ANGIOSPERM

any member of the more than 250,000 species of flowering plants (division Magnoliophyta), the largest and most diverse group within the kingdom Plantae. Angiosperms represent approximately 80 percent of all the known green plants now living. The angiosperms are vascular seed plants in which the ovule (egg) is fertilized and develops into a seed in an enclosed hollow ovary. The ovary itself is usually enclosed in a flower, that part of the angiospermous plant that contains the male or female reproductive organs or both. Fruits are derived from the maturing floral organs of the angiospermous plant and are therefore characteristic of angiosperms. By contrast, in gymnosperms (e.g., conifers), the other large group of vascular seed plants, the seeds do not develop enclosed within an ovary but are usually borne exposed on the surfaces of reproductive structures, such as cones, that originally produced the spores. Unlike such nonvascular plants as the bryophytes, in which all cells in the plant body participate in every function necessary to support, nourish, and extend the plant body (e.g., nutrition, photosynthesis, and cell division), angiosperms have evolved specialized cells and tissues that carry out these functions and have further evolved specialized vascular tissues that translocate the water and nutrients to all areas of the plant body. The specialization of the plant body, which has evolved as an adaptation to a principally terrestrial habitat, includes extensive root systems that anchor the plant and absorb water and minerals from the soil; a stem that supports the growing plant body; and leaves, which are the principal sites of photosynthesis for most angiospermous plants. Another significant evolutionary advancement over the nonvascular and the more primitive vascular plants is the presence of localized regions for plant growth, called meristems and cambia, which extend the length and width of the plant body, respectively. Except under certain conditions, these regions are the only areas in which cell division takes place in the plant body, although cell differentiation continues to occur over the life of the plant. The angiosperms dominate the Earth's surface and vegetation in more environments, particularly terrestrial habitats, than any other group of plants. As a result, angiosperms are the most important ultimate source of food for birds and mammals, including humans. In addition, the flowering plants are the most economically important group of green plants, serving as a source of pharmaceuticals, fibre products, timber, ornamentals, and other commercial products. Although in the past the taxonomy of the angiosperms was somewhat controversial, a consensus began to develop among plant taxonomists toward the end of the 20th century. The angiosperms came to be considered a group at the division level (comparable to the phylum level in animal classification systems) called Magnoliophyta. Throughout this article the orders or families are given, usually parenthetically, following the vernacular or scientific name of a plant. Following taxonomic conventions, genera and species are italicized. The higher taxa are readily identified by their suffixes: families end in -aceae, orders in -ales, subclasses in -idae, and classes in -opsida. For a comparison of angiosperms with the other major groups of plants, see plant, bryophyte, fern, lower vascular plant, and gymnosperm. any member of the more than 250,000 species of flowering plants (division Magnoliophyta) having roots, stems, leaves, and well-developed conductive tissues (xylem and phloem). Angiosperms are often differentiated from gymnosperms by their production of seeds within a closed chamber (the ovary), but this distinction is not always clear-cut. Compare gymnosperm. The Magnoliophyta division is composed of two classes, the Liliopsida, or monocotyledons, and the Magnoliopsida, or dicotyledons. The embryo of monocots possesses one seed leaf (cotyledon) and the dicot embryo develops two. Other developmental differences that are used to distinguish the monocots include flower parts in threes, scattered conducting strands in the stem, and the absence of a cambium (the cell layer responsible for secondary growth). Dicots have flower parts in fours or fives, conducting strands arranged in a cylinder, and a cambium. Less reliable traits include the prominent parallel veins in a monocot leaf and the net-veined pattern of the dicot leaf. Angiosperms reflect an immense diversity in habit, size, and form. Species of angiosperms have been assigned to more than 300 families growing on every continent, including Antarctica. Some survive up to 1,000 years, while others live only a few weeks. They range in size from the floating duckweed, about 0.02 inch (0.5 mm), to Eucalyptus regnans, which grows to 300 feet (90 m). Angiosperms have also adapted to an almost infinite variety of habitatsfrom freshwater lakes to brackish coastal waters; from rich, well-drained soils to arid desert regions and rocky ledges; from below sea level to high mountains. Some angiosperms propagate vegetatively; i.e., roots or stems (or sometimes other parts, or buds) can be separated from the main body and be planted. The great majority of angiosperms, however, reproduce principally by means of cross-pollination, i.e., sexually. This reproduction by seeds involves the specialized reproductive organs (stamens or carpels or both) that are present in all flowers. After pollination and fertilization take place, the seed-bearing ovary matures and becomes the fruit, which may be dry (as is the case in a grain of wheat) or fleshy (as in a watermelon, tomato, or grape). Additional reading General works Various aspects of plant life, including angiosperms, are dicussed in Peter H. Raven, Ray F. Evert, and Susan E. Eichhorn, Biology of Plants, 5th ed. (1992), an excellent general treatment; Armen Takhtajan (A.L. Takhtadzhian), Floristic Regions of the World (1986; originally published in Russian, 1978), a comparative study of the world's endemic floras, including the flowering plants; Knut Norstog and Robert W. Long, Plant Biology (1976); and D.J. Mabberley, The Plant-Book (1987), an excellent source for specific information on genera and families, arranged alphabetically with common-name entries. Descriptive, illustrated listings of plants may be found in Kenneth A. Beckett et al., The RHS Encyclopedia of House Plants (1987); Jacqueline Hriteau et al., The National Arboretum Book of Outstanding Garden Plants (1990); and Pierre Anglade (ed.), Larousse Gardening and Gardens (1990).Economically important plants of all families are addressed in John C. Roecklein and Pingsun Leung, A Profile of Economic Plants (1987); George Usher, A Dictionary of Plants Used by Man (1974); F.N. Howes, A Dictionary of Useful and Everyday Plants and Their Common Names (1974); Richard M. Klein, The Green World: An Introduction to Plants and People, 2nd ed. (1987); Charles B. Heiser, Jr., Of Plants and People (1985), and Seed to Civilization: The Story of Food, new ed. (1990); Beryl Brintnall Simpson and Molly Conner-Ogorzaly, Economic Botany: Plants in Our World (1986); N.T. Gill and K.C. Vear, Agricultural Botany, vol. 1, Dicotyledonous Crops, and vol. 2, Monocotyledonous Crops, 3rd ed. rev. (1980); Mas Yamaguchi, World Vegetables (1983); Martin Chudnoff, Tropical Timbers of the World (1984); Douglas Patterson, Commercial Timbers of the World, 5th ed. (1988); James M. Dempsey, Fiber Crops (1975); Leslie S. Cobley, An Introduction to the Botany of Tropical Crops, 2nd ed. rev. by W.M. Steele (1976); Steven Nagy and Philip E. Shaw, Tropical and Subtropical Fruits: Composition, Properties, and Uses (1980); J.A. Samson, Tropical Fruits, 2nd ed. (1986); Lawrence K. Opeke, Tropical Tree Crops (1982); Frederic Rosengarten, Jr., The Book of Edible Nuts (1984), and The Book of Spices, rev. ed. (1973, reissued 1981); J.W. Purseglove et al., Spices, 2 vol. (1981); and Gerhard Rbbelen, R. Keith Downey, and Amram Ashri (eds.), Oil Crops of the World (1989).Poisonous and medicinally useful plants are described in Walter H. Lewis and Memory P.F. Elvin-Lewis, Medical Botany: Plants Affecting Man's Health (1977); James W. Hardin and Jay M. Arena, Human Poisoning from Native and Cultivated Plants, 2nd ed. (1974); Kenneth P. Lampe and Mary Ann McCann, AMA Handbook of Poisonous and Injurious Plants (1985); and Will H. Blackwell, Poisonous and Medicinal Plants (1990).Insectivorous varieties are treated in Adrian Slack, Carnivorous Plants, rev. ed. (1988); James Pietropaolo and Patricia Pietropaolo, Carnivorous Plants of the World (1986); and B.E. Juniper, R.J. Robins, and D.M. Joel, The Carnivorous Plants (1989). General features V.H. Heywood (ed.), Flowering Plants of the World (1978, reissued 1985), is an illustrated guide to flowering plant families with distribution maps and uses. Flowering plants from several families are discussed together in Peter Bernhardt, Wily Violets & Underground Orchids (1989), a popular treatment of forest cyclicality, North American grassland pollination, Australian mistletoe, Amazonian giant water lilies, violets, and orchids. C.D. Sculthorpe, The Biology of Aquatic Vascular Plants (1967, reprinted 1985), is a comprehensive monograph on water plants. Structure and function Discussions of angiosperm anatomy are included in Arthur J. Eames, Morphology of the Angiosperms (1961, reprinted 1977), emphasizing floral structure and diversity; Katherine Esau, Anatomy of Seed Plants, 2nd ed. (1977), emphasizing developmental vegetative anatomy and growth; Ernest M. Gifford and Adriance S. Foster, Morphology and Evolution of Vascular Plants, 3rd ed. (1989), a succinct comparative treatment of reproductive and vegetative anatomy, morphology, and evolution; A. Fahn, Plant Anatomy, 4th ed. (1990), covering all aspects of reproductive and vegetative plant anatomy; F. Weberling, Morphology of Flowers and Inflorescences (1989; originally published in German, 1981), a thorough treatment of flower and inflorescence structure, development, and diversity; P.H. Raven, The Bases of Angiosperm Phylogeny: Cytology, Annals of the Missouri Botanical Garden, 62(3):724764 (1975), a comprehensive discussion of basic chromosome numbers in the angiosperms, although it does not go into great detail; Ghillean Tolmie Prance and Kjell B. Sandved, Leaves (1985); and Francis E. Putz and Harold A. Mooney (eds.), The Biology of Vines (1992). Reproduction K. Faegri and L. van der Pijl, The Principles of Pollination Ecology, 3rd rev. ed. (1979), provides a thorough treatment of pollination biology and plant-animal interactions. B.M. Johri (ed.), Embryology of Angiosperms (1984), summarizes recent advances in the knowledge of flowering plant reproduction. Also useful are Gwenda L. Davis, Systematic Embryology of the Angiosperms (1966); and Bastiaan Meeuse and Sean Morris, The Sex Life of Flowers (1984). Paleobotany and evolution The fossil record is analyzed in a number of books, including Armen Takhtajan (A.L. Takhtadzhian), Flowering Plants: Origin and Dispersal (1969; originally published in Russian, 2nd ed., 1961), and Evolutionary Trends in Flowering Plants (1991); Charles B. Beck (ed.), Origin and Early Evolution of Angiosperms (1976); Norman F. Hughes, Palaeobiology of Angiosperm Origins: Problems of Mesozoic Seed-Plant Evolution (1976); Wilson N. Stewart, Paleobotany and the Evolution of Plants (1983); and Else Marie Friis, William G. Chaloner, and Peter R. Crane (eds.), The Origins of Angiosperms and Their Biological Consequences (1987); and in such articles as Peter H. Raven and D.I. Axelrod, Angiosperm Biogeography and Past Continental Movements, Annals of the Missouri Botanical Garden, 61:539673 (1974); D.L. Dilcher, Early Angiosperm Reproduction: An Introductory Report, Review of Palaeobotany and Palynology, 27(34):291328 (1979); J. Muller, Fossil Pollen Records of Extant Angiosperms, The Botanical Review, 47(1):1142 (1981); Annals of the Missouri Botanical Garden, vol. 71, no. 2 (1984), an entire issue devoted to papers on angiosperm paleobotany; P.R. Crane, Phylogenetic Analysis of Seed Plants and the Origin of Angiosperms, Annals of the Missouri Botanical Garden, 72:716793 (1985); Peter K. Endress, The Early Evolution of the Angiosperm Flower, Trends in Ecology & Evolution, 2(10):300304 (1987), written for the general reader; and D.W. Taylor and L.J. Hickey, An Aptian Plant with Attached Leaves and Flowers: Implications for Angiosperm Origin, Science, 247(4943):702704 (1990). Classification Taxonomy is addressed by Arthur Cronquist, The Evolution and Classification of Flowering Plants, 2nd ed. (1988), and An Integrated System of Classification of Flowering Plants (1981), the full-dress exposition of his system; Adolf Engler, Syllabus der Pflanzenfamilien, vol. 2, Angiospermen, 12th ed. rev. by Hans Melchior (1964), the most recent edition of this work; J. Hutchinson, The Families of Flowering Plants, 3rd ed., 2 vol. (1973), the final version of his system; Tod Stuessy, Plant Taxonomy: The Systematic Evaluation of Comparative Data (1990); and Armen Takhtajan (A.L. Takhtadzhian), Sistema i filogeniia tsvetkovykh rastenii (1966), the first full-length version of his system, and Sistema magnoliofitov (1987), the most recent full-dress version. Short versions of Takhtajan's system have appeared at various times in English, e.g., Outline of the Classification of Flowering Plants (Magnoliophyta), The Botanical Review, 46(3):225359 (1980). Wendy B. Zomlefer, Guide to Flowering Plant Families (1994), treats families of the United States. Other journal articles on angiosperm taxonomy include Arthur Cronquist, Armen Takhtajan, and W. Zimmermann, On the Higher Taxa of Embryobionta, Taxon, 15:129134 (1966); Robert F. Thorne, A Phylogenetic Classification of the Angiospermae, Evolutionary Biology, 9:35106 (1976), An Updated Phylogenetic Classification of the Flowering Plants, Aliso, 13(2):365390 (1992), and Classification and Geography of the Flowering Plants, The Botanical Review, 58(3):225348 (1992); F. Ehrendorfer and R.M.T. Dahlgren (eds.), Symposium on New Evidence of Relationships and Modern Systems of Classifications of the Angiosperms, Nordic Journal of Botany, 3(1):1155 (1983), a series of papers that give useful discussions on and bibliographies of modern angiosperm classifications; and three articles from The Botanical Journal of the Linnean Society: E.J.H. Corner, Angiosperm Classification and Phylogeny: A Criticism, 82(1):8188 (1981); and R.M.T. Dahlgren, A Revised System of Classification of the Angiosperms, 80(2):91124 (1980), and Angiosperm Classification and Phylogeny: A Rectifying Comment, 82(1):8992 (1981). Paleobotany and evolution The origins and diversity of flowering plants can best be understood by studying their fossil history. The fossil record provides important data to help show when and where early angiosperms lived, why flowering plants came to exist, and from what group or groups of plants they evolved. The earliest plants generally accepted to be angiospermous are known from the Early Cretaceous Period (144 to 97.5 million years ago). Fossil pollen of angiosperms is first found in the Hauterivian and Barremian ages, which spanned from about 131 to 119 million years ago. A very few angiosperm leaves and flowers are found in layers dating to the early Aptian Age (about 119 to 113 million years ago). Many of the earliest fossils of angiosperms are most similar to small bushes or small herbaceous plants, such as those in the Chloranthaceae (Piperales), Ceratophyllaceae (Nymphaeales), and Ranunculaceae (Ranunculales) families. More diverse flora showing a larger variety of pollen, leaves, and reproductive organs with angiospermous affinities developed during the Albian Age (113 to 97.5 million years ago). From the end of the Albian (the close of the Early Cretaceous) and the beginning of the Late Cretaceous (97.5 to 66.4 million years ago), angiosperms further diversified and dispersed. Many woody angiosperms evolved at that time, as did several modern groups, such as the magnolia, laurel, sycamore, and rose families. Herbaceous plants, such as the water lilies (Nymphaeales), the family Ceratophyllaceae, and some of the early monocotyledons also persisted from the Albian until today. Angiosperms are thought to have radiated from the Equator and spread to either pole because some of the oldest and most diverse angiosperm floras are found in Africa near the Equator, followed by low-latitude, angiosperm-dominated floras in North America. The angiosperms developed a close association with insect pollinators early in their evolution. This promoted outcrossing resulting in genetically vigorous offspring. Also, the relatively short generation time in which the angiosperms reproducepermitting rapid population growth and easier colonization of disturbed habitatsgave the flowering plants an adaptive advantage over the gymnosperms, which were dominant during the Early Cretaceous. The seeds of angiosperms were small and were probably eaten and carried to new areas by animals. Thus, the angiosperms were able to migrate into and occupy new areas of the world. At the beginning of the Cenomanian Age (about 97.5 to 91 million years ago), angiosperms probably formed dominant pockets of vegetation along many low coastal tropical and warm temperate areas of the world. During the Cenomanian, the angiosperms also spread to inland continental areas as well as northward and southward along the coasts. By the Middle to Late Cenomanian (about 95 to 91 million years ago), angiosperms became the dominant form of vegetation in many areas of the world. One of the most conspicuous features of angiosperms is the flower. Most frequently flowers are brightly coloured, often scented structures containing nectar and the male and female reproductive organs. Because it is essential for the genetic integrity of a plant that it avoid pollinating itself or a nearby, possibly closely related, neighbour, pollen from one plant must be moved some distance to an unrelated flowering plant. Wind is often an effective but an imprecise pollination mechanism. Frequently, flowering plants are more accurately pollinated by animals, which carry the pollen some distance to another flower. Thus, development of showy flowers has involved the coevolution of insects or other animals and the early ancestors of the angiosperms. Various groups of extinct seed plants have been proposed as the ancestral stock at different times in the evolution of the angiosperms. The Pteridospermales (seed ferns) are a group of extinct early seed plants that resemble small trees and shrubs with fernlike foliage. They bore seeds on their leaves or in specialized structures derived from leaves and had specialized pollen-bearing organs or simple anthers. The ovules and pollen organs were separate reproductive units, and wind may have been the most common agent of pollen transfer. Some seed ferns of the Paleozoic Era (about 570 to 245 million years ago) contained pollen grains that were much too large to be effectively dispersed by the wind. These plants probably depended on insects to carry the pollen grains from one plant to another. The Cycadeoidophyta are a group of extinct seed plants that contain members that have widely different reproductive structures. In some, the female and male reproductive organs were separate, while in others, the reproductive structures were organized into a common reproductive unit in which the male organs surrounded the female organ. These reproductive organs sat on a receptacle similar to that in flowering plants and often were surrounded by sterile bracts or leaflike tissue, which may have opened to form a flowerlike structure in the genus Williamsoniella (Cycadeoidales). Some extinct Cycadeoidales may have been pollinated by insects. The female and male reproductive organs tend to be clustered when insect pollination is involved, which is probably why most flowers are bisexual. It is not clear whether the flowering plants are derived from the Pteridospermales or the Cycadeoidales; however, in both groups, the potential existed for the modification of the plant body and the reproductive tissue to be responsive to both the physical and biological environments of the Mesozoic Era (245 to 66.4 million years ago). The pollen evidence suggests that the Gnetales, a modern group of gymnosperms closely related to the angiosperms, were present during the Triassic Period (245 to 208 million years ago). Thus, the evolution that produced the plants which were eventually recognized as the angiosperms must have been taking place during the Triassic, Jurassic, and earliest Cretaceous periods (which span from 245 to 97.5 million years ago). The ancestral stock probably was a small to medium-size plant in which large leafy shoots contained individual fertile female, fertile male, and sterile leaves. The form of the plant was modified: the leaf size was reduced, and some shoots were modified so that the ovules remained enclosed inside the leaf tissue, which was shortened so that the ovule and pollen organs were borne close together. The sterile leaves may have been lost in some evolutionary lines or may have evolved into sepals and petals in others. The pollen-bearing organs (stamens) or ovule-bearing organs (carpels) may have been lost in some lines of evolution, resulting in unisexual flowers, or both may have been retained together in others to produce bisexual flowers. Those early lines of angiosperm evolution in which wind may have functioned in pollination retained small, inconspicuous, often unisexual flowers, while in those evolutionary lines that developed close associations with specific insect pollinators, the organs become dramatically modified. Small, inconspicuous bisexual or unisexual flowers are known from the Aptian Age. Large petals developed by the late Albian (about 105 million years ago). In insect-pollinated flowers and bisexual flowers that contain their characteristic nectaries, very large petals and anthers with abundant small pollen are known from the earliest Cenomanian Age. The presence of small inconspicuous unisexual flowers, probably pollinated by wind or water, from the Aptian and late Albian suggests that the form and mode of reproduction of angiosperms were beginning to diverge from those of their ancestors even before this is attested to in macrofossils. The special features of flowering plants that enhanced the coevolutionary links with animals evolved at various times in different groups of angiosperms. There were, however, three major nodes of coevolution in the development of flowering plants: the evolution of showy flowers attractive to animal (mainly insect) pollinators, the evolution of bilaterally symmetrical flowers with variously fused parts to direct the behaviour of particular animal pollinators (especially social insects and birds), and the evolution of larger energy-rich animals (especially mammals and birds) to disperse fruits and seeds. Each of these events had a dynamic effect on the evolution of angiosperms, increasing their diversity at different times in different groups and affecting their floral and fruit morphology in various ways. The early angiosperms appear to have had few and radially arranged flower parts. The flowers were unisexual or bisexual, endowed with superior ovaries, loosely closed to fully closed carpels, free flower parts, and small fruits and seeds. The fossil record of the early evolution of the flower demonstrates a tendency toward an increased number of flower parts, a loose to complete fusion of carpels, the development of a style, the elevation of the stigmatic surface upon the style, a slight increase in seed size, and a diversity of ways in which flowers were borne upon the plant. The evolution of both female and male reproductive organs in the same flower was both beneficial and problematic in the early angiosperms. Insects visiting a unisexual flower either picked up pollen or deposited pollen, depending on the sex of the flower visited. Insect visits therefore only randomly fertilized flowers as the insect alternated between male and female flowers. It became beneficial to the flower to evolve a place for both sexes in a single flower so that each insect visit would deposit and remove pollen. When both sexes are present in a single flower, however, there develops a strong possibility that the flower may pollinate itself, a situation that would cause inbreeding depression, thereby reducing the vigour of the offspring over successive generations. It was probably very early in the evolutionary history of flowering plants that self-incompatibility was evolved, a mechanism that prevents flowers or plants from self-pollinating. The pollen of many modern insect-pollinated bisexual flowers is incompatible with the flower in which it is produced. Another feature of flowers that developed as a result of insect pollination is pollen-tube competition. When a pollen load of 50200 pollen grains is deposited on a stigma at one time, each pollen grain grows a pollen tube into the stigmatic tissue. The pollen tubes that grow the fastest reach the ovules first and effect fertilization. It has been demonstrated that the pollen grain with the fastest-growing pollen tube carries genes that produce more vigorous offspring. By the Early Cenomanian the stigmas of some insect-pollinated flowers were elevated on styles, effectively establishing some distance for the pollen tubes to travel. This would establish pollen-tube competition as a selective mechanism within some early flowers. During the first 70 million years of angiospermous evolution all the known flowers were radially symmetrical. It is only in the Tertiary (66.4 to 1.6 million years ago) during the late Paleocene and early Eocene (63.6 to 52 million years ago) that the first evidence of bilaterally symmetrical flowers is found. The evolution of bilateral flowers, as, for example, those of the legumes and orchids, is an adaptation for specialized pollinators such as social insects (bees) and some birds. The sterile organs (sepals, petals) are modified to present a certain flower orientation to the pollinator, enabling the pollinator to enter the flower where the pollen organs and pollen-receptive tissue are positioned to maximize effective pollination. During the early Tertiary, the bilateral organization of floral organs coevolved with animal behaviour independently at different times and in various groups of angiosperms. The evolution of mammals and birds also influenced the evolution of flowering plants in the early Tertiary. During the first 7080 million years of their existence, the fruits and seeds of the angiosperms were small. The initial radiation of larger energy-rich fruits and seeds, such as the acorns, chestnuts, walnuts, legume pods, and the earliest grasses, took place during the Eocene. These fruits appeared over a short period of time contemporaneously with the diversification of seed- and fruit-eating mammals and birds. Seeds of fleshy fruits, such as grapes, also became common in the Eocene (about 45 million years ago). This fact demonstrates that a second important node of plant and animal coevolution developed about 5060 million years ago, when angiosperms began to produce fruits and seeds that were attractive to animals. The animals served as agents to carry fruits and seeds some distance from the parent plant, further enhancing the potential for outcrossing and aiding in the dispersal of angiospermous plants to new areas of the world. In summary, the evolutionary history of angiosperms is intimately, but not exclusively, tied to their coevolution with animal pollinators and agents of fruit and seed dispersal. Wind and water pollination and fruit and seed dispersal also continued throughout the entire evolutionary history of flowering plants. This network of evolutionary pressures resulted in the variety of flowers and fruits representative of present-day angiosperms. Accordingly, some of the most useful characters in determining the particular taxon to which living angiosperms belong are flowers, fruits, and seeds. The evolution of such vegetative characteristics as wood and leaves is more complex and less well understood. David L. Dilcher Classification Diagnostic classification The angiosperms are a well characterized, sharply defined group. There is not a single living plant species whose status as an angiosperm or non-angiosperm is in doubt. Even the fossil record provides no forms that connect with any other group, although there are of course some fossils of individual plant parts that cannot be effectively classified. Most typically, angiosperms are seed plants. This separates them from all other plants except the gymnosperms, of which the most familiar representatives are the conifers and cycads. The ovules (forerunners of the seeds) of angiosperms are characteristically enclosed in an ovary, in contrast to those of gymnosperms, which are exposed to the air at the time of pollination and never enclosed in an ovary. Pollen of angiosperms is received by the stigma, a specialized structure that is usually elevated above the ovary on a more slender structure known as the style. Pollen grains germinate on the stigma, and the pollen tube must grow through the tissues of the style (if present) and the ovary to reach the ovule. The pollen grains of gymnosperms, in contrast, are received at an opening (the micropyle) atop the ovule. The female gametophyte of angiosperms (called the embryo sac) is tiny and contains only a few (typically eight) nuclei; the cytoplasm associated more or less directly with these nuclei is not partitioned by cell walls. One of the several nuclei of the embryo sac serves as the egg in sexual reproduction, uniting with one of the two sperm nuclei delivered by the pollen tube. Two other nuclei of the embryo sac fuse with the second sperm nucleus from the pollen tube. This triple-fusion nucleus is characteristically the forerunner of a multicellular food-storage tissue in the seed, called the endosperm. The process in which both nuclei from the pollen tube fuse is referred to as double fertilization. This is perhaps the most characteristic single feature of angiosperms and is not shared with any other group. Gymnosperms, in sharp contrast, have a multicellular female gametophyte that consists of many hundreds or even thousands of cells. Double fertilization does not take place in this case, and the female gametophyte develops into the food-storage tissue of the seed. Furthermore, angiosperms have a more complex set of conducting tissues than do gymnosperms. The water-conducting tissue (xylem) ordinarily includes some long tubes called vessels. Only one small group of gymnosperms, the Gnetophyta, has vessels. The food-conducting tissue (phloem) of angiosperms characteristically has companion cells that bear a direct ontogenetic relationship to the sieve tubes through which the actual conduction takes place. The phloem of gymnosperms has less specialized sieve cells and lacks companion cells. Recognition that the angiosperms fall into two major groups, the dicotyledons and monocotyledons, has a long history and is embodied in all major systems of classification proposed since the late 18th century. The principal differences between the two groups, here treated as the classes Magnoliopsida and Liliopsida, respectively, are shown in the table below. All of the differences are individually subject to failure, but collectively they provide a reasonably clear distinction. Only the position of the order Nymphaeales remains controversial. The traditional and still dominant view among authorities is that the Nymphaeales are dicotyledons, but there is a persistent minority opinion that in spite of their two cotyledons they belong with the monocotyledons instead. Reproduction General features The vast array of angiosperm floral structures is for sexual reproduction. The angiosperm life cycle consists of a sporophyte phase and a gametophyte phase. The cells of a sporophyte body have a full complement of chromosomes (i.e., the cells are diploid, or 2n); the sporophyte is the typical plant body that we see when we look at an angiosperm. The gametophyte arises when cells of the sporophyte, in preparation for reproduction, undergo meiotic division and produce reproductive cells that have only half the number of chromosomes (i.e., haploid, or n). A two-celled microgametophyte called a pollen grain germinates into a pollen tube and through division produces the haploid sperm. (The prefix micro- denotes gametophytes emanating from a male reproductive organ.) An eight-celled megagametophyte called the embryo sac produces the egg. (The prefix mega- denotes gametophytes emanating from female reproductive organs.) Angiosperms are vascular plants, and all vascular plants have a life cycle in which the sporophyte phase (vegetative body) is the dominant phase and the gametophyte phase remains diminutive. In the nonvascular plants, such as the bryophytes, the gametophyte phase is dominant over the sporophyte phase. In bryophytes, the gametophyte produces its food by photosynthesis (is autotrophic) while the nongreen sporophyte is dependent on the food produced by the gametophyte. In nonseed vascular plants, such as ferns and horsetails, both the gametophyte and sporophyte are green and photosynthetic, and the gametophyte is small and without vascular tissue. In the seed plants (gymnosperms and angiosperms), the sporophyte is green and photosynthetic and the gametophyte depends on the sporophyte for nourishment. Within the seed plants, the gametophyte has become further reduced, with fewer cells comprising the gametophyte. The microgametophyte (pollen grain), therefore, is reduced from between 4 and 8 cells in the gymnosperms to a 3-celled microgametophyte in the angiosperms. A parallel reduction in the number of cells comprising a megagametophyte (ovule) has also taken place: between 256 and several thousand cells in the gymnosperms to an 8-celled megagametophyte in the angiosperms. The significance of the reduction in megagametophyte cells appears to be related to pollination and fertilization. In many gymnosperms, pollination leads to the formation of a large gametophyte with copious amounts of stored starch for the nourishment of the potential embryo regardless of whether fertilization of the ovule can actually take place (i.e., whether the pollen is from the same species as the ovule). If the pollen is from a different species, fertilization or embryo development fails, so that the stored food is wasted. In angiosperms, however, the megagametophyte and egg are mature before the food is stored, and this is not ever accomplished until after the egg has been adequately fertilized and an embryo is present. This reduces the chances that the stored food will be wasted. Figure 16: Typical angiosperm life cycle (see text). The process of sexual reproduction (Figure 16) depends on pollination to bring these gametophytes in close association so that fertilization can take place. Pollination is the process by which pollen that has been produced in the anthers is received by the stigma of the ovary. Fertilization occurs with the fusion of a sperm with an egg to produce a zygote, which eventually develops into an embryo. After fertilization, the ovule develops into a seed, and the ovary develops into a fruit. Anthers A transverse section of the anther reveals four areas of tissue capable of producing spores. These tissues are composed of microsporocytes, which are diploid cells capable of undergoing meiosis to form a tetrad (four joined cells) of haploid microspores. The microspores eventually separate and become pollen grains. During pollen development, the layer of cells beneath the dermis of the anther wall (the endothecium) develops thickenings in the cell walls. The cell layer immediately inside the endothecium (the tapetum) develops into a layer of nutritive cells that either secrete their contents into the area around the microsporocytes or lose their inner cell walls, dissociate from each other, and become amoeboid among the microsporocytes. The pollen grains develop a thick wall of at least two layers, the intine and the exine. The intine, or inner layer, consists primarily of cellulose and pectins. The exine, or outer layer, is composed of a highly decay-resistant chemical called sporopollenin. The exine has one or more thin areas, or pores, through which the pollen tubes germinate, and the thick area of the exine is usually highly sculptured. The number of pores and pattern of exine sculpturing are characteristic within an angiosperm family, genus, and often within a species. The terminology to describe the various sculpturing patterns and position and number of pores is highly complex and only a basic description as related to functional aspects of sculpturing is given here. For example, smooth or essentially smooth pollen is loosely correlated with wind pollination, as in oaks (Quercus) and grasses (corn, Zea mays). Many plants pollinated by birds, insects, and small mammals have highly sculptured patterns of spines, hooks, or sticky threadlike projections by which pollen adheres to the body of the foraging pollinator as it travels to other flowers. Each microspore (pollen grain) divides mitotically to form a two-celled microgametophyte; one cell is a tube cell (the cell that develops into a pollen tube), and the other is a generative cell, which will give rise to two sperm as a result of a further mitotic division. Thus, a mature microgametophyte consists of only three haploid cellsthe tube cell and two sperm. Most angiosperms shed pollen at the two-celled stage, but in some advanced cases it is shed at the mature three-celled stage. When the pollen grains are mature, the anther wall either splits open (dehisces) longitudinally or opens by an apical pore. Because the sporopollenin is resistant to decay, free pollen is well represented in the fossil record. The distinctive patterns of the exine are useful for identifying which species were present as well as suggesting the conditions of early climates. The proteins in the pollen walls are also a major factor in hay fever and other allergic reactions, and the spinose sculpturing patterns may cause physical irritation. Structure and function The wide diversity in the morphological features of the plant body has been discussed above. This section will outline the underlying structural (anatomic) diversity among angiosperms. Vegetative structures There are three levels of integrated organization in the vegetative plant body: organ, tissue system, and tissue. The organs of the plantthe roots, stems, and leavesare composed of tissue systems (dermal tissue, ground tissue, and vascular tissue; see below Tissue systems). The tissues of each of these systems are composed of cells of one or more types (parenchyma, collenchyma, and sclerenchyma; see below Tissue systems: Ground tissue). Tissues composed of only one cell type and performing only one function are simple tissues, while those composed of more than one cell type and performing more than one function, such as support and conduction, are complex tissues. Xylem and phloem are examples of complex tissues. The plant develops from a fertilized egg, called a zygote, which undergoes mitotic cell division to form an embryoa simple multicellular structure of undifferentiated cells (i.e., those that have not developed into cells of a specific type)and eventually a mature plant. The embryo consists of a bipolar axis that bears one or two cotyledons, or seed leaves; in most dicots the cotyledons contain stored food in the form of proteins, lipids, and starch, or they are photosynthetic and produce these products, whereas in most monocots and some dicots the endosperm stores the food and the cotyledons absorb the digested food. The embryos of dicotyledons have two seed leaves, while those of monocotyledons have only one. Figure 3: Apical meristems. (Left) The shoot apical meristem of Hypericum uralum As the embryo continues to develop and new cells arise, the angiospermous plant develops specialized regions in which only cell division takes place and other areas in which nonreproductive (vegetative) activities, such as metabolism, respiration, and storage, occur. The areas of dividing cells, essentially permanently embryonic tissue, are called meristems, and their cells are termed initials. In the embryo they are found at either end of the bipolar axis and are called apical meristems. As the plant matures