Angiosperms

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Angiosperms, commonly known as flowering plants, are the largest and most diverse group of plants, with 275,000 species representing more than 80 percent of all green plants. As a consequence, they are the principal components of the earth's terrestrial vegetation and are the most economically important group, as the principal source of foods, fiber products, timber, medicinals, dyes, spices, and ornamentals. In terms of number and phytomass, they contribute to the food chain in many ways. The vegetative parts are consumed by and support herbivores. Shoots and roots support vast numbers of insects and other invertebrates at vari

Grasses grow in a reclaimed coal strip mine near Westernport, Maryland. (NASA)

ous phases in their life cycles. Nectar-producing flowers support pollinating insects, birds, and small mammals such as rodents, bats, and marsupials. Fruits and seeds are an important food source for many birds, mammals, and some fish. Angiosperms are as important a food source to humans as they are to other animals, either through the consumption of grazing herbivores or through the direct consumption of fruits, seeds, and vegetables. The most important angiosperm families are the Poaceae (cereals, forage grasses, and bamboo), the Solanaceae (potato, eggplant, tomato, and chili and sweet peppers), Brassicaceae (broccoli, cabbage, cauliflower, radish, turnip, and other vegetables), Rosaceae (apples, cherries pears, plums, and many berries), Cucurbitaceae (melons, squashes, gourds, and the like), and Fabaceae (legumes and beans).

Angiosperms are a monophyletic group, and their unique features, besides the presence of flowers, are the carpel, which encloses the seeds and ripens as the fruit; the presence of a companion cell with each sieve tube element in the phloem, the photosynthate transport cells; and the presence of endosperm in the seeds that is the product of a triple fusion nucleus (that is, triploid) composed of two nuclei from the female parent and one from the male. The endosperm is generally the principal storage product (for example, the bulk of a grain of wheat) used by the developing embryo and seedling.

Growth form in the angiosperms varies from plants of less than 2 mm in watermeal (Wolffa, Araceae) to trees more than 100 m tall in mountain ash (Eucalyptus regnans, Myr-taceae). Within this range occur myriad sizes, from herbs to shrubs to trees, and forms, such as succulents in dry habitats (such as cacti), submerged aquatics both in freshwater and marine habitats (for example, sea grasses, or Zosteraceae), epiphytes (for example, most orchids and bromeliads), and insectivorous plants in bogs (such as sundews, Drosera; Venus's flytrap, Dionaea; pitcher plants, Sarracenia; and so forth). Life cycles also vary, ranging from herbaceous annuals that complete their life cycle within one growing season to herbaceous biennials (for example, beet, Beta vulgaris, and carrot, Daucus carrota) that grow vegetatively in the first season and then flower and fruit in the second season to herbaceous and woody perennials that grow for many years, often flowering and fruiting each year. Examples of perennial herbs are savanna grasses and lilies. In temperate areas, the aerial parts of perennials often die back, with new shoots produced the following year from underground rhizomes, bulbs, corms, tubers, or stolons.

In order to understand this diversity, it is necessary to understand the basic organization of the angiosperm plant body, which consists of three fundamental parts: stems, leaves, and roots. These organs constitute the vegetative plant body. Together the stem and its attached leaves constitute a shoot. The shoot grows from terminal buds and branches from lateral buds, which are located in the axil of each leaf, where it meets the stem. This attachment point is a node, and the area between nodes, if present, is an internode. Simply varying internode length from very long to very short can lead to vines (for example, grape, Vitis vinifera) versus rosette plants (for example, lettuce, Lactuca sativa). These positional relationships of leaf stem and axillary bud allow us to interpret the diversity of the shoot system so that we find leaves that no longer look like leaves, and stems that are so modified that they look like leaves. Thus spines are modified leaves (for example, cacti), thorns are modified shoots (for example, honey locust, Gledis-tia tricanthos), and prickles are merely epidermal proliferations (for example, roses). Stems may be flattened into cladodes that look just like leaves, as in many epiphytic cacti, or modified into tendrils as in passion flower (Passiflora spp.). Stems may be modified into storage organs such as bulbs, corms, rhizomes, and tubers.

Branching is of two principal types: monopodial and sympodial. In monopodial branching, there is a main terminal bud that grows as a leader shoot with lateral axillary branches remaining subordinate (for example, hickory trees, Carya). In sympodial branching there is no continuously growing leader shoot, because the terminal bud is determinate (that is, usually turning into a flower— inflorescence), and an axillary bud takes over as a new leader shoot, also known as a renewal shoot. Many trees combine both types of branching, with the main stem being monopo-dial and side branches being sympodial (for example, the pagoda tree, Terminalia catapa).

There are two basic types of root systems: a primary root system and an adventitious root system. The primary root system consists of a prominent tap root from which many lateral roots originate, giving rise to more lateral roots until a highly branched root system is developed. In contrast, adventitious roots arise from stems, are sparingly branched, and are often short lived. They are found mostly in the monocots (grasses, lilies, orchids, bromeliads, and so forth). Both types of roots may be modified into fleshy storage organs. Examples of these tuberous roots are the tap roots of carrots and beets and the adventitious roots of cassava (Manihot esculenta). The roots of most angiosperms are mycorrhizal.

Just as there is wide variation in the vege tative plant parts, there is similar variation in the reproductive parts. The reproductive unit of the angiosperms is the flower. Many vegetative shoots eventually become flower buds that have determinate growth. Flowers may occur singly or in clusters, known as an inflorescence. A basic flower consists of four sets of organs successively and alternately attached in whorls to a short receptacle. From the base upward, these are the sepals, petals, stamens (male, pollen-producing organs), and carpels (female, seed-producing organs). Collectively, the sepals are the calyx, the petals the corolla, the stamens the androecium, and the carpels the gynoecium. The calyx and corolla are known collectively as the perianth. When all sets are present, the flower is complete. In contrast, an incomplete flower is missing one of the sets. A perfect flower has both stamens and carpels; an imperfect flower is missing either stamens (a carpellate flower) or carpels (a staminate flower). Often, when the stamens are sterile, they become petaloid and are known as staminodes (for example, most rose cultivars). Species with either perfect flowers or both types of imperfect flowers are monoecious, whereas those species with only staminate or carpellate flowers are dioecious. Floral organs may be fused. When members of the same type are fused, they are connate (fused laterally) with each other (for example, the petals of bindweed, Convolulus). When members of different types are fused, they are adnate to one another (for example, stamens adnate to the petals in mints, Lamiaceae). When the perianth and androecium are adnate to the gynoecium, the flower is termed epig-ynous, and when free from the gynoecium, hypogenous. The flowers may be radially symmetrical or actinomorphic (polysymmetric), or bilaterally symmetrical or zygomorphic (mono-symmetric).

The potential combinations of missing parts, symmetry, and fusions are the basis for angiosperm floral diversity. Most of this diversity appears to be in response to specialized pollination mechanisms involving the transfer of pollen from the stamens to the stigmas of the gynoecium. Wind-pollinated flowers are simple, very often unisexual, and have either reduced perianth parts or none at all. In contrast, animal-pollinated flowers are generally showy and elaborate, with varying degrees of fusion of parts and monosymmetry accompanied by nectaries. Along with this floral diversity is a range of inflorescence diversity. There are two major types of inflorescences based upon monopodial and sympodial branching. In monopodial or indeterminate inflorescences, the apex does not form a terminal flower. The first flowers to form and open are at the base of the inflorescence, and, thus, the last formed and youngest flowers are in the center. These include racemes, spikes, aments (catkins), umbels, and heads. In racemes, each flower is borne at the end of a stalk or pedicel. Spikes and aments are racemes with sessile flowers. Spikes are erect; aments are pendulous and associated with wind pollination (for example, oaks, Quercus spp.). Umbels have basal flowers with longer pedicels that those of the apical flowers, so that all flowers are presented collectively in one plane for pollination (for example, carrot). Heads have nonpedi-cellate flowers and are contracted spikes that appear spherical to flat, where the inflorescence itself may appear as a single flower (for example, sunflower, Helianthus annus). In sympodial or determinate inflorescences (cymes), the apex forms a terminal flower. The first flowers to form and open are those at the apex, and, thus, the last formed and youngest flowers are on the outside. Parallels to umbels and heads are formed by varying pedicel lengths but can be distinguished by the presence of older, centrally located flowers.

Fruits are a ripened carpel or, in the case of fused carpels, a ripened gynoecium. There are many types of fruits, and fruit classification is highly complex. However, there are a few common and overlying themes. When a flower has only a single carpel or several fused carpels, the fruit is a simple fruit, as in peaches and oranges. When the flower has several separate carpels, the fruit is an aggregate fruit, as in blackberries, in which each unit is a carpel. When an inflorescence ripens as a whole, the fruit is a multiple fruit, as in mulberries and pineapple. Fruits are also classified as dry or fleshy. Simple dry fruits are either dehiscent or indehiscent. Capsules are dehiscent fruits formed by several fused carpels that open either longitudinally (for example, lilies) or apically by an operculum (for example, Brazil nut, Bertholletia excelsa). Follicles are single carpels with a single line of dehiscence (for example, milkweed, Asclepias spp.), in contrast to legumes with two lines of dehiscence per single carpel. Achenes are indehiscent fruits formed by a single carpel, whereas nuts are formed by fused carpels. Winged achenes are samaras, as in ashes (Fraxinus spp.). Schizocarps are formed when the fused carpels separate at maturity but remain indehiscent. In maples (Acer spp.), the individual schizocarps are winged. Another specialized indehiscent fruit type is the caryopsis, found in the grasses, in which the single seed is fused to the fruit wall. Fleshy fruits are berries, drupes, and pomes. Berries are many-seeded fruits formed by one carpel or several fused carpels; they are found in many textures (for example, melons, oranges, tomatoes, and so forth). Drupes (stone fruits) are a single, carpelled simple fruit with a fleshy outer wall and a woody, inner wall (plums, cherries, peaches, and so forth). Druplet is used for the individual carpels of aggregate fruit (for example, raspberries). Pomes are formed from epig-ynous flowers in which the adnate perianth parts become fleshy (for example, apple and pears). Fruits serve as dispersal units and as agents of dispersal. They also often provide nutrients for developing seedlings.

The classification of the angiosperms is currently in a state of flux as a result of new molecular data. However, this data has yet to be widely applied and has not yet been thoroughly integrated. Consequently, the brief outline given here may well be subject to change in the near future. For example, the angiosperms have traditionally been divided into two classes: the Magnoliopsida or Dicotyledons and the Liliopsida or Monocotyledons. It is now well established that the former is artificial, because the monocots clearly are derived from within the dicots; this, however, has yet to be incorporated in the classification.

Class Magnoliopsida is the largest group, with about 200,000 species in 10,500 genera and 316 families in 65 orders. The class is divided into a minimum of six subclasses: Magnoliidae, Hamamelidae, Caryophyllidae, Dilleniidae, Rosidae, and Asteridae. The Magnoliidae have in the main perfect flowers with separate carpels and monosulcate pollen. Many members are considered archaic and are found in the Pacific Basin and particularly New Caledonia. The Magnoliidae, with about 12,000 species in 39 families and 8 orders, have been considered a basal group of angiosperms, and recent evidence supports earlier suggestions that the subclass is para-phyletic. It includes many herbaceous or semiherbaceous members that appear to form a monophyletic group, chief among which are the Piperales, Ranunculales (buttercups), Papaverales (poppies), Laurales (laurels), and Nymphaeales (water lilies). The Hamamel-idae, with about 3,500 species in 26 families and 11 orders, are characterized by more or less reduced flowers with a poorly developed or missing perianth. The flowers are often unisexual and the inflorescence is an ament (catkin). The principal orders of the Hamamelidae are the Fagales (oaks and beeches), Juglandales (walnuts and hickories), and Urticales (mulberries, figs, nettles, cannabis, and so forth). The latter appears from molecular evidence to be misplaced here. The Hamamelidae are mainly temperate, with tropical members occurring at high elevations and Urticales occurring in the lowland tropics.

The Caryophyllidae, with about 11,000 species in 14 families and 3 orders, usually contains betalins instead of anthocyanins, often have the petals missing and the sepals appearing as petals, and have perisperm as the seed storage tissue instead of endosperm. The principal order is the Caryophyllales, which includes pinks, cacti, pokeweeds, and amaranths. The Dilleniidae, with about 26,000 species in 77 families and 13 orders, have a persistent calyx and style in fruit and centrifugal stamens. The principal orders are the Theales, Malvales (with kapok, cotton, okra, balsa, and so forth), Nepenthales (with pitcher plants and sundews, although the order is probably misplaced), Violales (with squashes, passion flowers, begonias, and so forth), Capparales (with capers and the mustard family), Ericales (blueberries, cranberries, and so forth), and Ebenales (with ebony, persimmons, and so forth). The Rosidae is the largest subclass, with more than 65,000 species in 116 families and 18 orders. Definitive characters are lacking, and the subclass is most likely not natural. Principal orders include the Fabales (legumes), Rosales (with roses, apples, pears and most drupaceous fruits and crassulas, gooseberries, and so forth), Myrtales (with eucalypts and pomegranates), Santalales (most families of which are parasitic), Euphorbiales (euphorbs and boxwood), Sapindales (with maples, horse chestnuts, poison ivy, mahoganies, citrus, and so forth), and Apiales (aralias, celery, rhubarb, and so forth). The latter is most certainly misplaced, based upon chemistry and molecular data and probably belongs in the Asteridae. The Asteridae, with more than 60,000 species in 49 families and 11 orders, have tubular corollas and generally only two fused carpels. Principal orders include the Solanales (potatoes, morning glories, and so forth), Lamiales (mints and borages), Scrophulariales (gesne-riads, ashes, olives, scrophs, bignons, and so forth), Campanulales, and Asterales (with a single family, the Asteraceae or sunflowers, with some 20,000 species).

Class Liliopsida is characterized by flowers with the parts in threes or multiples of three, no truly woody members, one cotyledon or seed leaf, and the mature root system composed only of adventitious roots. There are approximately 7,500 species in 3,000 genera and 106 families in 23 orders. The class is divided into five subclasses: Alismatidae, Arecidae, Commelinidae, Zingiberidae, and Liliidae. Most of these are artificial, and the Zingiberidae is a subset of the Commelin-idae, as is part of the Arecidae (the palms). The Arales (aroids) of the Arecidae are part of the Alismatidae. Future formal classifications of the Liliopsida will certainly reflect these changes based upon phylogenetic analyses of morphological and molecular data. Briefly, the Liliidae contains the lilies, agaves, aloes, irises, orchids, and many other commonly cultivated ornamentals. The Zin-giberidae contains the gingers, bananas, stre-litzias, heliconias, marantas, and cannas. The Commelinidae contains the bromeliads, water hyacinths, pondweeds, xyrises, grasses, sedges, rushes, and restios. The Alismatidae contains many freshwater aquatic monocots as well as the only marine angiosperms.

—Dennis Wm. Stevenson

See also: Biogeography; Botany; Extinction, Direct Causes of; Fungi; Gymnosperms; Phylogeny; Plate Tectonics

Bibliography

Angiosperm Phylogeny Group. 1998. "An Ordinal Classification for the Families of Flowering Plants." Annals of the Missouri Botanical Garden 85: 531-553; Cronquist, Arthur. 1981. The Evolution and Classification of Flowering Plants. New York: Columbia University Press; Endress, Peter. 1994. Diversity and Evolutionary Biology of Tropical Flowers. Cambridge: Cambridge University Press; Gifford, Ernst M., and Adriance S. Foster. 1989. Comparative Morphology of Vascular Plants, 3d ed. San Francisco: W. H. Freeman; Kubitzki, Klaus, ed. 1993, 1998. The Families and Genera of Vascular Plants. Vol. II: Dicotyledons: Mag-nolid, Hamamelid and Caryophyllid Families. Vol. III: Monocotyledons: Lilianae (except Orchidaceae). Vol. IV: Monocotyledons: Alismatanae andCommelinanae (except Gramineae). Berlin: Springer-Verlag; Leins, Peter. 2000. Blüte und Frucht: Morphologie, Entwicklungsgeschichte, Phylogenie, Funktion, Ökologie. Stuttgart: E. Schweizerbart'sche; Takhtajan, Armen Leonovich. 1997. Diversity and Classification of Flowering Plants. New York: Columbia University Press; Walters, Kerry S., and Harriet J. Gillett, eds. 1998. 1997IUCN Red List of Threatened Plants. Gland, Switzerland: IUCN World Conservation Union; Weberling, Focko. 1989. Morphology of Flowers and Inflorescences. Cambridge: Cambridge University Press.

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