Distribution and abundance
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- OpenStax - Biology 2e - Angiosperms
- Nature - The ancestral flower of angiosperms and its early diversification
- National Center for Biotechnology Information - PubMed Central - Origin of Angiosperms: Problems, Challenges, and Solutions
- USDA Agricultural Research Service - Evolutionary Patterns and Biogeochemical Significance of Angiosperm Root Traits (PDF)
- Biology LibreTexts - Angiosperms
- University of Wisconsin - Department of Botany - The Angiosperms
- University of Nevada, Las Vegas - Angiosperms
- PNAS - Angiosperms versus gymnosperms in the Cretaceous
Thediversity of form within the angiosperms has contributed to their successfulcolonization of morehabitats than any other group of land plants.Gymnosperms (the nonfloweringseed 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 ofcycads, 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 todiverse or extreme habitats. The absence ofxylemvessels in most gymnosperms, and hence the less efficientwater transport system than that found in the angiosperms, is one example. In fact, the only gymnosperms with vessels, theGnetales, is the only group that containsvines 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 theWinteraceae. 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 stableenvironment for growth. Thus, restraints imposed by anatomy andlife 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 worldwidedistribution. The only area without angiosperms is the southern region of theAntarctic 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 ofsalt marshes, tidal marshes, andmangrove 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, theeelgrasses (Zostera andPhyllospadix; Zosteraceae). The various terrestrial biomes (defined primarily based upon the type of vegetation and climate) are composed mainly of herbaceous and woody angiosperms, except fortaiga (boreal forest),temperate rainforest, and juniper savanna, whereconifers (a gymnospermous division) dominate the woody component and angiosperms dominate the herbaceous andshrub components.
Morphological and habitat diversity, together withcosmopolitan distribution, contributes to the wide ecological tolerance of the angiosperms—adapting to Alpinetundra regions and salt marshes, from theArctic Circle to the lowland tropicalrainforests. 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 areautotrophs: they are green plants (primary producers) that usesolar radiation,carbon dioxide, water, and minerals to synthesize organic compounds; oxygen is a by-product of these metabolic reactions. The few exceptions are eithermycoheterotrophs (e.g., theIndian pipeMonotropa uniflora;Ericaceae) that use connections withmycorrhizalfungi (fungi that form an association with the roots of certain plants) to obtain carbohydrates orparasitic plants that develop specialized roots (haustoria), which penetrate the hostplant and absorb food and other materials (e.g., thedodder [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 offood for animals and other living organisms.Organic compounds (carbon-containingcompounds, principallycarbohydrates) 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 mostheterotrophic 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 intochemical energy, which is stored in the tissues of the plant. The trapped energy is transferred from one organism to the next asherbivoresconsume the plant,carnivores consume herbivores, and so on up thefood chain. In atemperate forest, a single angiospermtree may support many thousands of animals (the majority beinginsects,birds, andmammals), 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, andseeds) also provide an energy source for many animals. Thepollen supports many pollinating insects, particularlybees.
The flowers provide food from floralnectaries thatsecrete sugars andamino acids. These flowers often produce fragrances that attract pollinators which feed on thenectar. Nectar-feeding animals include manyinsect groups (bees,butterflies,moths,flies, and evenmosquitoes), manymammal groups (bats, smallrodents, and smallmarsupials), and birds (honeyeaters,hummingbirds, andsunbirds). 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. Inreturn for this food source, they attack and destroy animals of all sizes as well as other plants that contact theacacia 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 somefish. 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 angiospermspropagate.
Another aspect of angiosperm diversity is found in the production ofsecondary compounds, such asalkaloids,quinones,essential oils, andglycosides. Angiosperms have evolved acomprehensive 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 theterrestrialbiosphere, 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 manyprimates,reptiles, andamphibians. Thetankbromeliad, which traps water in its crowns, provides a habitat forsalamanders,frogs, and many aquatic insects andlarvae. Theanimal inhabitants of the water-filled insectivorouspitcher plant leaves have adapted to the hostile environment of the leaves’ digestive fluids.