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(also Magnoliophyta or Anthophyta), a division of seed plants. Angiosperms are characterized by the presence of a true flower, which differs from the strobiles of gymnosperms in that the megasporophyll is converted into a carpel. The carpel concresces at the edges to form a closed cavity in which the ovules develop. Double fertilization occurs in all angiosperms. There are approximately 250,000 species, distributed throughout the world. The plants are particularly widespread in humid tropics. Angiosperms are divided into two classes: dicotyledons and monocotyledons. The Angiospermae arose in the Lower Cretaceous. Of all the divisions of the plant kingdom, angiosperms play the largest role in human life.
(also Magnoliophyta; also known as flowering plants or angiosperms), the group of higher plants that produce a flower. The Angiospermae embrace more than 400 families, including more than 12,000 genera and probably no fewer than 235,000 species. They include considerably more species than all other groups of higher plants taken together.
Distinctive features. The ovules of the Angiospermae, in contrast to those of the Gymnospermae, are contained by a more or less closed ovary chamber formed in one or several fused carpels. The most distinctive feature of the Angiospermae and the most important characteristic that distinguishes the flower from the gymnosperm strobilus is the stigma, which in primitive plants extends along the suture of the carpel. The gametophytes in the Angiospermae, being extremely simplified and tiny, are able to develop at a considerably faster rate than those of the Gymnospermae. They are formed as a result of a minimal number of mitotic divisions, using a minimal amount of matter. Even the more complex female gametophyte (embryo sac) develops as a result of only three mitotic divisions (preceded by two meiotic divisions of the megaspore). In contrast, in the Gymnospermae the female gametophyte develops as a result of at least nine divisions. The development of the male gametophyte of flowering plants, together with the process of gametogenesis, is reduced to only two mitotic divisions. Because of the sharp curtailment of the process of individual development (ontogenesis) and extreme reduction, the gametophytes of flowering plants lost their gametangia—antheridia and archegonia. Gametogenesis occurs at such an early stage in the development of the gametophyte that gametangia cannot develop, even in an embryonic form. As a result, the formation of the gametes themselves is curtailed, especially of the male gametes, or sperm. Thus the male gametes have become greatly reduced.
Another distinguishing feature of the Angiospermae is double fertilization. This process greatly differentiates them from all other plant groups in the world. One of two sperm nuclei unites with the ovule (strictly speaking, this is fertilization, or syngamy), and the other unites with the two polar nuclei (trigamy). Thus, syngamy results in the formation of the zygote, and trigamy results in the formation of the primary nucleus of the endosperm, with its characteristic triploid number of chromosomes. The endosperm serves as the nutritive tissue for the developing embryo. Triple fusion probably arose as a result of the extreme reduction of the female gametophyte, which usually has no nutrient reserves. It represents an effective adaptation to rapidly compensate for this deficiency.
The seeds of flowering plants are enclosed in the fruit. In the simpler plants, such as the magnolia, peony, and lily, the fruits are exposed and the seeds therefore are the organs of dispersal. In the more specialized plants, such as the Compositae or Gramineae, the fruit is enclosed and serves as the organ of dispersal.
Unlike the other higher plants, flowering plants have sieve elements in the phloem and are equipped with companion cells. Vessels are also present, except in some primitive groups. In regard to their level of evolutionary development, flowering plants occupy a position in the plant kingdom analogous to that of mammals in the animal kingdom.
Paleontological history. The origin of the Angiospermae dates from the early Cretaceous, about 125 million years ago. The possibility of a pre-Cretaceous origin has been rejected by such modern scholars as D. Axelrod (USA), N. Hughes (Great Britain), and J. Doyle and L. Hickey (USA). Early Cretaceous flowering plants were not as diverse as previously believed. The pollen grains in the earliest flowering plants were monocolpate, that is, of a primitive type. Only a few types of leaves had developed, and their venation as a whole lacked any specific order (Doyle and Hickey, 1972, 1976).
In the early Cretaceous, flowering plants were quite rare, occupying an insignificant place in the plant cover of the earth. However, one of the most intense and abrupt terrestrial changes in the plant world occurred during the mid-Cretaceous (approximately 110 million years ago). In a relatively short period of geological time—several million years—flowering plants spread over the entire world and reached the arctic and antarctic. One of the basic conditions for the rapid distribution of flowering plants was their great evolutionary flexibility, which was reflected in the unusually varied and numerous adaptations to the most diverse ecological conditions. Animal pollinators, especially suctorials, have played a large role in the evolution and mass distribution of flowering plants. As a result of adaptive radiation, flowering plants became capable of forming greatly diversified groupings belonging to the most varied ecosystems. In contrast to the Gymnospermae, which include no true herbaceous species, the Angiospermae include a large number of various herbs, including epiphytes. They constitute the only group of plants capable of forming complex communities in which plants of other groups are mainly absent. The origin of such communities led to the more intensive use of environmental resources by flowering plants, as well as to their distribution over new territories and adaptation to new habitats.
Origin. Despite the variance in the external form and internal structure of the Angiospermae, it is now generally believed that the hypothesis of their origin from various gymnosperm ancestors or various divisions of higher plants (the theory of the polyphy-letic origin of flowering plants) is erroneous and contradicts findings of comparative morphology and taxonomy. A great number of morphological, anatomical, and embryological characteristics of the most diverse families and orders (including features not related to each other in ontogenesis and in the evolutionary process) point to the common origin of all flowering plants. This is proven by the fact that double fertilization, accompanied by the formation of the triploid endosperm, is a process unique to the flowering plants and common to all of them.
Findings in taxonomy also attest to the common origin of flowering plants. Even the most singular and seemingly isolated groups of flowering plants within the taxonomic system are related to each other. While these plant groups at first might seem to prove the theory of independent origin, close study of any flowering plant and broad comparision with other groups will reveal the plant’s natural place among the Angiospermae.
Nevertheless, the question of the probable ancestors of flowering plants remains open to this day. It is generally recognized that not a single group of higher plants existing today could be the ancestor of the Angiospermae. Among the extinct Gymnospermae, the closest to the Angiospermae are the Bennettitales, which usually had bisporangiate strobili. However, despite their superficial similarity, great differences exist between the bisporangiate strobili of the Bennettitales and the flower of the magnolia and related plants. This fact indicates that the flower and the Bennettitales strobili followed different evolutionary paths from the very beginning. Thus, the direct ancestors of flowering plants are unknown. However, findings from comparative morphology provide a basis for the supposition that the ancestors of flowering plants were probably closely linked with the class of ferns Lyginopteridopsida and possibly comprised one of the branches of this primitive group of gymnosperms. This is suggested by the external integument of the flowering plant, which has been the subject of research by a number of botanists, including H. Gaussen of France (1946), A. L. Takhtadzhian (Takhtajan) of the USSR (1950, 1964), J. Walton of Great Britain (1953), and G. L. Stebbins of the USA (1974). These scientists have proposed that the integument comes from the cupule, which is characteristic of the more highly developed species of Lyginopteridopsida, such as Caytoniales, Pteridospermes, and Medullosaceae. The absence of paleontological data concerning the first Angiospermae and the intermediate group between them and their Gymnospermae ancestors can most likely be explained by the fact that they grew in the mountains, that is, under conditions unfavorable for precipitation and the burial of plant remains. This theory has been proposed by C. Arnold of the USA (1947), V. A. Vakhrameev of the USSR (1947), and Takhtadzhian (1948). Moreover, it is surmised that their apparently small populations did not play a noticeable role in the vegetative cover of the earth, which also should have decreased chances of burial.
The basic morphological characteristics of the Angiospermae can best be explained by their neotenic origin. The imprint of neoteny is found on the organization of both the sporophyte and gametophyte of flowering plants. The neotenic origin of the flower and of the male and female gametophytes is most evident. The flower can be examined as a neotenic form of a shortened spore-bearing shoot from a primitive gymnosperm that has been specialized in a new direction. The stamen and the carpel of the flowering plant probably correspond not so much to the adult microsporophylls and megasporophylls of hypothetical ancestors of flowering plants as to an early, juvenile stage in their development. Finally, the leaves and the vascular system of the axial organs of flowering plants also bear the imprint of a neotenic origin.
Neoteny is usually related to limiting factors in the environment such as a shortage of moisture, low temperatures, or a short growing season, and for this reason it is natural to suppose that flowering plants arose under conditions of ecological stress. Apparently they were formed under conditions of temporary dryness in a monsoonal climate and most likely on unprotected slopes.
As C. Darwin wrote in a letter to the Swiss paleontologist O. Heer in 1875, flowering plants must have developed in some isolated region, but because of geographical changes they were able to pass beyond the boundaries of this region and spread rapidly over the world. On the basis of an analysis of geographic distribution and phylogenetic relationships of the more primitive groups of flowering plants existing today, Takhtadzhian proposed in 1957 that this isolated region most probably was located somewhere in Southeast Asia. The most likely region for the formation and initial distribution of flowering plants was the territory in the southeastern section of the ancient continent of Laurasia, which corresponded to the southeastern sections of China, Indochina, the Malay Peninsula, the Phillipine Islands (or only their southern part), and parts of the Greater Sunda Islands.
It is quite probable that Southeast Asia is in fact the place of origin and dispersal of the flowering plants, and not a museum of living fossils, as Stebbins claimed in 1974. Numerous facts support Takhtadzhian’s theory. Although extensive territories in Western Godwanaland (in Africa and especially in South America) preserved mesophytic tropical forest flora relatively well and with great variations in form, the greatest quantity of primitive forms, many more than in America and especially Africa, were preserved in Southeast Asia and neighboring areas. In those cases where primitive groups were preserved in Southeast Asia and neighboring regions and also on territories corresponding to Western Gondwanaland, namely Southeast Asia and neighboring territories, they are represented in the latter regions in greater variety and by more primitive specimens. Thus, the primitive families of Magnoliaceae and Winteraceae are absent in Africa, and in America they are represented by a smaller number of genera and species and by less primitive taxa. Southeast Asia and neighboring regions have not only many primitive families, but also the more primitive representatives of many more advanced families and genera of both dicots and monocotyledons.
The primary type of flowering plant. Not a single extant flowering plant has retained all of its primitive features because all taxa, even the most primitive, became specialized in a particular area. However, by summarizing the most archaic characteristics dispersed among the Magnoliaceae, Winteraceae, Degeneriaceae, and other primitive families, it is possible to re-create some of the features of the early Angiospermae. Probably these were woody plants, most likely small trees, but not shrubs and certainly not subshrubs, as Stebbins claims (1974), because the subshrub is undoubtedly a secondary life form. The xylem lacked sieve vessels. The leaves were evergreen (as in the case with almost all gymnosperms), alternate, more or less xeromorphic, with pinnate venation. The stomata had guard cells adjoining ordinary cells on the lengthwise axis. The flowers of the early Angiospermae probably grew in primitive cymose inflorescences; they were androgynous and had a moderately elongate receptacle, on which sepals, stamens, and carpels were spirally arranged. The flowers did not have petals; petals developed later, primarily from the stamens. Pollination was carried out by insects, probably by beetles. The pollen grains were monocolpate and had a smooth exine and still lacked the stylar layer in the ectexine that is characteristic of flowering plants. Seeds with a well-developed layer of living parenchymatous cells were dispersed by birds. The plants bore multiple fruits. If a fossilized example of this hypothetical primary type of flowering plant were found, taxonomists would classify it in the order Magnoliales.
According to all data, the evolution of flowering plants followed a path of extensive adaptive radiation at a very rapid pace from the very beginning. This is explained by ecological as well as genetic and cytogenetic factors. In particular, a large role was played by aneuploid chromosomal rearrangement and by polyploidization. As a result, as early as the mid-Cretaceous, flowering plants had achieved a great variety of forms and had adapted to the growing diversity of ecological regions. By this time all the basic taxa of Angiospermae had arisen, down to many genera. Since the mid-Cretaceous, flowering plants have been predominant in the world plant cover, and they play an important role even in coniferous forests. The evolution of the earth’s animal kingdom, especially of insects, birds, and mammals, is closely related to that of flowering plants.
Classification and philogeny. The classification of flowering plants is based on the synthesis of data from comparative morphology, anatomy, embryology, cytology, genetics, biochemistry, and plant geography. The subdivision of flowering plants into classes, subclasses, orders, and families is based on the morphology of the flower and inflorescences, the fine structure of pollen grains, the structure and development of the ovule and of the male and female gametophytes, the anatomy of the seed coat, the structure and degree of development of the embryo, the number of cotyledons, the presence or absence of endosperm, the structure and development of the stomate apparatus, and the anatomy of the xylem and phloem. The study of the ultrastructure of plastids is becoming more and more important.
The Angiospermae can be divided into two classes: dicotyledons (dicots) and monocotyledons (monocots). The embryo of dicots has two cotyledons, which usually germinate on the surface of the soil. The cotyledons usually have three main vascular bundles. The embryo of monocots, on the other hand, has one cotyledon, which germinates in the soil in most cases. The cotyledons usually have two main vascular bundles.
The leaves of dicots are mostly pinnately and sometimes pal-mately veined. The petiole is usually clearly differentiated. There are usually one to three leaf traces. The leaves of monocots usually have parallel venation. As a rule there is no differentiation of the petiole and blade. There are usually many leaf traces.
In dicots, the prophylls and bracts are usually paired and distributed laterally, while in monocots they are usually single and located on the ventral side of the shoot.
The vascular system of dicots usually consists of one ring of bundles, usually with a cambium. The phloem usually contains parenchyma. The bark and medulla are usually highly differentiated. In monocots, the vascular system usually consists of individual bundles or of one or two rings of bundles, sometimes more. The phloem contains no parenchyma, and there is usually no clear differentiation of the bark and medulla.
The primary radicle of dicots usually develops into the main root, from which smaller side roots grow. The root cap and epidermis usually originate together during ontogenesis, with the exception of the order Nympheales. In monocots, however, the primary radicle withers away early and is replaced by a system of adventitious roots. The root cap and epidermis originate separately during ontogenesis.
Dicots are woody or herbaceous plants and sometimes include secondary dendroid forms, for example, the genus Haloxylon. Monocots include herbs and some secondary dendroid plants, for example, palms.
The parts of dicotyledonous flowers grow in multiples of five and sometimes of four. Those of some primitive groups grow in multiples of three. The parts of monocotyledonous flowers grow in multiples of three, rarely of four or two and never of five.
There is no single feature, therefore, that clearly differentiates the two classes of flowering plants. Dicots and monocots are distinguished essentially by a combination of features. There is an exception for each of the morphological characteristics mentioned.
Monocots are derived from dicots and probably began developing as a separate group at the very beginning of the evolution of the Angiospermae. They could only be derived from those dicots that had an apocarpous gynoecium and monocolpate pollen grains. Among the modern dicots, members of the order Nymphaeales have the largest number of characteristics in common with monocots. However, these are all specialized aquatic plants and therefore cannot be studied as the probable ancestors of monocots. There is reason to believe, however, that monocots and the Nymphaeales both originated from a more primitive terrestrial herbaceous dicot. The nearest ancestors of monocots were most likely terrestrial plants that adapted to permanent or temporary watery conditions. Primary monocots were probably perennial rhizomatous herbs having entire arc-veined elliptical leaves. A cross section of the stem would show scattered vascular bundles and a residual cambium. The flowers located in the uppermost inflorescences had parts growing in threes, with a perianth in two rings. They had an androecium of primitive ribbonlike stamens and an apocarpous gynoecium of primitive carpels. The pollen grain had a single furrow and was binucleate when mature. The seed contained a large quantity of endosperm.
The classes of dicots and monocots are further subdivided into subclasses, orders (sometimes united in superorders), families, genera, and species, with all types of intermediate classifications. The Angiospermae are classified according to several systems today. The following is a short presentation of the system of Takhtadzhian, on which several reference works are based. Only classes, subclasses, and the most important orders and families are included.
Class I. Dicots (Magnoliopsida or Dicotyledones). The class contains as many as 360 families and about 170,000 species.
Subclass I. Magnoliidae. For the most part, the Magnoliidae are woody plants, some of which lack vessels. The leaves and stems frequently have secretory cells, and the stomates most frequently have two guard cells. The flowers are mostly bisexual and frequently spirally arranged or coiled. The mature pollen is bicellular and occasionally tricellular. The coat of the pollen grains usually has a single furrow or is of a derivative type. The gynoecium is mainly apocarpous. The ovules usually have a double integument and are crassinucellate (the mother cell of the megaspore is separated from the epidermus of the megasporangia by one or several layers of cells). The endosperm is usually cellular. As a rule, the seeds have a small embryo and a large quantity of endosperm. The subclass consists of eight orders.
(1) Magnoliales. Of all the extant flowering plants, the Magnoliales are the most primitive. The families in this order include Winteraceae, Degeneriaceae, Magnoliaceae, Annonaceae, and Myristicaceae.
(2) iniciales. Closely related to Magnoliales, the iniciales probably share a common origin with the Winteraceae. Families that constitute the order include Illiciaceae and Schisandraceae.
(3) Laurales. This order is closely related to Magnoliales but is more highly developed. Its families include Monimiaceae, Calycanthaceae, and Lauraceae.
(4) Piperales. The order Piperales is closely related to Laurales, with which it shares a common origin. The order is very specialized. Its families include Saururaceae and Piperaceae.
(5) Aristolochiales. This order evidently originated directly from Magnoliales. Its only family is Aristolochiaceae.
(6) Rafflesiaceae. Probably derived from ancestors of the Aristolochiales, the order consists of nonchlorophyllous parasitic herbs. The families include Rafflesiaceae and Hydnoraceae.
(7) Nymphaeales. This order is probably descended from the most ancient vessel-less representatives of Magnoliales. It consists of perennial aquatic herbs. The most important families include Cabombaceae and Nymphaeaceae.
(8) Nelumbonales. The taxonomic position and origin of this order are not totally clear. The single family, Lotaceae, consists of one genus, Lotus. Sometimes the order is included with the family Nymphaeaceae, but there are many important differences between the two.
Subclass 2. Ranunculidae. This subclass is very closely related to the Magnoliidae, with which it is frequently classified; however, it is more highly developed. It primarily includes herbs. All representatives possess vessels. Secretory cells in the parenchymatous tissue are found only in the Menispermaceae. The stomata are of various types and usually lack subsidiary cells. The flowers are bisexual or unisexual and are frequently arranged spirally or in whorls. The mature pollen is primarily binucleate. The coat of the pollen grains has three furrows or is of a derivative type; it never has only a single furrow. The ovules usually have a double integument or are crassinucellate; in some cases they are tenuinucellate (the mother cell of the megaspore is located directly under the epidermis of the megasporangium). The seeds usually have small embryos, and most have a large quantity of endosperm; however, the endosperm is absent in some individuals.
The Ranunculidae embrace three orders: Ranunculales, Papaverales, and Sarraceniales.
(1) Ranunculales. This order probably has the same origin as iniciales. The most important families are Menispermaceae, Ranunculaceae, and Berberidaceae.
(2) Papaverales. This order is closely related to Ranunculales. The most important families are Papaveraceae and Fumariaceae, both of which are frequently included in a single family.
(3) Sarraceniales. This order consists of very specialized insectivorous herbs. The herbs have retained some primitive features that indicate that they are related to Ranunculales. The most important family is Sarraceniaceae.
Subclass 3. Hamamelididae. This subclass consists mostly of woody plants. The plants contain vessels, except for the order Trochodendrales. The stomata have two or more subsidiary cells; some species lack subsidiary cells. The flowers are mostly anemophilous, more or less reduced, and usually unisexual. The perianth is usually poorly developed. The flowers lack petals and frequently calyxes. The mature pollen is usually binucleate and with three furrows or derivative. The gynoecium is syncarpous. The ovules have two integuments and in most cases are crassinucellate. The fruits are usually monospermous. The seed may contain abundant or scant endosperm or none at all. The subclass consists of eight orders.
(1) Trochodendrales. This order occupies an intermediate position between Magnoliales on the one hand and Cercidiphyllales and Hamamelidales on the other. Its families include Trochodendraceae and Tetracentraceae.
(2) Cercidiphyllales. Closely related to Trochodendrales, the order Cercidyphyllales includes the family Cercidiphyllaceae.
(3) Hamamelidales. This order serves as the link between Trochodendrales and all the following orders of this subclass. It is probably derived from the closest ancestors of Trochodendrales, which had entomophilous flowers with an apocarpous gynoecium. The most important families are Hamamelidaceae and Platanaceae.
(4) Urticales. This order is related to Hamamelidales and probably is derived from it. Its families include Ulmaceae, Moraceae, Cannabaceae, and Urticaceae.
(5) Casuarinales. This order probably is derived from Hamamelidales. Its single family is Casuarinaceae.
(6) Fagales. It is probable that the order Fagales is derived from Hamamelidales. Its families include Fagaceae and Betulaceae.
(7) Myricales. This order has much in common with Casuarinales and Betulaceae on the one hand and with Juglandales on the other. Its families include Myricaceae.
(8) Juglandales. Both Myricales and Fagales have much in common with Juglandales. The families include Rhoipteleaceae and Juglandaceae.
Subclass 4. Caryophyllidae. This subclass consists mostly of herbaceous plants, subshrubs, and low shrubs; it also includes a few small trees. The leaves are entire and always have vessels. The segments of the vessels have scalariform or simple perforations. The stomata usually are surrounded by two or three or sometimes four subsidiary cells; occasionally these cells are absent. The flowers are bisexual or sometimes unisexual; they are mostly apetalous. The mature pollen is trinucleate and tricolpate or derivative. The gynoecium is apocarpous or occasionally syncarpous. The ovules usually have two integuments and are crassinucellate. The seeds usually have curved peripheral embryos, frequently with a perisperm.
The Caryophyllidae embrace three orders: CaryophyHales, Polygonales, and Plumbaginales.
(1) Caryophyllales. Most likely this order descends from Ranunculales. Its most obvious ties are with the family Phytolaccaceae. The most important families are Phytolaccaceae, Nyctaginaceae, Aizoaceae, Cactaceae, Portulacaceae, Caryophyllaceae, Amaranthaceae, and Chenopodiaceae.
(2) Polygonales. This order is closely related to Caryophyllales, and the two probably share a common origin. Its families include Polygonaceae.
(3) Plumbaginales. This order evidently shares a common origin with Caryophyllales. Its families include Plumbaginaceae.
Subclass 5. Dilleniidae. This subclass consists of trees, shrubs, and herbs. The leaves are entire or segmented. The stomata can be of various types but usually have no subsidiary cells. Vessels are found in all species, and the vessel segments have scalariform or simple perforations. The flowers are either bisexual or unisexual, with double perianths. Occasionally they are apetalous. The more primitive families frequently possess a spiral or cyclical perianth. When the androecium consists of many stamens, it develops in centrifugal sequence. The mature pollen is binucleate and occasionally trinucleate. It may have three furrows or be derived from this type. The gynoecium is syncarpous or occasionally apocarpous. The ovules usually have a double integument and are mostly crassinucellate. The seeds usually have an endosperm.
The subclass consists of 14 orders.
(1) Dilleniales. This order, which links Magnoliales with Theales and Viólales, includes the family Dilleniaceae.
(2) Paeoniales. The most important family of the Paeoniales is Paeoniaceae. The order is closely related to Dilleniales.
(3) Theales. This order is closely related to Dilleniales and is probably derived from its primitive representatives. The most important families are Ochnaceae, Dipterocarpaceae, Theaceae, and Hypericaceae (Guttiferae).
(4) Vialales. This order is closely related to Theales, with which it shares a common origin in the order Dilleniales. The most important families are Flacourtiaceae, Violaceae, Cistaceae, Passifloraceae, Caricaceae, and Cucurbitaceae.
(5) Begoniales. Probably derived from Viólales, this order includes the families Datiscaceae and Begoniaceae.
(6) Capparales. This order is descended from primitive representatives of Violales. The most important families are Capparaceae, Brassicaceae (Cruciferae), and Resedaceae.
(7) Tamaricales. Although descended from Viólales, this order is very specialized. Its families include Tamaricaceae, Fouquieriaceae, and Frankeniaceae.
(8) Salicales. This order is descended from Flacourtiaceae and most probably from the ancestors of the modern genus Idesia. Its main family is Salicaceae.
(9) Ericales. This order is related to Theales, with which it shares a common origin from Dilleniales; it is closely linked with Dilleniales through the primitive family Actinidiaceae. The most important families include Actinidiaceae, Clethraceae, Ericaceae, Empetraceae, Epacridaceae, Diapensiaceae, and Curillaceae.
(10) Ebenales. This order is descended from Theales. The most important families are Styracaceae, Ebenaceae, and Sapotaceae.
(11) Primulales. This order is related to Ebenales and shares a common origin with it from Theales. The most important families are Myrsinaceae and Primulaceae.
(12) Malvales. It is possible that this order is derived from some intermediate group between the primitive Theales and Violales. The most important families are Tiliaceae, Sterculiaceae, Bombacaceae, and Malvaceae.
(13) Euphorbiales. Close links have been discovered between this order and both Malvales and Violales. The order probably originated from some extinct intermediate group between the other two. The most important families are Buxaceae, Daphniphyllaceae, and Euphorbiaceae.
(14) Thymelaeales. This order has much in common with Euphorbiales, to which it is very closely related, and less in common with Málvales. All three orders share a common origin. One of the major families is Thymelaeaceae.
Subclass 6. Rosidae. This subclass consists of trees, shrubs, and herbs. The leaves are entire or segmented. The stomata are of various types; subsidiary cells are usually absent, although occasionally there are two. Vessels are present, and vessel segments have simple or, occasionally, scalariform perforations. The flowers may be bisexual, with a double perianth, or asepalous. When the androecium consists of many stamens it develops in centripetal sequence. The mature pollen is most frequently binucleate. The cover of the pollen grain is triaperturate or derived from this type. The gynoecium is apocarpous or syncarpous. The ovules usually have a double integument and are crassinucellate. Seeds can be either with or without endosperm.
The subclass embraces more than 20 orders.
(1) Saxifragales. This order is related to Dilleniales through the Cunoniaceae and related families and probably shares a common origin with it. The most important families are Cunoniaceae, Escalloniaceae, Grossulariaceae, Hydrangeaceae, Pittosporaceae, Crassulaceae, and Saxifragaceae.
(2) Rosales. It is probable that this order shares a common origin with Saxifragales, with which it is closely related. The most important family is Rosaceae.
(3) Nepenthales. This order, probably descended from Saxifragales, includes the families Droseraceae and Nepenthaceae.
(4) Podostemales. This order is related to Saxifragales and especially to Crassulaceae. The herbs, which are often extremely small, are adapted to life in rapidly moving water. Its families include the Podostemaceae.
(5) Fabales. This order is closest to the primitive representatives of Saxifragales but is much more advanced. It consists of the single family Fabaceae (Leguminosae), which is frequently divided into the separate families Mimosaceae, Caesalpiniaceae, and Fabaceae (Papilionaceae).
(6) Connarales. This order has much in common with Saxifragales, especially with the family Cunoniaceae and, in part, with the order Fabales. Its families include Connaraceae.
(7) Proteales. This is a very isolated order whose phylogenetic links are not fully clear. Its chemical composition resembles that of Fabales, but morphologically the order has the most in common with primitive families of Saxifragales. All three orders probably had a common origin. The families include Proteaceae.
(8) Myrtales. This order probably descends from the Saxifragales. The most important families are Lythraceae, Sonneratiaceae, Punicaceae, Rhizophoraceae, Combretaceae, Myrtaceae, Melastomataceae, Onagraceae, and Lecythidaceae (Lecythis).
(9) Hippuridales. This order, related to Myrtales, includes the families Haloragaceae, Gunneraceae, and Hippuridaceae.
(10) Rutales. It is probable that this order descends from primitive Saxifragales. The most important families are Anacardiaceae, Burseraceae, Simaroubaceae, Rutaceae, and Meliaceae.
(11) Sapindales. This order is related to Rutales. The most important families are Staphyleaceae, Aceraceae, Sapindaceae, and Hippocastanaceae.
(12) Geraniales. Related to Rutales, this order includes mostly herbs. The most important families are Linaceae, Erythroxylaceae, Zygophyllaceae, Oxalidaceae, Geraniaceae, Tropaeolaceae, and Balsaminaceae.
(13) Polygalales. This order is closely related to Geraniales, especially the family Malpighiaceae, which could justifiably be placed in either of the two orders. The most important families are Malpighiaceae, Vochysiaceae, and Polygalaceae.
(14) Cornales. Descended from the primitive Saxifragales, this order embraces the families Davidiaceae, Nyssaceae, Cornaceae, Garryaceae, Alangiaceae, and Mastixiaceae.
(15) Araliales. This order is very close to Cornales but is more highly advanced. Its families include Araliaceae and Apiaceae (Umbelliferae).
(16) Celastrales. It is most likely that this order is descended from the most primitive Saxifragales. The most important families are Aquifoliaceae, Icacinaceae, and Celastraceae.
(17) Rhamnales. This order is closely related to Celastrales. The most important families are Rhamnaceae and Vitaceae.
(18) Oleales. This order is related to Celastrales, and the two probably share a common origin from Saxifragales. The families include Oleaceae.
(19) Santalales. The primitive representatives of this order are related to the primitive families of Celastrales. Both orders probably share a common origin. The most important families are Olacaceae, Santalaceae, Loranthaceae, Viscaceae, and Balanophoraceae.
(20) Elaeagnales. This is a taxonomically isolated order whose genetic ties are not yet clear. It has features in common with the orders Thymelaeales, Myrtales, Rhamnales, and Proteales, but it is possible that it originated independently from Saxifragales. Its families include Elaeagnaceae.
Subclass 7. Asteridae. This subclass consists of herbs and some trees and shrubs. The leaves are entire or segmented. The stoma usually has two, often four, and occasionally six subsidiary cells. Vessels are always present, and the vessel segments have simple or sometimes scalariform perforations. The flowers are bisexual and almost always sympetalous. There is usually the same number of stamens as corolla lobes; sometimes there are fewer stamens. The mature pollen is trinucleate or binucleate. The coat of the pollen grains has three furrows or is derived from this type. The gynoecium is syncarpous and morphologically seems to be always paracarpous. The ovules have a simple integument and are tenuinucellate or sometimes crassinucellate. Seeds can be either with or without endosperm. There are seven orders.
(1) Dipsacales. The primitive representatives of this order have much in common with the order Cornales but are more closely related to the order Saxifragales and probably are descended from its primitive representatives. Its families include Caprifoliaceae, Adoxaceae, Valerianaceae, and Dipsacaceae.
(2) Gentianales. This order shares a common origin with Dipsacales. The most important families are Loganiaceae, Rubiaceae, Apocynaceae, Asclepiadaceae, and Gentianaceae.
(3) Polemoniales. This order is closely related to Gentianales but is more advanced. The most important families are Convolvulaceae, Cuscutaceae, Polemoniaceae, Hydrophyllaceae, and Boraginaceae.
(4) Scrophulariales. This order is related to the order Polemoniales, with which it shares a common origin. The most important families are Solanaceae, Buddlejaceae, Scrophulariaceae, Bignoniaceae, Pedaliaceae, Gesneriaceae, Orobanchaceae, Lentibulariaceae, Acanthaceae, and Plantaginaceae.
(5) Lamiales. It is probable that this order is directly descended from the order Scrophulariales, with which it is closely related. The most important families are Lamiaceae (Labiatae) and Verbenaceae.
(6) Campanulales. This order is closely related to the order Polemoniales, and the two are probably descended from the nearest ancestors of Gentianales. The most important families are Campanulaceae, Stylidiaceae, and Goodeniaceae.
(7) Asterales. This order has much in common with Campanulales. Both are probably descended from the nearest ancestor of Gentianales. Asteraceae (Compositae) is the only family.
Class II. Liliopsida (Monocotyledones). The monocots consist of about 70 families, embracing more than 65,000 species.
Subclass 1. Alismatidae. This subclass includes aquatic or bog herbs. The stomata have two and occasionally four subsidiary cells. Vessels are absent or are found only in the roots. The mature pollen is usually trinucleate. The coat of pollen grains usually has a single furrow. It may be biporous or multiporous or without furrows and without pores. The gynoecium is usually apocarpous and sometimes syncarpous. The ovules have a double integument and are crassinucellate or sometimes tenuinucellate. The endosperm is nuclear or helobial. The seeds lack endosperm.
The subclass consists of three orders: Alismatales, Hydrocharitales, and Najadales.
(1) Alismatales. Several primitive features are observed in this order, for example, the presence of forms with an apocarpous gynoecium, as well as specialized characteristics, in particular, the absence of endosperm. The families include Butomaceae, Limnocharitaceae, and Alismataceae.
(2) Hydrocharitales. This order descends from the most immediate ancestors of the order Alismatales. Its families include Hydrocharitaceae.
(3) Najadales. This order is closely related to Alismatales and is probably descended from its most immediate ancestors. The most important families are Scheuchzeriaceae, Juncaginaceae, Aponogetonaceae, Zosteraceae, Potamogetonaceae, Zannichelliaceae, and Najadaceae.
Subclass 2. Liliidae. This subclass consists of herbs or secondary dendroid forms. The stomata usually lack subsidiary cells, although they sometimes have two and even four. Vessels are located either only in the roots or in all vegetative organs. The perianth is well developed; usually the sepals resemble the petals. The mature pollen is usually binucleate, although it is occasionally trinucleate. The coat of the pollen grains has a single furrow or occasionally none. The gynoecium is syncarpous and occasionally apocarpous. The ovules have a double or sometimes simple integument; they are crassinucellate, or on occasion tenuinucellate. The endosperm is nuclear or helobial. The seed has much endosperm, but the order Zingiberales has a perisperm with the remnants of an endosperm or only a perisperm.
The sublcass consists of four orders: Triuridales, Liliales, Zingiberales, and Orchidales.
(1) Triuridales. This order includes very specialized saprophytic herbs that have retained such primitive features as an apocarpous gynoecium and seed with a large quantity of endosperm. Its families include Triuridaceae.
(2) Liliales. This order is differentiated from Alismatales by the presence of endosperm in the seeds and from Triuridales by the binucleate pollen. All three orders may share a common origin. The most important families are Liliaceae, Iridaceae, Alstroemeriaceae, Philesiaceae, Xanthorrhoeaceae, Haemodoraceae, Hypoxidaceae, Velloziaceae, Amaryllidaceae, Asparagaceae, and Dioscoreaceae.
(3) Zingiberales. It is probable that this order descends from the order Liliales. The most important families are Musaceae, Zingiberaceae, Cannaceae, and Marantaceae.
(4) Orchidales. This order is most closely related to the family Hypoxidaceae of Liliales. Its families include Orchidaceae.
Subclass 3. Commelinidae. This subclass includes herbs, which sometimes possess a woody stalk, for example, the family Bambusaceae. The stomata almost always have subsidiary cells, usually two. Vessels are present in all vegetative organs and are absent very rarely. The mature pollen is binucleate or trinucleate. The coat of the pollen grains has a single furrow or pore or sometimes four pores. The gynoecium is syncarpous. The ovules have a double or, very rarely, single integument and are usually crassinucellate. The endosperm is nuclear or sometimes helobial, for example, in Juncales, Bromeliales, and Eriocaulales. The seed contains a farinaceous endosperm.
The subclass consists of seven orders.
(1) Juncales. This order has much in common with the family Liliaceae and probably is descended from its ancestors. Its families include Juncaceae.
(2) Cyperales. It is probable that this order is descended from the more primitive Juncales. Its families include Cyperaceae.
(3) Bromeliales. This order shares with Juncales a common origin from the ancestors of the lily type. The order includes the family Bromeliaceae.
(4) Commelinales. This order shares a common origin with Bromeliales. The most important families are Commelinaceae and Xyridaceae.
(5) Eriocaulales. Most likely this order probably shares a common origin with Commelinales. It includes the family Eriocaulaceae.
(6) Restionales. This order shares a common origin with Commelinales. It has features characteristic of Poales. The most important families are Restionaceae and Flagellariaceae.
(7) Poales. According to all data, this order descends directly from Restionales and most likely from ancestors of the family Flagellariaceae. Its families include Poaceae (Gramineae).
Subclass 4. Arecidae. This subclass includes herbs or secondary dendroid forms. The stomata have subsidiary cells, most frequently four. There are vessels in all vegetative organs or only in the roots, for example, the order Arales. Flowers are usually unisexual. The perianth consists of sepals and petals that are very similar; occasionally it is reduced or absent. The flowers are aggregated into paniculate or spherical inflorescences or spadices that usually have a spathe. The mature pollen is usually binucleate. The coat of the pollen grains is of various types; it most frequently is monosolpate. The gynoecium is syncarpous and occasionally apocarpous, for example, in some palms. The ovules have a double integument and are crassinucellate or sometimes tenuinucellate. The endosperm is usually nuclear. The seeds have endosperm, usually a large quantity.
The subclass Arecidae consists of five orders.
(1) Arecales. This order probably shares a common origin with Liliales. Its families include Arecaceae (Palmae).
(2) Cyclanthales. This order shares a common origin with Arecales. Its families include Cyclanthaceae.
(3) Arales. It is probable that this order shares a common origin with Arecales and Cyclanthales through the nearest ancestors of Liliales. Its families include Araceae and Lemnaceae.
(4) Pandanales. This order is closest to Cyclanthales. Its families include Pandanaceae.
(5) Typhales. Most likely this order shares a common origin with Pandanales. Its families include Sparganiaceae and Typhaceae.
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A. L. TAKHTADZHIAN