly in the axil of an See also:ordinary foliage-leaf; it is then termed axillary. Generally, however, the flower-bearing portion of the plant is sharply distinguished from the foliage leaf-bearing or vegetative portion, and forms a more or less elaborate branch-system in which the bracts are small and scale-like. Such a branch-system is called an inflorescence. The primary function of the flower is to See also:bear the spores. These, as in Gymnosperms, are of two kinds, microspores or See also:pollen-grains, borne in the stamens (or microsporophylls) and megaspores, in which the egg-See also:cell is developed, contained in the ovule, which is borne enclosed in the carpel (or megasporophyll). The flower may consist only of spore-bearing leaves, as in See also:willow, where each flower comprises only a few stamens or two carpels. Usually, however, other leaves are present which are only indirectly concerned with the reproductive See also:process, acting as protective organs for the sporophylls or forming an attractive envelope. These form the perianth and are in one series, when the flower is termed monochlamydeous, or in two series (dichlamydeous). In the second case the outer series (calyx of sepals) is generally green and leaf-like, its function being to protect the See also:rest of the flower, especially in the bud; while the inner series (corolla of petals) is generally See also: The development of the micro-Stamen sporangia and the contained spores (pollen-grains) and pollen. is closely comparable with that of the microsporangia in Gymnosperms or heterosporous ferns. The pollen is set See also:free by the opening (dehiscence) of the anther, generally by means of See also:longitudinal slits, but sometimes by pores, as in the See also:heath family (See also:Ericaceae), or by valves, as in the See also:barberry. It is then dropped or carried by some See also:external See also:agent, See also:wind, water or some member of the See also:animal kingdom, on to the receptive surface of the carpel of the same or another flower. The carpel, or aggregate of carpels forming the See also:pistil or See also:gynaeceum, comprises an ovary containing one or more ovules and a receptive surface or stigma; the stigma is sometimes carried up on a See also:style. The mature pollen-See also:grain is, like other spores, a single cell; except in the case of some submerged aquatic plants, it has a See also:double See also:wall, a thin delicate wall of unaltered See also:cellulose, the endospore or intine, and a tough outer cuticularized exospore or extine. The exospore often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for the distinction of genera or higher groups. Germination of the microspore begins before it leaves the pollen-sac. In very few cases has anything representing prothallial development been observed; generally a small cell (the antheridial or generative cell) is cut off, leaving a larger See also:tube-cell. When placed on the stigma, under favourable circumstances, the pollen-grain puts forth a pollen-tube which grows down the tissue of the style to the ovary, and makes its way along the See also:placenta, guided by projections or hairs, to the mouth of an ovule. The See also:nucleus of the tube-cell has meanwhile passed into the tube, as does also the generative nucleus which divides to form two male- or sperm-cells. The male-cells are carried to their destination in the tip of the pollen-tube. The ovary contains one or more ovules borne on a pla- centa, which is generally some part of the ovary-wall. The development of the ovule, which represents the macrosporangium, is very similar to the process in Gymnosperms; when mature it consists of one or two coats surrounding the central nucellus, except at the See also:apex where an opening, the micropyle, is See also:left. The nucellus is a cellular tissue enveloping one large cell, the embryo-sac or macrospore. The germination of the macrospore consists in the repeated See also:division of its nucleus to form two groups of four, one group at each end of the embryo-sac. One nucleus from each group, the polar nucleus, passes to the centre of the sac, where the two fuse to form the so-called definitive nucleus. Of the three cells at the micropylar end of the sac, all naked cells (the so-called egg-apparatus), one is the egg-cell or oosphere, the other two, which may be regarded as representing abortive egg-cells (in rare cases capable of fertilization), are known as synergidae. The three cells at the opposite end are known as antipodal cells and become invested with a cell-wall. The gametophyte or prothallial See also:generation is thus extremely reduced, consisting of but little more than the male and See also:female sexual cells—the two sperm-cells in the pollen-tube and the egg-cell (with the synergidae) in the embryo-sac. At the See also:period of fertilization the embryo-sac lies in See also:close proximity tube has penetrated, the separating cell-wall becomes absorbed, and the male or sperm-cells are ejected into the embryo-sac. Guided by the synergidae one male-cell passes into the oosphere with which it fuses, the two nuclei uniting, while the other fuses with the definitive nucleus, or, as it is also called, the endosperm nucleus. This remarkable double fertilization as it has been called, although only recently discovered, has been proved to take place in widely-separated families, and both in Monocotyledons and Dicotyledons, and there is every See also:probability that, perhaps with See also:variations, it is the normal process in Angiosperms. Af ter impregnation the fertilized oosphere immediately surrounds itself with a cell-wall and becomes the oospore which by a process of growth forms the embryo of the new plant. The endosperm-nucleus divides rapidly to produce a cellular tissue which fills up the interior of the rapidly-growing embryo-sac, and forms a tissue, known as endosperm, in which is stored a See also:supply of nourishment for the use later on of the embryo. It has long been known that after fertilization of the egg has taken place, the formation of endosperm begins from the endosperm nucleus, and this had come to be regarded as the recommencement of the development of a prothallium after a pause following the reinvigorating See also:union of the polar nuclei. This view is still maintained by those who differentiate two acts of fertilization within the embryo-sac, and regard that of the egg by the firstmale-cell, as the true or generative fertilization, and that of the polar nuclei by the second male gamete as a vegetative fertilization which gives a stimulus to development in correlation with the other. If, on the other See also:hand, the endosperm is the product of an See also:act of fertilization as definite as that giving rise to the embryo itself, we have to recognize that twin-plants are produced within the embryo-sac—one, the embryo, which becomes the angiospermous plant, the other, the endosperm, a See also:short-lived, undifferentiated See also:nurse to assist in the See also:nutrition of the former, even as the subsidiary embryos in a pluri-embryonic Gymnosperm may facilitate the nutrition of the dominant one. If this is so, and the endosperm like the embryo is normally the product of a sexual act, hybridization will give a hybrid endosperm as it does a hybrid embryo, and herein (it is suggested) we may have the explanation of the phenomenon of See also:xenia observed in the mixed endosperms of hybrid races of See also:maize and other plants, regarding which it has only been possible hitherto to assert that they were indications of the See also:extension of the See also:influence of the pollen beyond the egg and its product. This would not, however, explain the formation of fruits intermediate in size and colour between those of crossed parents. The signification of the coalescence of the polar nuclei is not explained by these new facts, but it is noteworthy that the second male-cell is said to unite sometimes with the apical polar nucleus, the See also:sister of the egg, before the union of this with the basal polar one. The See also:idea of the endosperm as a second subsidiary plant is no new one; it was suggested long ago in explanation of the coalescence of the polar nuclei, but it was then based on the See also:assumption that these represented male and female cells, an assumption for which there was no See also:evidence and which was inherently improbable. The See also:proof of a coalescence of the second male nucleus with the definitive nucleus gives the conception a more See also:stable basis. The antipodal cells aid more or less in the process of nutrition of the developing embryo, and may undergo multiplication, though they ultimately disintegrate, as do also the synergidae. As in Gymnosperms and other groups an interesting qualitative See also:change is associated with the process of fertilization. The number of chromosomes (see PLANTS: See also:Cytology) in the nucleus of the two spores, pollen-grain and embryo-sac, is only See also:half the number found in an ordinary vegetative nucleus; and this reduced number persists in the cells derived from them. The full number is restored in the See also:fusion of the male and female nuclei in the process of fertilization, and remains until the formation of the cells from which the spores are derived in the new generation. In several natural orders and genera departures from the course of development just described have been noted. In the natural order See also:Rosaceae, the series Querciflorae, and the very anomalous genus See also:Casuarina and others, instead of a single macrospore a more or less extensive sporogenous tissue is formed, but only one cell proceeds to the formation of a functional female cell. In Casuarina, Juglans and the order Corylaceae, the pollen-tube does not enter by means of the micropyle, but passing down the ovary wall and through the placenta, enters at the chalazal end of the ovule. Such a method of entrance is styled chalazogamic, in contrast to the porogamic or ordinary method of approach by means of the micropyle. The result of fertilization is the development of the ovule into the seed. By the segmentation of the fertilized egg, now invested by cell-membrane, the embryo-plant arises. A varying Pistil and embryo-sac. Fertlllsa- to the opening of the micropyle, into which the pollen- Ebry- number of transverse segment-walls transform it into o' a See also:pro-embryo—a cellular See also:row of which the cell nearest the micropyle becomes attached to the apex of the embryo-sac, and thus fixes the position of the developing, embryo, while the terminal cell is projected into its cavity. In Dicotyledons the shoot of the embryo is wholly derived from the terminal cell of the pro-embryo, from the next cell the root arises, and the remaining ones form the suspensor. In many Monocotyledons the terminal cell forms the-cotyledonary portion alone of the shoot of the embryo, its axial part and the root being derived from the adjacent cell; the cotyledon is thus a terminal structure and the apex of the primary stem a lateral one—a See also:condition in marked contrast with that of the Dicotyledons. In some Monocotyledons, however, the cotyledon is not really terminal. The primary root of the embryo in all Angiosperms points towards the inicropyle. The developing embryo at the end of the suspensor grows out to a varying extent into the forming endosperm, from which by surface absorption it derives See also:good material for growth; at the same time the suspensor plays a See also:direct part as a See also:carrier of nutrition, and may even develop, where perhaps no endosperm is formed, special absorptive "suspensor roots" which invest the developing embryo, or pass out into the See also:body and coats of the ovule, or even into the placenta. In some cases the embryo or the embryo-sac sends out suckers into the nucellus and ovular integument. As the embryo develops it may absorb all the See also:food material available, and See also:store, either in its cotyledons or in its hypocotyl, what is not immediately required for growth, as reserve-food for use in germination, and by so doing it increases in size until it may fill entirely the embryo-sac; or its absorptive See also:power at this See also:stage may be limited to what is necessary for growth and it remains of relatively small size, occupying but a small area of the embryo-sac, which is otherwise filled with endosperm in which the reserve-food is stored. There are also intermediate states. The position of the embryo in relation to the endosperm varies, sometimes it is internal, sometimes external, but the significance of this has not yet been established. The formation of endosperm starts, as has been stated, from the endosperm nucleus. Its segmentation always begins before that of the egg, and thus there is timely preparation for the See also:nursing of the young embryo. If in its extension to contain the new formations within it the embryo-sac remains narrow, endosperm formation proceeds upon the lines of a cell-division, but in wide embryo-sacs the endosperm is first of all formed as a layer of naked cells around the wall of the sac, and only gradually acquires a pluricellular character, forming a tissue filling the sac. The function of the endosperm is primarily that of nourishing the embryo, and its basal position in the embryo-sac places it favourably for the absorption of food material entering the ovule. Its duration varies with the precocity of the embryo. It may be wholly absorbed by the progressive growth of the embryo within the embryo-sac, or it may persist as a definite and more or less conspicuous constituent of the seed. When it persists as a massive See also:element of the seed its nutritive function is usually apparent, for there is accumulated within its cells reserve-food., and according to the dominant substance it is starchy, oily, or See also:rich in cellulose, See also:mucilage or proteid. In cases where the embryo has stored reserve food within itself and thus provided for self-nutrition, such endosperm as remains in the seed may take on other functions, for instance, that of water-absorption. Some deviations from the usual course of development may be noted. Parthenogenesis, or the development of an embryo from an egg-cell without the latter having been fertilized has been de-scribed in See also:species of Thalictrusn, Antennaria and Alchemilla. Polyembryony is generally associated with the development of cells other than the egg-cell. Thus in Erythronium and Limnocharis the fertilized egg may form a See also:mass of tissue on which several embryos are produced. Isolated cases show that any of the cells within the embryo-sac may exceptionally form an embryo, e.g. the synergidae in species of See also:Mimosa, See also:Iris and See also:Allium, and in the last-mentioned the antipodal cells also. In Coelebogyne (See also:Euphorbiaceae) and in See also:Funkia (See also:Liliaceae) polyembryony results from an adventitious See also:production of embryos from the cells of the nucellus around the See also:top of the embryo-sac. In a species of Allium, embryos have been found developing in the same individual from the egg-cell, synergids, antipodal cells and cells of the nucellus. In two Malayan species of Balanophora, the embryo is developed from a cell of the endosperm, which is formed from the upper polar nucleus only, the egg apparatus becoming disorganized. The last-mentioned case has been regarded as representing an apogamous development of the sporophyte from the gametophyte comparable to the cases of See also:apogamy described in Ferns. But the great diversity of these abnormal cases as shown in the examples cited above suggests the use of great caution in formulating definite morphological theories upon them. As the development of embryo and endosperm proceeds withinthe embryo-sac, its wall enlarges and commonly absorbs the substance of the nucellus (which is likewise enlarging) to near its outer limit, and combines with it and the integument Fruit and to form the seed-coat; or the whole nucellus and even seed. the integument may be absorbed. In some plants the nucellus is not thus absorbed, but itself becomes a seat of de-posit of reserve-food constituting the perisperm which may coexist with endosperm, as in the water-See also:lily order, or may alone form a food-reserve for the embryo, as in Canna. Endospermic food-reserve has evident advantages over perispermic, and the latter is comparatively rarely found and only in non-progressive series. Seeds in which endosperm or perisperm or both exist are commonly called albuminous or endospermic, those in which neither is found are termed exalbuminous or exendospermic. These terms, extensively used by systematists, only refer, however, to the grosser features of the seed, and indicate the more or less evident occurrence of a food-reserve; many so-called exalbuminous seeds show to microscopic examination a distinct endosperm which may have other than a nutritive function. The presence or See also:absence of endosperm, its relative amount when present, and the position of the embryo within it, are valuable characters for the distinction of orders and groups of orders. Meanwhile the ovary wall has developed to form the fruit or pericarp, the structure of which is closely associated with the manner of See also:distribution of the seed. Frequently the influence of fertilization is See also:felt beyond the ovary, and other parts of the flower take part in the formation of the fruit, as the floral receptacle in the See also:apple, See also:strawberry and others. The character of the seed-coat bears a definite relation to that of the fruit. Their function is the twofold one of protecting the embryo and of aiding in dissemination; they may also directly promote germination. If the fruit is a dehiscent one and the seed is therefore soon exposed, the seed-coat has to provide for the See also:protection of the embryo and may also have to secure dissemination. On the other hand, indehiscent fruits See also:discharge these functions for the embryo, and the seed-coat is only slightly developed. Dissemination is effected by the agency of msse water> of See also:air, a of animals—and fruits and seeds are atnonion.7ntherefore grouped in respect of this as liydrophilous, anemophilous and zooidiophilous. The needs for these are obvious—buoyancy in water and resistance to wetting for the first, some form of See also:parachute for the second, and some attaching mechanism or attractive structure for the third. The methods in which these are provided are of See also:infinite variety, and any and every part of the flower and of the inflorescence may be called into requisition to supply the See also:adaptation (see FRUIT). Special outgrowths, arils, of the seed-coat are of frequent occurrence. In the feature of fruit and seed, by which the distribution of Angiosperms is effected, we have a distinctive character of the class. In Gymnosperms we have seeds, and the carpels may become modified and close around these, as in Pinus, during the process of ripening to form an See also:imitation of a See also:box-like fruit which subsequently opening allows the seeds to See also:escape; but there is never in them the closed ovary investing from the outset the ovules, and ultimately forming the ground-See also:work of the fruit. Their fortuitous dissemination does not always bring seeds upon a suitable nidus for germination, the primary essential of which is a sufficiency of moisture, and the duration of vitality of the embryo is a point of See also:interest. Some (iermina- seeds retain vitality for a period of many years, thoughseeddon. . there is no See also:warrant for the popular notion that genuine " See also:mummy See also:wheat " will germinate; on the other hand some seeds lose vitality in little more than a See also:year. Further, the older the seed the more slow as a general See also:rule will germination be in starting, but there are notable exceptions. This pause, often of so long duration, in the growth of the embryo between the time of its perfect development within the seed and the moment of germination, is one of the remarkable and distinctive features of the life of Spermatophytes. The aim of germination is the fixing of the embryo in the soil, effected usually by means of the root, which is the first part of the embryo to appear, in preparation for the See also:elongation of the epicotyledonary portion of the shoot, and there is infinite variety in the details of the process. In albuminous Dicotyledons the cotyledons act as the absorbents of the reserve-food of the seed and are commonly brought above ground (epigeal), either withdrawn from the seed-coat or carrying it upon them, and then they serve as the first green organs of the plant. The part of the stem below the cotyledons (hypocotyl) commonly plays the greater part in bringing this about. Ex-albuminous Dicotyledons usually store reserve-food in their cotyledons, which may in germination remain below ground (hypogeal). In albuminous Monocotyledons the cotyledon itself, probably in consequence of its terminal position, is commonly the agent by which the embryo is thrust out of the seed, and it may function solely as a feeder, its extremity developing as a sucker through which the endosperm is absorbed, or it may become the first green See also:organ, the terminal sucker dropping off with the seed-coat when the endosperm is exhausted. Exalbuminous Monocotyledons are either hydrophytes or strongly hygrophilous plants and have often See also:peculiar features in germination. Distribution by seed appears to satisfy so well the requirements of Angiosperms that distribution by vegetative buds is only an occasional process. At the same time every bud on a vegetative shoot has the capacity to form a new plant if placed rro- duction. in suitable conditions, as the horticultural practice of See also:propagation by cuttings shows; in nature we see plants spreading by the rooting of their shoots, and buds we know may be freely formed not only on stems but on leaves and on roots. Where detachable buds are produced, which can be transported through the air to a distance, each of them is an incipient shoot which may have a root, and there is always reserve-food stored in some part of it. In essentials such a bud resembles a seed. A relation between such vegetative distribution buds and production of flower is usually marked. Where there is free formation of buds there is little flower and commonly no seed, and the converse is also the case. Viviparous plants are an See also:illustration of substitution of vegetative buds for flower. The position of Angiosperms as the highest plant-group is unassailable, but of the point or points of their origin from the general stem of the plant kingdom, and of the path Phylogeny or paths of their evolution, we can as yet say little. and Until well on in the Mesozoic period See also:geological See also:history taxonomy. tells us nothing about Angiosperms, and then only by their vegetative organs. We readily recognize in them now-adays the natural classes of Dicotyledons and Monocotyledons distinguished alike in vegetative and in reproductive construction, yet showing remarkable parallel sequences in development; and we see that the Dicotyledons are the more advanced and show the greater capacity for further progressive evolution. But there is no See also:sound basis for the assumption that the Dicotyledons are derived from Monocotyledons; indeed, the palaeontological evidence seems to point to the Dicotyledons being the older. This, however, does not entitle us to assume the origin of Monocotyledons from Dicotyledons, although there is manifestly a temptation to connect helobic forms of the former with ranal ones of the latter. There is no doubt that the phylum of Angiosperms has not sprung from that of Gymnosperms. Within each class the flower-characters as the essential feature of Angiosperms supply the See also:clue to phylogeny, but the uncertainty regarding the construction of the primitive angiospermous flower gives a fundamental point of divergence in attempts to construct progressive sequences of the families. Simplicity of flower-structure has appeared to some to be always primitive, whilst by others it has been taken to be always derived. There is, however, abundant evidence that it may have the one or the other character in different cases. Apart from this, botanists are generally agreed that the concrescence of parts of the flower-whorls—in the gynaeceum as the seed-covering, and in the corolla as the seat of attraction, more than in the androecium and the calyx—is an indication of advance, as is also the concrescence that gives the condition of epigyny. Dorsiventrality is also clearly derived from radial construction, and anatropy of the ovule has followed atropy. We should expect the albuminous See also:state of the seed to be an antecedent one to the ex-albuminous condition, and the See also:recent discoveries in fertilization tend to confirm this view. Amongst Dicotyledons the gamopetalous forms are admitted to be the highest development and a dominant one of our epoch. Advance has been along two lines, markedly in relation to insect-pollination, one of which has culminated in thehypogynous epipetalous bicarpellate forms with See also:dorsiventral often large and loosely arranged flowers such as occur in Scrophvlariaceae, and the other in the epigynous bicarpellate small-flowered families of which the See also:Compositae represent the most elaborate type. In the polypetalous forms progression from hypogyny.to epigyny is generally recognized, and where dorsiventrality with insect-pollination has been established, a dominant group has been developed as in the See also:Leguminosae. The starting-point of the class, however, and the position within it of apetalous families with frequently unisexual flowers, have provoked much discussion. In Monocotyledons a similar advance from hypogyny to epigyny is observed, and from the dorsiventral to the radial type of flower. In this connexion it is noteworthy that so many of the higher forms are adapted as bulbous geophytes, or as aerophytes to special xerophilous conditions. The Gramineae offer a prominent example of a dominant self-pollinated or wind-pollinated family, and this may find explanation in a multiplicity of factors. Though best known for his artificial (or sexual) system, Linnaeus was impressed with the importance of elaborating a natural system of arrangement in which plants should be arranged according to their true See also:affinities. In his Philosophia Botanica (1751) Linnaeus. grouped the genera then known into sixty-seven orders (fragmenta), all except five of which are Angiosperms. He gave names to these but did not characterize them or See also:attempt to arrange them in larger groups. Some represent natural groups and had in several cases been already recognized by See also:Ray and others, but the See also:majority are, in the See also:light of modern knowledge, very mixed. Well-defined polypetalous and gamopetalous genera sometimes occur in the same order, and even Monocotyledons and Dicotyledons are classed together where they have some striking physiological character in See also:common. Work on the lines suggested by the Linnaean fragmenta was continued in See also:France by See also:Bernard de See also:Jussieu and his See also:nephew, See also:Antoine See also:Laurent, and the arrangement suggested by the latter in his Genera Plantarum secundum Ordines Naturales disposita (1789) is the first which can claim to be a natural system. The orders are carefully characterized, and those of Angiosperms are grouped in fourteen classes under the two main divisions Monocotyledons and Dicotyledons. The former comprise three classes, which are distinguished by the relative position of the stamens and ovary; the eleven classes of the latter are based on the same set of characters and fall into the larger subdivisions Apetalae, Monopetalae and Polypetalae, characterized respectively by absence, union or freedom of- the petals, and a subdivision, Didines Irregulares, a very unnatural group, Including one class only. A. P. de See also:Candolle introduced several improvements into the system. In his arrangement the last sub-division disappears, and the Dicotyledons fall into two groups, a larger containing those in which both calyx and corolla are present in the flower, and a smaller, Monochlamydeae, representing the Apetalae and Diclines Irregulares of Jussieu. The dichlamydeous group is subdivided into three, Thalamiflorae, Calyciflorae and Corolliflorae, depending on the position and union of the petals. This, which we may distinguish as the See also:French system, finds its most perfect expression in the classic Genera Plantarum (1862–1883) of See also:Bentham and See also: Inferae. Gamopetalae Heteromerae. Bicarpellatae. Monochlamydeae in eight series. Monocotyledons in seven series. Of the Polypetalae, series 1, Thalamiflorae, is characterized by hypogynous petals and stamens, and contains 34 orders distributed in 6 larger groups or cohorts. - Series 2, Disciflorae, takes its name from a development of the floral axis which forms a ring or See also:cushion at the See also:base of the ovary or is broken up into glands; the ovary is See also:superior. It contains 23 orders in 4 cohorts. Series 3, Calyciflorae, has petals and stamens perigynous, or sometimes superior. It contains 27 orders in 5 cohorts. Of the Gamopetalae, series 1, Inferae, has an interior ovary and stamens usually-as many as the corolla-lobes. It contains 9 orders in 3 cohorts. Series 2, Heteromerae, has generally a superior ovary, stamens as many as the corolla-lobes or more, and more than two carpels. It contains 12 orders in 3 cohorts. Series 3, Bicarpellatae, has generally a superior ovary and usually two carpels. It contains 24 orders in 4 cohorts. The eight series of Monochlamydeae, containing 36 orders, form groups characterized mainly by differences in the ovary and ovules, and are now recognized as of unequal value. The seven series of Monocotyledons represent a sequence beginning with the most complicated epigynous orders, such as Orchideae and Scitamineae, and passing through the petaloid hypogynous orders (series Coronarieae) of which Liliaceae is the representative to See also:juncaceae and the palms (series Calycinae) where the perianth loses its petaloid character and thence to the Aroids, See also:screw-pines and others where it is more or less aborted (series Nudiflorae). Series 6, Apocarpeae, is characterized by 5 carpels, and in the last series Glumaceae, great simplification in the flower is associated with a grass-like habit. The sequence of orders in the polypetalous subdivision of Dicotyledons undoubtedly represents a progression from simpler to more elaborate forms, but a great See also:drawback to the value of the system is the inclusion among the Monochlamydeae of a number of orders which are closely allied with orders of Polypetalae though differing in absence of a corolla. The See also:German systematist, A. W. Eichler, attempted to remove this disadvantage which since the time of Jussieu had characterized the French system, and in 1883 grouped the Dicotyledons in two subclasses. The earlier Choripetalae embraces the Polypetalae and Monochlamydae of the French systems. It includes 21 series, and is an attempt to arrange as far as possible in a linear series those orders which are characterized by absence or freedom of petals. The second subclass, Gamopetalae, includes 9 series and culminates in those which show the most elaborate type of flower, the series Aggregatae, the See also:chief representative of which• is the great and wide-spread order Compositae. A modification of Eichler's system, embracing the most recent views of the affinities of the orders of Angiosperms, has been put forward by Dr Adolf Engler of See also:Berlin, who adopts the suggestive names Archichlamydeae and Metachlamydeae for the two subdivisions of Dicotyledons. Dr Engler is the See also:principal editor of a large series of volumes which, under the See also:title See also:Die naturlichen Pflanzenfamilien, is a systematic See also:account of all the known genera of plants and represents the work of many botanists. More recently in Das Pflanzenreich the same author organized a series of complete monographs of the families of seed-plants. As an attempt at a phylogenetic arrangement, Engler's system is now preferred by many botanists. More recently a startling novelty in the way of system has been produced by See also:van Tieghem, as follows: Monocotyledons. Liorhizal Dicotyledons. Dicotyledons. Additional information and CommentsThere are no comments yet for this article.
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