ANGIOSPERMS. The botanical term receptacle, and arrEppa, seed) was coined in the form Angiosper mae by Paul Hermann in 169o, as the name of that one of his primary divisions of the plant kingdom which included flowering plants possessing seeds enclosed in capsules, in contradistinction to his Gymnospermae, or flowering plants with fruits containing only one seed or dividing into distinct one-seeded portions—the whole fruit or each of its pieces being here regarded as a naked seed. These terms were maintained by Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Their use with any approach to their modern scope only became possible after Robert Brown had es tablished in 1827 the existence of truly naked seeds in the cycads and conifers, entitling them to be correctly called gymnosperms. From that time onwards, so long as these gymnosperms were, as was usual, reckoned as dicotyledonous flowering plants, the term angiosperm was used antithetically by botanical writers, but with varying limitation, as a group-name for other dicotyledonous plants. Wilhelm Hof meister's brilliant discovery in 1851 of the changes occurring in the embryo-sac of flowering plants, and his determination of the correct relationships of these with the cryptogams (non-flowering plants), fixed the true position of gymnosperms as a class distinct from Dicotyledons, and the term angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than gymnosperms, and as including, therefore, the classes of Dicoty ledons and Monocotyledons. This is the sense in which the term is used in this article.
The trend of the evolution of the plant kingdom has been in the direction of the establishment of a vegetation of fixed habit and adapted to the vicissitudes of a life on land, and the angiosperms are the highest expression of this evolution and constitute the dominant vegetation of the earth's surface at the present epoch. There is no land-area, from the poles to the equator, where plant life is possible, upon which angiosperms are not found. They occur also abundantly in the shallows of rivers and fresh-water lakes, and in less numbers in salt lakes and in the sea ; such aquatic angiosperms are not, however, primitive forms, but are derived from immediate land-ancestors. Associated with this diversity of habitat is great variety in general form and manner of growth. The familiar duckweed which covers the surface of a pond consists of a tiny green thalloid shoot, one, that is, which shows no distinction of parts—stem and leaf, and a simple root growing vertically downwards into the water. The great forest tree has a shoot, which in the course perhaps of hundreds of years, has developed a wide-spreading system of trunk and branches, bearing on the ultimate twigs or branchlets innumer able leaves, while beneath the 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 rare cases the main axis is unbranched and ends in a flower, as, for instance, in the tulip, where scale-leaves, forming the under ground bulb, green foliage-leaves and coloured floral leaves are borne on one and the same axis. Generally, flowers are formed only on shoots of a higher order, of ten only on the ultimate branches of a much branched system. A potential branch or bud, either foliage or flower, is formed in the axil of each leaf ; some times more than one bud arises, as for instance in the walnut, where two or three stand in vertical 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 adventitious ; this occurs in many fruit trees, poplars, elms and others. For instance, the young" shoots seen springing from the ground around an elm are not seedlings but root-shoots. Frequently, as in many Dicotyledons, the primary root, the original root of the seedling, persists throughout the life of the plant, forming, as often in biennials, a thickened tap-root, as in carrot, or in perennials, a much-branched root-system. In many Dicotyledons and most Monocotyledons, the primary root soon perishes and its place is taken by adventitious roots developed from the stem.
Flower.—Themost characteristic feature of the angiosperm is the flower, which shows remarkable variety in form and elabo ration and supplies the most trustworthy characters for the dis tinction of the orders and families into which the group is divided. The flower is a shoot (a stem bearing leaves) with a special form associated with the special function of ensuring the ferti lization of the 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 the violet, a flower arises singly in the axil of an ordinary f oliage 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 bear the spores (minute one-celled re productive organs) . These, as in gymnosperms, are of two kinds, microspores or pollen-grains, borne in the stamens (or micro sporophylls) and megaspores, in which the egg-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 the willow, where each flower comprises only a few stamens or two carpels. Usually, however, other leaves are pres ent which are only indirectly concerned with the reproductive process, acting as protective organs for the sporophylls or form ing 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 rest of the flower, especially in the bud ; while the inner series (corolla of petals) is generally white or brightly coloured, and more delicate in structure, its function being to attract the particular insect or bird by agency of which transfer ence of pollen (pollination) is effected. The insect, or bird, is attracted by the colour and scent of the flower, and frequently also by nectar which is secreted in some part of the flower. (For fur ther details on the form and arrangement of the flower and its parts, see FLOWER.) Stamen and Pollen.—Eachstamen generally bears four pol len-sacs (microsporangia) which are associated to form the anther and carried up on a stalk or filament. The development of the microsporangia and the contained spores (pollen-grains) is closely comparable with that of the microsporangia in gymnosperms or heterosporous ferns. The pollen is set free by the opening (dehiscence) of the anther, generally by means of longitudinal slits, but sometimes by pores, as in the heath family (Ericaceae), or by valves, as in the barberry. It is then dropped or carried by some external agent, wind, water, or some member of the animal kingdom, on to the receptive surface of the carpel of the same or another flower. The carpel, or aggregate of carpels forming the pistil or gynaeceum, comprises an ovary containing one or more ovules and a receptive surface or stigma; the stigma is sometimes carried up on a style. The mature pollen-grain is, like other spores, a single cell; except in the case of some submerged aquatic plants, it has a double wall, a thin delicate wall of unaltered 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 of ten 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 the development of tissue, com parable with the prothallium of a fern, been observed ; generally a small cell (the antheridial or generative cell) is cut off, leaving a larger 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 placenta, guided by projections or hairs, to the mouth of an ovule. The 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.
Fertilization.—Atthe period of fertilization the embryo-sac lies in close proximity to the opening of the micropyle, into which the pollen-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 f uses with the endosperm-nucleus. After impregnation the ferti lized oosphere immediately surrounds itself with a cell-wall and becomes the oospore, which by a process of growth forms the em bryo 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, endosperm, in which is stored a supply of nourishment for the use later on of the embryo.
We can recognize, therefore, two products of fertilization—one, the embryo, which becomes the future plant ; the other, the endo sperm, a short-lived undifferentiated nurse to assist in the nutri tion of the former. The endosperm, like the embryo, being the product of a sexual act, hybridization will give a hybrid endosperm as it does a hybrid embryo, and herein we may have the explanation of the phenomenon of "xenia" observed in the mixed endosperms of hybrid races of maize and other plants. The antipodal cells aid more or less in the process of nutrition of the developing embryo, and may undergo multiplication, though they ultimately disinte grate, as do also the synergidae. As in gymnosperms and other groups an interesting qualitative change is associated with the process of fertilization. The number of chromosomes (see CYTOLOGY) in the nucleus of the two spores, pollen-grain and embryo-sac is only 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 fusion of the male and female nuclei in the process of fertilization and re mains until the formation of the cells from which the spores are derived in the new generation.
The above is a general account representing the normal sequence of events, but various departures have been noted. Thus in the family Rosaceae, the order Fagales and the anomalous genus Casuarina and others, instead of a single macrospore a sporo genous tissue is formed, but only one cell proceeds to the forma tion of a functional female cell. In Casuarina, Juglans and the family 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 opposite, or 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 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 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-sac 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 ab sorption 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 element of the seed its nutritive function is usually apparent, for there is accumulated within its cells reserve-food, and according to the dominant sub stance it is starchy, oily, or rich in cellulose, 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 described in species of Thalictrum, Antennaria, and Alchemilla. Polyembryony is generally associated with the development of cells other than the egg-cell. Thus in Erythronium and Limnoch aris the fertilized egg may form a 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 Mimosa, Iris, and Allium, and in the last-mentioned the antipodal cells also. In Coelebogyne (Euphor biaceae) and in Funkia (Liliaceae) polyembryony results from an adventitious production of embryos from the cells of the nucellus around the 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 apog amous development of the sporophyte from the gametophyte comparable to the cases of apogamy described in ferns. But the great diversity of these abnormal cases as shown in the examples cited above suggests the use of much caution in formulating definite morphological theories upon them.
As the development of embryo and endosperm proceeds within the embryo-sac its wall enlarges and commonly absorbs the sub stance of the nucellus (which is likewise enlarging) to near its outer limit, and combines with it and the integument to form the seed-coat ; or the whole nucellus and even the integument may be absorbed. In some plants the nucellus is not thus absorbed, but it self becomes a seat of deposit of reserve-food constituting the perisperm which may coexist with endosperm, as in the water-lily family (Nymphaeaceae), or may alone form a food-reserve for the embryo, as in Canna. Endospermic food-reserve has evi dent advantages over perispermic, and the latter is compara tively rarely found. 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 ex endospermic. The presence or absence of endosperm, its relative amount when present, and the position of the embryo within it are valuable characters for the distinction of families and orders, or groups of families. Meanwhile the ovary-wall has developed to form the fruit or pericarp, the structure of which is closely asso ciated with the manner of distribution of the seed. Frequently the influence of fertilization is felt beyond the ovary, and other parts of the flower take part in the formation of the fruit, as the floral receptacle in the apple, strawberry, and others. The character of the seed-coat bears a definite relation to that of the fruit. Its function is the twofold one of protecting the embryo and of aiding in dissemination; it 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 protection of the embryo and may also have to secure dissemination. On the other hand, inde hiscent fruits discharge these functions for the embryo, and the seed-coat is only slightly developed.
Dissemination.—Dissemination is effected by an explosive mechanism resident in the fruit or seed, or by aid of some external agency—water, air, or animals. The need for this is obvious— buoyancy in water and resistance to wetting for the first, some sort of parachute for the second, and some attaching mechanism or attractive structure for the third. The methods in which these are provided are of infinite variety, and any and every part of the flower and of the inflorescence may be called into requisition to supply the adaptation (see FRUIT). Special outgrowths, arils, of the seed-coat are of frequent occurrence. In the feature of the fruit and seed, by which the distribution of angiosperms is effected, we have a distinctive character of 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 imi tation of a box-like fruit which, subsequently opening, allows the seeds to escape ; but there is never in them the closed ovary invest ing from the outset the ovules and ultimately forming the ground work of the fruit.
Germination of Seed.—Their fortuitous dissemination does not always bring seeds upon a suitable nidus for germination, the first need for which is a sufficiency of moisture, and the duration of vitality of the embryo is a point of interest. Some seeds retain vitality for a period of many years, though there is no warrant for the popular notion that genuine "mummy wheat" will germinate; on the other hand some seeds lose vitality in a very short time. Further, the older the seed the more slow as a general rule will germination be in starting; but there are notable exceptions. This pause, often of so long duration, in the growth of the embryo be tween 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 seed-plants (spermatophytes) . The aim of germination is the fixing of the embryo in the soil, effected us ually by means of the root, which is the first part of the embryo to appear, in preparation for the elongation of the portion of the shoot above the cotyledons (epicotyl), and there is infinite variety in the details of the process. In endospermic Dicotyledons the cotyledons act as the absorbents of the reserve-food of the seed and are commonly brought above ground (epigeal), either with drawn 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. Exendospermic Dicotyledons usually store reserve-food in their cotyledons, which may in germination remain below ground (hypogeal). In endospermic Monocotyle dons 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 organ, the terminal sucker drop ping off with the seed-coat when the endosperm is exhausted. Exendospermic Monocotyledons are either hydrophytes or strongly hygrophilous plants and have often peculiar features in germi nation.
A great drawback to the value of this system is the inclusion among the Monochlamydeae of a number of families which are closely allied with families of Polypetalae though differing in ab sence of a corolla. The German systematist, A. W. Eichler, at tempted 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 subclass, Chori petalae, embraces the Polypetalae and Monochlamydeae of the French and English systems, and is an attempt to arrange as far as possible in a linear series those families which are characterized by absence or freedom of petals. A modification of Eichler's sys tem, embracing more recent views of the affinities of the families of angiosperms, has been put forward by Dr. Adolf Engler of Berlin, who adopts the suggestive names Archichlamydeae and Metachlamydeae for the two subdivisions of Dicotyledons. Engler regarded his system as phylogenetic ; simplicity of floral structure was considered primitive and the earlier orders (as groups of families are now termed) in his system are thus characterized. But he recognized that a simple type of flower may also appear in an advanced group and is then obviously derived ; and many botanists would regard all families characterized by simple flowers as derived from more advanced forms.
Divergent views as to the origin of the angiosperms have some bearing on this problem. The angiosperms appear in the Creta ceous period as a well-developed and widely distributed group of plants with the characters of the existing class. The fossil remains may be distributed among existing families and genera and give no clue to the early history of the group.
The great antiquity of the other class of seed-plants, the gym nosperms, has suggested that the origin of the angiosperms should be sought within it. Thus Richard Wettstein seeks to derive a simple form of flower from an ancestor, resembling Ephedra (Gnetaceae), and regards the simple flower as primitive in the angiosperms. A view which has found considerable support re gards the extinct Mesozoic Cycadeoidea, a group allied to the cycads, as the nearest approach known to the ancestral angiosperm stock. The "flower" of Bennettites (see PALAEOBOTANY), which has an elongated axis bearing sterile leaves below and micro- and macro-sporophylls above, is regarded as the forerunner of the angiosperm flower as exhibited, for instance, in Magnolia, a perfect bisexual flower with free sepals, petals, stamens, and carpels fol lowing in spiral succession on an elongated floral receptacle. On this theory, which has been developed by E. A. N. Arber and J. W. Parkin (see Journ. Linn. Soc. [Bot.] xxxviii. 29, 1907), this type of flower represents the starting point of a phylogenetic system of classification of Dicotyledons. Such are the systems elaborated by C. E. Bessey in America, Hans Hallier in Germany, and recently by John Hutchinson in England.
It is, however, probable that the original stock of the angio sperms has not yet been traced. A vast number of forms must have arisen and become extinct in the course of evolution of the group, and among these may be some of which certain of the exist ing simple-flowered groups are surviving representatives.
The relation of the two divisions, Dicotyledons and Monocoty ledons, has also been much debated. Had each group a separate origin or has one been derived from the other? Botanists who up hold the Bennettites ancestry of Dicotyledons would derive the Monocotyledons from the base of the dicotyledonous stock. On the other hand strong arguments have been adduced in favour of its distinct origin, the two divisions representing a parallel de velopment.
A phylogenetic tree including the whole plant-kingdom has re cently been elaborated by Carl Mez and H. Ziegenspeck, based on the reaction to serum of members of the various families (see Von C. Mez and H. Ziegenspeck, "Der Konigsberger Serodiagnos tische Stammbaum," Mez, Botanisches Archiv. xiii. 482 [1926]).
BIBLIOGRAPHY.-The reader will find in the following works details Bibliography.-The reader will find in the following works details of the subject and references to the literature: Bentham and Hooker, Genera Plantarum (1862-83) ; A. W. Eichler, Bluthendiagramme (Leipzig, ; Engler and Prantl, Die naturlichen Pflanzen fawilien (Leipzig, 1887-99) ; A. Engler, Syllabus der Pflanzenfamilien, ed. io (1924) ; H. Solereder, Systematic Anatomy of the Dicotyledons, Eng. ed. (Oxford, 1908) ; Coulter and Chamberlain, Morphology of Angiosperms (1903) ; A. B. Rendle, Classification of Flowering Plants. i. Monocotyledons, 1904, ii. Dicotyledons (Cambridge, 1925) ; H. Hallier, "Provisional Scheme of the Natural (Phylogenetic) System of Flowering Plants," New Phytologist, iv. (19os) ; Knuth, Handbook of Flower Pollination, Eng. trans. (Oxford, 1906-o9) ; C. E. Bessey, "The phylogenetic taxonomy of flowering plants," Annals Missouri Botan. Garden. ii. (1915) ; R. Wettstein, Handbuch der Systematischen Botanik. ed. 2 (1923) ; A. Arber, Monocotyledons: A morphological study (Cambridge, 1925) ; J. Hutchinson, Families of Flowering Plants, i. Dicotyledons (1926) . B. B. ; A. B. R.)