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Introduction
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (/ˌændʒiəˈspərmiː/),commonly called angiosperms
They include all forbs (flowering plants without a woody stem), grasses and grass-like plants, a vast majority of broad-leaved trees, shrubs & vines, and most aquatic plants
The term "angiosperm" is derived from the Greek words ἀγγεῖον /angeion ('container, vessel') and σπέρμα / sperma ('seed'), meaning that the seeds are enclosed within a fruit
They are by far the most diverse group of land plants with 64 orders, 416 families, approximately 13,000 known genera and 300,000 known species
Angiosperms were formerly called Magnoliophyta (/mæɡˌnoʊliˈɒfətə, -əˈfaɪtə/)
Angiosperms are distinguished from the other seed-producing plants, the gymnosperms, by having flowers, xylem consisting of vessel elements instead of tracheids, endosperm within their seeds, and fruits that completely envelop the seeds
The ancestors of flowering plants diverged from the common ancestor of all living gymnosperms before the end of the Carboniferous, over 300 million years ago
In the Cretaceous, angiosperms diversified explosively, becoming the dominant group of plants across the planet
Agriculture is almost entirely dependent on angiosperms, and a small number of flowering plant families supply nearly all plant-based food and livestock feed
Rice, maize, and wheat provide half of the world's calorie intake, and all three plants are cereals from the Poaceae family (colloquially known as grasses)
Other families provide materials such as wood, paper and cotton, and supply numerous ingredients for traditional and modern medicines
Flowering plants are also commonly grown for decorative purposes, with certain flowers playing a significant role in many cultures
Out of the "Big Five" extinction events in Earth's history, only the End-Cretaceous extinction event had occurred while angiosperms dominated plant life on the planet
Today, Holocene extinction affects all kingdoms of complex life on Earth, and conservation measures are necessary to protect plants in their habitats in the wild (in situ), or failing that, ex situ in seed banks or artificial habitats
Otherwise, around 40% of plant species may become extinct due to human actions such as habitat destruction, introduction of invasive species, unsustainable logging and collection of medicinal or decorative plants
Further, climate change is starting to impact plants and is likely to cause many species to become extinct by 2100
Distinguishing features
Angiosperms are terrestrial vascular plants; like the gymnosperms, they have roots, stems, leaves, and seeds. They differ from other seed plants in several ways:
Ecological diversity
The largest angiosperms are Eucalyptus gum trees of Australia, & Shorea faguetiana, dipterocarp rainforest trees of Southeast Asia, both of which can reach almost 100 m in height
The smallest are Wolffia duckweeds which float on freshwater, each plant less than 2 mm across
Considering their method of obtaining energy, some 99% of flowering plants are photosynthetic autotrophs, deriving their energy from sunlight and using it to create molecules such as sugars
The remainder are parasitic, whether on fungi like the orchids for part or all of their life-cycle, or on other plants, either wholly like the broomrapes, Orobanche, or partially like the witchweeds, Striga
In terms of their environment, flowering plants are cosmopolitan, occupying a wide range of habitats on land, in fresh water and in the sea
On land, they are the dominant plant group in every habitat except for frigid moss-lichen tundra and coniferous forest
The seagrasses in the Alismatales grow in marine environments, spreading with rhizomes that grow through the mud in sheltered coastal waters
Some specialised angiosperms are able to flourish in extremely acid or alkaline habitats
The sundews, many of which live in nutrient-poor acid bogs, are carnivorous plants, able to derive nutrients such as nitrate from the bodies of trapped insects
Other flowers such as Gentiana verna, the spring gentian, are adapted to the alkaline conditions found on calcium-rich chalk and limestone, which give rise to often dry topographies such as limestone pavement
As for their growth habit, the flowering plants range from small, soft herbaceous plants, often living as annuals or biennials that set seed and die after one growing season, to large perennial woody trees that may live for many centuries and grow to many metres in height. Some species grow tall without being self-supporting like trees by climbing on other plants in the manner of vines or lianas
Taxonomic diversity
The number of species of flowering plants is estimated to be in the range of 250,000 to 400,000
This compares to around 12,000 species of moss and 11,000 species of pteridophytes.
The APG system seeks to determine the number of families, mostly by molecular phylogenetics
In the 2009 APG III there were 415 families
The 2016 APG IV added five new orders (Boraginales, Dilleniales, Icacinales, Metteniusales and Vahliales), along with some new families, making a total of 64 angiosperm orders and 416 families
The diversity of flowering plants is not evenly distributed
Nearly all species belong to the eudicot (75%), monocot (23%), and magnoliid (2%) clades
The remaining five clades contain a little over 250 species in total; i.e. less than 0.1% of flowering plant diversity, divided among nine families
The 25 most species-rich of 443 families, containing over 166,000 species between them in their APG circumscriptions, are:
Evolution
History of classification
Main article: Plant taxonomy
The botanical term "angiosperm", from Greek words angeíon (ἀγγεῖον 'bottle, vessel') and spérma (σπέρμα 'seed'), was coined in the form "Angiospermae" by Paul Hermann in 1690, including only flowering plants whose seeds were enclosed in capsules
The term angiosperm fundamentally changed in meaning in 1827 with Robert Brown, when angiosperm came to mean a seed plant with enclosed ovules
In 1851, with Wilhelm Hofmeister's work on embryo-sacs, Angiosperm came to have its modern meaning of all the flowering plants including Dicotyledons and Monocotyledons
The APG system treats the flowering plants as an unranked clade without a formal Latin name (angiosperms)
A formal classification was published alongside the 2009 revision in which the flowering plants rank as the subclass Magnoliidae
From 1998, the Angiosperm Phylogeny Group (APG) has reclassified the angiosperms, with updates in the APG II system in 2003, the APG III system in 2009, and the APG IV system in 2016
Phylogeny
External
In 2019, a molecular phylogeny of plants placed the flowering plants in their evolutionary context:
Internal
The major groups of living angiosperms are:
Fossil history
Main article: Fossil history of flowering plants
Adaptive radiation in the Cretaceous created many flowering plants, such as Sagaria in the Ranunculaceae
Fossilised spores suggest that land plants (embryophytes) have existed for at least 475 million years
However, angiosperms appear suddenly and in great diversity in the fossil record in the Early Cretaceous (~130 mya)
Molecular evidence suggests that the ancestors of angiosperms diverged from the gymnosperms during the late Devonian, about 365 million years ago
A Bayesian analysis of 52 angiosperm taxa suggested that the crown group of angiosperms evolved between 178 million years ago and 198 million years ago
By the late Cretaceous, angiosperms appear to have dominated environments formerly occupied by ferns and cycads
Large canopy-forming trees replaced conifers as the dominant trees close to the end of the Cretaceous, 66 million years ago
The radiation of herbaceous angiosperms occurred much later
Reproduction
Flowers
Main articles: Flower and Plant reproductive morphology
Angiosperm flower showing reproductive parts and life cycle
The characteristic feature of angiosperms is the flower
Its function is to ensure fertilization of the ovule and development of fruit containing seeds
It may arise terminally on a shoot or from the axil of a leaf
The flower-bearing part of the plant is usually sharply distinguished from the leaf-bearing part, and forms a branch-system called an inflorescence
Flowers produce two kinds of reproductive cells. Microspores, which divide to become pollen grains, are the male cells; they are borne in the stamens.
The female cells, megaspores, divide to become the egg cell
They are contained in the ovule and enclosed in the carpel; one or more carpels form the pistil
The flower may consist only of these parts, as in wind-pollinated plants like the willow, where each flower comprises only a few stamens or two carpels
In insect- or bird-pollinated plants, other structures protect the sporophylls and attract pollinators. The individual members of these surrounding structures are known as sepals and petals (or tepals in flowers such as Magnolia where sepals and petals are not distinguishable from each other)
The outer series (calyx of sepals) is usually green and leaf-like, and functions to protect the rest of the flower, especially the bud
The inner series (corolla of petals) is, in general, white or brightly coloured, is more delicate in structure, and attracts pollinators by colour, scent, and nectar
Most flowers are hermaphrodite, producing both pollen and ovules in the same flower, but some use other devices to reduce self-fertilization
Heteromorphic flowers have carpels and stamens of differing lengths, so animal pollinators cannot easily transfer pollen between them
Homomorphic flowers may use a biochemical self-incompatibility to discriminate between self and non-self pollen grains. Dioecious plants such as holly have male and female flowers on separate plants
Monoecious plants have separate male and female flowers on the same plant; these are often wind-pollinated, as in maize, but include some insect-pollinated plants such as Cucurbita squashes
Fertilisation and embryogenesis
Main articles: Fertilization and Plant embryogenesis
Double fertilization requires two sperm cells to fertilise cells in the ovule
A pollen grain sticks to the stigma at the top of the pistil, germinates, and grows a long pollen tube. A haploid generative cell travels down the tube behind the tube nucleus
The generative cell divides by mitosis to produce two haploid (n) sperm cells. The pollen tube grows from the stigma, down the style and into the ovary
When it reaches the micropyle of the ovule, it digests its way into one of the synergids, releasing its contents including the sperm cells. The synergid that the cells were released into degenerates; one sperm makes its way to fertilise the egg cell, producing a diploid (2n) zygote
The second sperm cell fuses with both central cell nuclei, producing a triploid (3n) cell
The zygote develops into an embryo; the triploid cell develops into the endosperm, the embryo's food supply
The ovary develops into a fruit. and each ovule into a seed
Fruit and seed
The fruit of the horse chestnut tree, showing the large seed inside the fruit, which is dehiscing or splitting open.
Main articles: Fruit and Seed
As the embryo and endosperm develop, the wall of the embryo sac enlarges and combines with the nucellus and integument to form the seed coat. The ovary wall develops to form the fruit or pericarp, whose form is closely associated with type of seed dispersal system
Other parts of the flower often contribute to forming the fruit. For example, in the apple, the hypanthium forms the edible flesh, surrounding the ovaries which form the tough cases around the seeds
Apomixis, setting seed without fertilization, is found naturally in about 2.2% of angiosperm genera.[68] Some angiosperms, including many citrus varieties, are able to produce fruits through a type of apomixis called nucellar embryony.[69]
Interactions with humans
Main article: Human uses of plants
Practical uses
Agriculture is almost entirely dependent on angiosperms, which provide virtually all plant-based food and livestock feed. Much of this food derives from a small number of flowering plant families
For instance, half of the world's calorie intake is supplied by just three plants - wheat, rice and maize
Flowering plants provide a diverse range of materials in the form of wood, paper, fibers such as cotton, flax, and hemp, medicines such as digoxin and opioids, and decorative and landscaping plants
Coffee and hot chocolate are beverages from flowering plants
Cultural uses
Bird-and-flower painting: Kingfisher and iris kachō-e woodblock print by Ohara Koson (late 19th century)
Both real and fictitious plants play a wide variety of roles in literature and film
Flowers are the subjects of many poems by poets such as William Blake, Robert Frost, and Rabindranath Tagore
Bird-and-flower painting (Huaniaohua) is a kind of Chinese painting that celebrates the beauty of flowering plants
Flowers have been used in literature to convey meaning by authors including William Shakespeare
Flowers are used in a variety of art forms which arrange cut or living plants, such as bonsai, ikebana, and flower arranging. Ornamental plants have sometimes changed the course of history, as in tulipomania
Many countries and regions have floral emblems; a survey of 70 of these found that the most popular flowering plant family for such emblems is Orchidaceae at 15.7% (11 emblems), followed by Fabaceae at 10% (7 emblems), and Asparagaceae, Asteraceae, and Rosaceae all at 5.7% (4 emblems each)
Conservation
Further information: Conservation biology and Effects of climate change on plant biodiversity
Viola calcarata, a species known to be highly vulnerable to climate change
Human impact on the environment has driven a range of species extinct and is threatening even more today. Multiple organizations such as IUCN and Royal Botanic Gardens, Kew suggest that around 40% of plant species are threatened with extinction
The majority are threatened by habitat loss, but activities such as logging of wild timber trees and collection of medicinal plants, or the introduction of non-native invasive species, also play a role
Relatively few plant diversity assessments currently consider climate change,[ yet it is starting to impact plants as well: according to IPCC, about 3% of flowering plants are very likely to be driven extinct within a century at 2 °C , and 10% at 3.2 °C
In worst-case scenarios, half of all tree species, or of all plant species, may be driven extinct by climate change over that timeframe
Conservation in this context is the attempt to prevent extinction, whether in situ by protecting plants and their habitats in the wild, or ex situ in seed banks or as living plants
Some 3000 botanic gardens around the world maintain living plants, including over 40% of the species known to be threatened, as an "insurance policy against extinction in the wild"
The United Nations' Global Strategy for Plant Conservation asserts that "without plants, there is no life"
It aims to "halt the continuing loss of plant diversity" throughout the world
Flowering plants, the angiosperms, are the most diverse group of plants on our planet
Today, they dominate most vegetation types, but their origin continues to remain a mystery. However, we continue to gain knowledge about their early evolution and history. It seems increasingly probable that their origin is associated with climatic and environmental changes in tropical areas and was coeval with the breakup of the supercontinent Gondwana. The first angiosperms appeared in the fossil record about 135 million years ago based on the occurrence of their rare pollen grains in fossil assemblages of North Gondwana and southwest Europe. Their evolution may be associated with climate perturbation and an overall change in wetland to mesophytic habitats, as this group is adapted to tolerate a seasonally dry climate. Soon after the first early angiosperms in the late Valanginian, higher angiosperms, the eudicots, are part of the fossil record of Africa. These initial flowering plants had small inconspicuous flowers and small fruits, and were most probably of small growth stature, likely herbs and shrubs. After angiosperms colonized mineral soils across the landscape, they expanded their habitats to aquatic environments and evolved strategies for their rapid dispersal in these settings. By the mid-Cretaceous (90–100 Ma), angiosperms conquered higher latitudes in both hemispheres and expanded into various tropical to warm temperate (= subtropical) environments. Chloranthoids, laurels, and plane trees experienced their heyday. In the Late Cretaceous, core∗ and higher eudicots evolved rapidly, and nearly all extant angiosperm families appeared by the end of the Cretaceous. Angiosperm clades developed a physiology capable of overcoming drought conditions by the Cenomanian. However, their expansion and colonization of mesophytic upland habitats only took place in the Late Cretaceous. Seasonally dry habitats, such as savannas, were inhabited by angiosperms in the Late Cretaceous, with the first evidence of graminoids.
Source:
When and Why Nature Gained Angiosperms Jiří Kvaček1 , Clement Coiffard2 , Maria Gandolfo3 , Alexei B. Herman4, Julien Legrand5 , Mário Miguel Mendes6 , Harufumi Nishida5 , Sun Ge7 and Hongshan Wang8
... from ebook Nature Through Time Virtual Field Trips Through the Nature of the Past, 1st ed. 2020
Patterns of Biodiversity
Patterns of Biodiversity
Estimated percentage representation of flowering plants (angiosperms) at different times in geological history and at different latitudes. Angiosperms have always been most abundant in the low-latitude (tropical) regions. From Crane and Lidgard.
Source: Biogeography : an ecological and evolutionary approach. C. Barry Cox,
Peter D. Moore, Richard Ladle., Ninth edition, Chichester, UK; Hoboken, NJ : John Wiley &
Sons, 2016, p131.
NOTES
The Angiosperm Phylogeny Group (APG) is an informal international group of systematic botanists who collaborate to establish a consensus on the taxonomy of flowering plants (angiosperms) that reflects new knowledge about plant relationships discovered through phylogenetic studies.
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