Distribution and abundance
- Also called:
- flowering plant
The diversity of form within the angiosperms has contributed to their successful colonization of more habitats than any other group of land plants. Gymnosperms (the nonflowering seed plants) are only woody plants with a few woody twining vines. There are few herbaceous or aquatic gymnosperms; most gymnosperms do not occur as swampy vegetation or in marine habitats. With the exception of cycads, gymnosperms have simple leaves, and none are modified as spines, tendrils, or storage organs.
The absence of substantial diversity in the vegetative features of gymnosperms appears to have limited their ability to adapt to diverse or extreme habitats. The absence of xylem vessels in most gymnosperms, and hence the less efficient water transport system than that found in the angiosperms, is one example. In fact, the only gymnosperms with vessels, the Gnetales, is the only group that contains vines and the only group that deviates from the usually woody trunk growth form. The absence of vessels in angiosperms, however, is rare; the few groups without vessels are small trees or shrubs with limited distribution, as in the Winteraceae. Another factor contributing to the limited distribution of gymnosperms is that they do not produce reproductive structures until several years after the seed germinates; therefore, a woody habit is required to achieve sexual maturity. Finally, the gymnosperms also require a relatively stable environment for growth. Thus, restraints imposed by anatomy and life cycle have probably limited morphological diversity among the gymnosperms.
The wide variation in the angiosperm form is reflected in the range of habitats in which they grow and their almost complete worldwide distribution. The only area without angiosperms is the southern region of the Antarctic continent, although two angiosperm groups are found in the islands off that continent. Angiosperms dominate terrestrial vegetation, particularly in the tropics, although submerged and floating aquatic angiosperms do exist throughout the world. Angiosperms are the principal component of salt marshes, tidal marshes, and mangrove marshes. The only vascular marine plants are a few submerged marine angiosperms that occur in shallow waters of coastal areas throughout the world—for example, the eelgrasses (Zostera and Phyllospadix; Zosteraceae). The various terrestrial biomes (defined primarily based upon the type of vegetation and climate) are composed mainly of herbaceous and woody angiosperms, except for taiga (boreal forest), temperate rainforest, and juniper savanna, where conifers (a gymnospermous division) dominate the woody component and angiosperms dominate the herbaceous and shrub components.
Morphological and habitat diversity, together with cosmopolitan distribution, contributes to the wide ecological tolerance of the angiosperms—adapting to Alpine tundra regions and salt marshes, from the Arctic Circle to the lowland tropical rainforests. The importance of angiosperms in the terrestrial portion of the biosphere is rarely rivaled by any other group of organisms.
All but a few angiosperms are autotrophs: they are green plants (primary producers) that use solar radiation, carbon dioxide, water, and minerals to synthesize organic compounds; oxygen is a by-product of these metabolic reactions. The few exceptions are either mycoheterotrophs (e.g., the Indian pipe Monotropa uniflora; Ericaceae) that use connections with mycorrhizal fungi (fungi that form an association with the roots of certain plants) to obtain carbohydrates or parasitic plants that develop specialized roots (haustoria), which penetrate the host plant and absorb food and other materials (e.g., the dodder [Cuscuta species; Convolvulaceae]).
Importance
Contribution to food chain
Because angiosperms are the most numerous component of the terrestrial environment in terms of biomass and number of individuals, they provide an important source of food for animals and other living organisms. Organic compounds (carbon-containing compounds, principally carbohydrates) not only are used by the plant itself for synthesizing cellular structures and for fueling their basic metabolisms but also serve as the only source of energy for most heterotrophic organisms. (Heterotrophs require an organic source of carbon that has originated as part of another living organism, in contrast to autotrophs, which require only an inorganic source of carbon—CO2.) Solar energy is trapped by the photosynthetic pigments in the plant cells and converted into chemical energy, which is stored in the tissues of the plant. The trapped energy is transferred from one organism to the next as herbivores consume the plant, carnivores consume herbivores, and so on up the food chain. In a temperate forest, a single angiosperm tree may support many thousands of animals (the majority being insects, birds, and mammals), a relationship that underscores the basic importance of the angiosperms to the food chain and the ecological web.
The angiosperm body contributes to the food chain in many ways. The vegetative parts (the nonreproductive organs, such as stems and leaves) are consumed by, and support, plant-eating animals. Vast numbers of insects and other invertebrates depend on shoots for food during all or part of their life histories. The reproductive organs (flowers, fruits, and seeds) also provide an energy source for many animals. The pollen supports many pollinating insects, particularly bees.
The flowers provide food from floral nectaries that secrete sugars and amino acids. These flowers often produce fragrances that attract pollinators which feed on the nectar. Nectar-feeding animals include many insect groups (bees, butterflies, moths, flies, and even mosquitoes), many mammal groups (bats, small rodents, and small marsupials), and birds (honeyeaters, hummingbirds, and sunbirds). Nectaries also occur on the nonfloral, or vegetative, parts of some angiosperms, such as the leaves and the petioles of bull’s-horn thorn (Acacia collinsii; Fabaceae). Ants live inside the hollow modified spinous structures of bull’s-horn thorn and feed on the nectar. In return for this food source, they attack and destroy animals of all sizes as well as other plants that contact the acacia plant. In doing so, the ants protect the bull’s-horn thorn from herbivores and other plants competing for the available space, light, and minerals.
Fruits produced by angiosperms are the principal food for many bats, birds, mammals, and even some fish. Seeds are also an important food source for many animals, particularly small rodents and birds. These animals often carry the fruits and seeds of the angiosperms they consume to new areas, where the angiosperms propagate.
Another aspect of angiosperm diversity is found in the production of secondary compounds, such as alkaloids, quinones, essential oils, and glycosides. Angiosperms have evolved a comprehensive array of unpalatable or toxic secondary plant compounds that protect the plants from foraging herbivores. Some insects, however, successfully store these secondary compounds in their tissues and use them as protection from predation.
As the principal component of the terrestrial biosphere, the angiosperm flora determines many features of the habitat, some of which are available food, aspects of the forest canopy, and grazing land. They supply nesting sites and materials for a wide range of birds and mammals, and they are the principal living spaces for many primates, reptiles, and amphibians. The tank bromeliad, which traps water in its crowns, provides a habitat for salamanders, frogs, and many aquatic insects and larvae. The animal inhabitants of the water-filled insectivorous pitcher plant leaves have adapted to the hostile environment of the leaves’ digestive fluids.
Significance to humans
Angiosperms are as important to humans as they are to other animals. Angiosperms serve as the major source of food—either directly or indirectly through consumption by herbivores—and, as mentioned above, they are a primary source of consumer goods, such as building materials, textile fibers, spices and herbs, and pharmaceuticals.
Among the most important food plants on a global scale are cereals from the grass family (Poaceae); potatoes, tomatoes, eggplant, and chili peppers from the potato family (Solanaceae); legumes or beans (Fabaceae); pumpkins, melons, and gourds from the squash family (Cucurbitaceae); broccoli, cabbage, cauliflower, radish, and other vegetables from the mustard family (Brassicaceae); and almonds, apples, apricots, cherries, loquats, peaches, pears, raspberries, and strawberries from the rose family (Rosaceae). Members of many angiosperm families are used for food on a local level, such as ullucu (Ullucus tuberosus; Basellaceae) in the Andes and cassava (Manihot esculenta; Euphorbiaceae) throughout the tropics. Tropical angiosperm trees are an important source of timber in the tropics and throughout the world.
Dennis William Stevenson
The flowering plants have a number of uses as food, specifically as grains, sugars, vegetables, fruits, oils, nuts, and spices. In addition, plants and their products serve a number of other needs, such as dyes, fibers, timber, fuel, medicines, and ornamentals. Many plants serve more than one function. For example, the seeds of the kapok fruit (Ceiba pentandra; Malvaceae) yield a water-repellent fiber used in sound and thermal insulation and an edible oil used in cooking, lubricants, and soap; the oil cake is rich in protein and is fed to livestock; and the soft, light wood is used to make furniture and boats.
The angiospermous plant converts the energy of the sun into starch, the energy-rich storage form of sugar, and reserves it in the endosperm of the seed for the time when the seedling germinates and grows. Among the most economically important grains throughout the world are corn (Zea mays), wheat (Triticum), rice (Oryza), barley (Hordeum), oats (Avena), sorghum (Sorghum), and rye (Secale), all members of the grass family, Poaceae.
Corn provides food for humans and domesticated animals, and its derivatives (e.g., cornstarch and corn oil) are used in making cosmetics, adhesives, varnishes, paints, soaps, and linoleum. Among the many cultivars of Zea mays, dent corn, variety indentat, is a widely used feed type in the United States. Wheat, barley, and rye are all members of the same tribe (Triticeae) within the family Poaceae. Wheat is among the oldest of the cultivated food crops. Barley is used for human consumption, livestock feed, and malting. Rye is usually used as a livestock feed, but can be used in baking and distilling liquor. Rice is a semiaquatic annual grass and is one of the major cereal crops of the world.
Vegetables constitute perhaps the greatest diversity of form and nutritional content and are grown for one or more of their parts—the flowers, shoots, or leaves; or the underground parts, such as tuberous roots, bulbs, rhizomes, corms, and tubers.
The globe, or French, artichoke (Cynara scolymus; Asteraceae) is an immature flower bud and receptacle overlaid by bracts. Asparagus (Asparagus officinalis; Asparagaceae) is a perennial plant cultivated for its succulent green cladodes that arise from underground stems called crowns.
The mustard family (Brassicaceae) contains a number of important vegetables—broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, and kohlrabi—all members of Brassica oleraceae and comprising a group of vegetables called the cole crops, a term that probably reflects the fact that they are principally stem plants. The flower heads and stalks of broccoli and cauliflower are eaten, the two plants differing in that the white head of the cauliflower consists of malformed (hypertrophied) flowers that form in dense clusters. Brussels sprouts continually form many small heads in the axils of the leaves throughout the growing season. The cabbage head is a large terminal bud.
The edible portion of celery (Apium graveolens; Apiaceae) is the petiole (leaf stalk) that arises from a compact stem. Rhubarb (Rheum rhabarbarum; Polygonaceae) is a leafy plant also grown for its leaf petioles.
Parsley (Petroselinum crispum; Apiaceae), spinach (Spinacia oleracea; Amaranthaceae), and Swiss chard (Beta vulgaris, variety cicla; Amaranthaceae) are cultivated for their leaves, and the leek (Allium porrum; Amaryllidaceae), a close relative of the onion, is cultivated for its leaf bases.
Root crops are grown for their fleshy subterranean storage bodies: tuberous roots, bulbs, rhizomes, corms, and tubers. The potato is a tuber found in Solanaceae, the potato family. Other important root crops include the carrot (Daucus carota; Apiaceae), beet (Beta vulgaris; Amaranthaceae), and sweet potato (Ipomoea batatas; Convolvulaceae), as well as the radish (Raphanus sativus), turnip (Brassica rapa, variety rapa), and rutabaga (B. napus, variety napobrassica) of the mustard family (Brassicaceae).
Bulb crops are underground leafy scales attached to short compressed stems; food is stored in the leaves rather than the roots, causing them to enlarge into bulbs. Onions and garlic (Allium cepa and A. sativum, respectively; Amaryllidaceae) are the most obvious examples of the bulb vegetable.
Many plants classified popularly as vegetables are in actuality fruits because they develop from the reproductive structures of the plant. The genus Cucurbita (Cucurbitaceae) includes the pumpkins, squashes, and gourds, of which C. moschata (winter squash, or crookneck pumpkin), C. pepo (summer squash, or marrow), and C. mixta (the pumpkin, or mixta squash) are some of the common types. The cucumber (Cucumis sativus; Cucurbitaceae) produces a fruit that develops from a branching vine. Okra (Abelmoschus esculentus; Malvaceae) is a warm-weather crop that produces small fruit pods. Breadfruit (Artocarpus altilis; Moraceae), a plant native to the Pacific Islands, is a staple crop, providing a rich source of calcium and starch.
The common bean (Phaseolus vulgaris), including the French, or kidney, bean, the string bean, and the navy bean, is the edible fleshy pod containing the bean seeds. It provides a good source of protein. Lima beans (P. lunatus) probably originated in Central America and are now found in the United States, the lowland tropics, and Africa. The garden, or English, pea (Pisum sativum; Fabaceae) is an annual cool-weather plant cultivated for its edible green seed or pod. The pea is found throughout most temperate and tropical regions.
The family Solanaceae contains a number of important fruit vegetables—eggplants (aubergines), peppers, and tomatoes—all herbaceous plants, which are perennial in the tropics and annual in temperate zones. The pepper (Capsicum) includes the sweet, or bell, pepper (which is green when immature but red or yellow when ripe), and the red, or chili, pepper. Pepper plants are cultivated for their fruits, some of which are extremely pungent because of the presence of capsaicin found in the septa, in the placenta, and, to a lesser extent, in the seeds, but not in the wall, of the fruit. The tomato (Solanum lycopersicum), native to South America, was at one time wrongly reported to bear poisonous fruits. The fruit is a fleshy berry invested with many small seeds.
Plants cultivated for their fruits are found in temperate, tropical, or subtropical regions. Temperate plants are generally deciduous and either tolerate or require a cool period for growth. Apples (Malus) and pears (Pyrus) are important pome fruits of the family Rosaceae. Some well-known stone fruits of the family include the peaches and nectarines (Prunus persica), plums (P. domestica), and cherries (P. avium). Other temperate fruits grown on bushes, vines, or low plants include grapes (Vitis; Vitaceae) and strawberries (Fragaria; Rosaceae), as well as blueberries (Vaccinium) and cranberries (V. macrocarpon), both from Ericaceae.
Tropical fruits tend to be grown on evergreen plants and can survive temperatures only above freezing. Subtropical plants are either deciduous or tropical and are not as susceptible to temperatures slightly below freezing. Citrus (Rutaceae), avocados (Persea americana; Lauraceae), olives (Olea; Oleaceae), dates (Phoenix dactylifera; Arecaceae), figs (Ficus; Moraceae), pineapples (Ananas comosus; Bromeliaceae), bananas (Musa; Muscaceae), and papayas (Carica; Caricaceae) are tropical and subtropical plants.
Commercially important plants cultivated for the nuts and hard seeds they produce are almonds (Prunus dulcis; Rosaceae), walnuts (Juglans; Juglandaceae), pecans (Carya illinoinensis; Juglandaceae), macadamias (Macadamia; Proteaceae), and hazelnuts (Corylus; Betulaceae).
Sugarcane (Saccharum officinurum; Poaceae) and sugar beet (Beta vulgaris; Amaranthaceae) are rich sources of natural sugar.
Peanuts (Arachis) and soybeans (Glycine), both members of Fabaceae, the legume family, produce edible seeds that are important for their rich supply of protein or oil. Other plants rich in oil and important economically are the castor bean (Ricinus; Euphorbiaceae), coconut (Cocos nucifera; Arecaceae), flax (Linum usitatissimum; Linaceae), olives, oil palm (Elaeis guineensis; Arecaceae), sesame (Sesamum; Pedaliaceae), and sunflowers (Helianthus; Asteraceae).
As noted earlier, some plants produce toxic secondary compounds for protection. Some of the secondary compounds produced by angiosperms are not toxic, however; in fact, many are found in herbs and spices—for example, cloves, the dried flower buds of Syzygium aromaticum (Myrtaceae). The use of herbs and spices in cooking predates recorded history. Herbs are usually leaves or young shoots of nonwoody plants, although bay leaves and a few other leaves from woody plants are also considered herbs. Spices are the highly flavored, aromatic parts of plants that are usually high in essential oil content. Spices are derived from roots, rhizomes, bark, seeds, fruits, and flower parts. The search for spices and alternative shipping routes for spices played a major role in world exploration in the 13th to 15th century. Many beverages are also derived from angiosperms; these include coffee (Coffea arabica; Rubiaceae), tea (Camellia sinensis; Theaceae), many soft drinks (e.g., root beer from the roots of Sassafras albidum; Lauraceae), and most alcoholic beverages (e.g., beer and whiskey from cereal grains and wine from grapes).
The angiosperms provide valuable pharmaceuticals. With the exception of antibiotics, almost all medicinals either are derived directly from compounds produced by angiosperms or, if synthesized, were originally discovered in angiosperms. This includes some vitamins (e.g., vitamin C, originally extracted from fruits); aspirin, originally from the bark of willows (Salix; Salicaceae); narcotics (e.g., opium and its derivatives from the opium poppy, Papaver somniferum; Papaveraceae); and quinine from Cinchona (Rubiaceae) bark. Some angiosperm compounds that are highly toxic to humans have proved to be effective in the treatment of certain forms of cancer, such as acute leukemia (vincristine from the Madagascar periwinkle, Catharanthus roseus; Apocynaceae), and of heart problems (digitalis from foxglove, Digitalis purpurea; Plantaginaceae). Muscle relaxants derived from curare (Strychnos toxifera; Loganiaceae) are used during open-heart surgery.
The contribution of the angiosperms to biodiversity and habitat is so extremely important that human life is totally dependent on it. A significant loss of angiosperms would reduce the variety of food sources and oxygen supply in a habitat and drastically alter the amount and distribution of the world’s precipitation. Many sources of food and medicine doubtless remain to be discovered in this group of vascular plants.
