Orchid

Flowering Plants" redirects here. For the book by G. Ledyard Stebbins, see Flowering Plants: Evolution Above the Species Level.

Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (/ˌændʒiəˈspərmiː/),[5][6] commonly called angiosperms. They include insect-pollinated herbs such as buttercups, pond plants such as water lilies, wind-pollinated grasses, and trees such as apple and oak. 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.[7] Angiosperms were formerly called Magnoliophyta (/mæɡˌnoʊliˈɒfətə, -əˈfaɪtə/).[8]

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, but the earliest angiosperm fossils are in the form of pollen around 134 million years ago during the Early Cretaceous. Over the course of the Cretaceous, angiosperms diversified explosively, becoming the dominant group of plants across the planet by the end of the period, corresponding with the decline and extinction of previously widespread gymnosperm groups

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,[11] 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.[13] 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.[15] 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,[17] 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.[18]

Taxonomic diversity

The number of species of flowering plants is estimated to be in the range of 250,000 to 400,000.[19][20][21] This compares to around 12,000 species of moss[22] and 11,000 species of pteridophytes.[23] The APG system seeks to determine the number of families, mostly by molecular phylogenetics. In the 2009 APG III there were 415 families.[24] 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.[1]

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,[25] 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.[33] The term angiosperm fundamentally changed in meaning in 1827 with Robert Brown, when angiosperm came to mean a seed plant with enclosed ovules.[34][35] 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.[35][36] The APG system[24] 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.[37] The Cronquist system, proposed in 1968 and published in full in 1981, is still widely used but is no longer believed to accurately reflect phylogeny. From 1998, the Angiosperm Phylogeny Group (APG) has reclassified the angiosperms, with updates in the APG II system in 2003,[38] the APG III system in 2009,[24][39] and the APG IV system in 2016.[1] Traditionally, the flowering plants were divided into the Dicotyledoneae or Magnoliopsida, and the Monocotyledoneae or Liliopsida. The dicots most often have two cotyledons, or embryonic leaves, within each seed. The monocots usually have only one.[40] The APG showed that the monocots are a clade, but that the dicots are paraphyletic

Fossil history

Main article: Fossil history of flowering plants

Fossilised spores suggest that land plants (embryophytes) have existed for at least 475 million years.[46] However, angiosperms appear suddenly and in great diversity in the fossil record in the Early Cretaceous.[47] This poses such a problem for the theory of gradual evolution that Charles Darwin called it an "abominable mystery".[48]

Several disputed claims of pre-Cretaceous angiosperm fossils have been made, such as the upper Triassic Sanmiguelia lewisi.[49] Oleanane, a secondary metabolite produced by many flowering plants, has been found in Permian deposits of that age together with fossils of gigantopterids,[50] extinct seed plants that share many features with flowering plants.[51] Molecular evidence suggests that the ancestors of angiosperms diverged from the gymnosperms during the late Devonian, about 365 million years ago.[52] Pollen that looks much like that of angiosperms has been found in the Middle Triassic (247.2–242.0 Ma).[52][53]

The Caytoniales, a group of Triassic seed ferns, may be close relatives of angiosperms.[54] Nanjinganthus dendrostyla from Early Jurassic China seems to share many exclusively angiosperm features, such as flower-like structures and a thickened receptacle with ovules, and thus might represent a crown-group or a stem-group angiosperm,[55] but other researchers contend that the structures are decomposed conifer cones.[56][57]

The oldest fossils definitively attributable to angiosperms are reticulated monosulcate pollen from the late Valanginian (Early or Lower Cretaceous - 140 to 133 million years ago) of Italy and Israel, likely representing basal angiosperms.[56] The earliest macrofossil confidently identified as an angiosperm, Archaefructus liaoningensis, is dated to about 125 million years ago in the Cretaceous.[58]

Micropetasos flowers have been found encased in amber dated to 100 million years ago. The amber had frozen the act of sexual reproduction in the process of taking place. Microscopic images showed tubes growing out of pollen and penetrating the flower's stigma. The pollen had stuck to the flower's style and calyx, suggesting that it was sticky and carried by insects.[59] 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.[60]

The great angiosperm radiation, when a great diversity of angiosperms appears in the fossil record, occurred in the mid-Cretaceous, approximately 100 million years ago. However, a study in 2007 estimated that the divergence of the five most recent of the eight main groups, namely the genus Ceratophyllum, the family Chloranthaceae, the eudicots, the magnoliids, and the monocots, occurred around 140 million years ago.[61]

By the late Cretaceous, angiosperms appear to have dominated environments formerly occupied by ferns and cycadophytes. Large canopy-forming trees replaced conifers as the dominant trees close to the end of the Cretaceous, 66 million years ago or even later, at the beginning of the Paleogene.[62] The radiation of herbaceous angiosperms occurred much later.[63]

Reproduction

Flowers

Main articles: Flower and Plant reproductive morphology

The characteristic feature of angiosperms is the flower. Its function is to ensure fertilization of the ovule and development of fruit containing seeds.[64] It may arise terminally on a shoot or from the axil of a leaf.[65] The flower-bearing part of the plant is usually sharply distinguished from the leaf-bearing part, and forms a branch-system called an inflorescence.[36]

Flowers produce two kinds of reproductive cells. Microspores, which divide to become pollen grains, are the male cells; they are borne in the stamens.[66] 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.[66]

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.[36] 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.[67][68] The inner series (corolla of petals) is, in general, white or brightly colored, is more delicate in structure, and attracts pollinators by colour, scent, and nectar.[69][70]

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.[71] Monoecious plants have separate male and female flowers on the same plant; these are often wind-pollinated,[72] as in maize,[73] but include some insect-pollinated plants such as Cucurbita squashes.[74][75]

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.[76]

Fruit and seed

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.[77]

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.[78]

Apomixis, setting seed without fertilization, is found naturally in about 2.2% of angiosperm genera.[79] Some angiosperms, including many citrus varieties, are able to produce fruits through a type of apomixis called nucellar embryony.[80]

Uses

Main article: Human uses of plants

Agriculture is almost entirely dependent on angiosperms, which provide virtually all plant-based food, and a significant amount of livestock feed. Of all the families of plants, the Poaceae, or grass family is by far the most important, providing the bulk of all feedstocks (rice, maize, wheat, barley, rye, oats, pearl millet, sugar cane, sorghum). The Fabaceae, or legume family, comes in second place. Also of high importance are the Solanaceae, or nightshade family (including potatoes, tomatoes, and peppers); the Cucurbitaceae, or gourd family (including pumpkins and melons); the Brassicaceae, or mustard plant family (including rapeseed and the many varieties of the cabbage species Brassica oleracea); and the Apiaceae, or parsley family. Many of our fruits come from the Rutaceae, or rue family, including oranges, lemons, and grapefruits, and the Rosaceae, or rose family which provides apples, pears, cherries, apricots, and plums.[81][82] Flowering plants provide 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.[83] Both real and fictitious plants play a wide variety of roles in literature and film.[84] Flowers are the subjects of many poems by poets such as William Blake, Robert Frost, and Rabindranath Tagore.