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Flowering plant Temporal range: Late Valanginian – present,
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Diversity of angiosperms | |
Scientific classification ![]() | |
Kingdom: | Plantae |
Clade: | Tracheophytes |
Clade: | Spermatophytes |
Clade: | Angiosperms |
Groups (APG IV)[1] | |
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Synonyms | |
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (/ˌændʒiəˈspɜːrmiː/),[5][6] commonly called angiosperms. The term "angiosperm" is derived from the Greek words angeion ('container, vessel') and sperma ('seed'), and refers to those plants that produce their seeds 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]
Like gymnosperms, angiosperms are seed-producing plants. They are distinguished from gymnosperms by characteristics including flowers, endosperm within their seeds, and the production of fruits that contain 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. The closest fossil relatives of flowering plants are uncertain and contentious.
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 explosively diversified, 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. The origin and diversification of the angiosperms is often known as "Darwin's abominable mystery".[9]
Description
Angiosperm derived characteristics
Angiosperms differ from other seed plants in several ways, described in the table below. These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans.[a]
Feature | Description |
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Flowering organs | Flowers, the reproductive organs of flowering plants, are the most remarkable feature distinguishing them from the other seed plants. Flowers provided angiosperms with the means to have a more species-specific breeding system, and hence a way to evolve more readily into different species without the risk of crossing back with related species. Faster speciation enabled the Angiosperms to adapt to a wider range of ecological niches. This has allowed flowering plants to largely dominate terrestrial ecosystems, comprising about 90 percent of all plant species.[10] |
Stamens with two pairs of pollen sacs | Stamens are much lighter than the corresponding organs of gymnosperms and have contributed to the diversification of angiosperms through time with adaptations to specialised pollination syndromes, such as particular pollinators. Stamens have also become modified through time to prevent self-fertilization, which has permitted further diversification, allowing angiosperms eventually to fill more niches. |
Reduced male gametophyte, three cells | The male gametophyte in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants.[11] The smaller size of the pollen reduces the amount of time between pollination (the pollen grain reaching the female plant) and fertilization. In gymnosperms, fertilization can occur up to a year after pollination, whereas in angiosperms, fertilization begins very soon after pollination.[12] The shorter amount of time between pollination and fertilization allows angiosperms to produce seeds earlier after pollination than gymnosperms, providing angiosperms a distinct evolutionary advantage. |
Closed carpel enclosing the ovules (carpel or carpels and accessory parts may become the fruit) | The closed carpel of angiosperms also allows adaptations to specialised pollination syndromes and controls. This helps to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilised, the carpel and some surrounding tissues develop into a fruit. This fruit often serves as an attractant to seed-dispersing animals. The resulting cooperative relationship presents another advantage to angiosperms in the process of dispersal. |
Reduced female gametophyte, seven cells with eight nuclei | The reduced female gametophyte, like the reduced male gametophyte, may be an adaptation allowing for more rapid seed set,[citation needed] eventually leading to such flowering plant adaptations as annual herbaceous life-cycles, allowing the flowering plants to fill even more niches. |
Endosperm | In general, endosperm formation begins after fertilization and before the first division of the zygote. Endosperm is a highly nutritive tissue that can provide food for the developing embryo, the cotyledons, and sometimes the seedling when it first appears. |
Vascular anatomy
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Stem-histology-cross-section-tag.svg/220px-Stem-histology-cross-section-tag.svg.png)
1. pith, 2. protoxylem, 3. xylem, 4. phloem, 5. sclerenchyma (bast fibre), 6. cortex, 7. epidermis
Angiosperm stems are made up of seven layers as shown on the right. The amount and complexity of tissue-formation in flowering plants exceeds that of gymnosperms.
In the dicotyledons, the vascular bundles of the stem are arranged such that the xylem and phloem form concentric rings.[13] The bundles in the very young stem are arranged in an open ring, separating a central pith from an outer cortex. In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue known as cambium. By the formation of a layer of cambium between the bundles (interfascicular cambium), a complete ring is formed, and a regular periodical increase in thickness results from the development of xylem on the inside and phloem on the outside. The soft phloem becomes crushed, but the hard wood persists and forms the bulk of the stem and branches of the woody perennial. Owing to differences in the character of the elements produced at the beginning and end of the season, the wood is marked out in transverse section into concentric rings, one for each season of growth, called annual rings.
Among the monocotyledons, the bundles are more numerous in the young stem and are scattered through the ground tissue. They contain no cambium and once formed the stem increases in diameter only in exceptional cases.
Reproductive anatomy
The characteristic feature of angiosperms is the flower. Flowers show remarkable variation in form and elaboration, and provide the most trustworthy external characteristics for establishing relationships among angiosperm species. The function of the flower is to ensure fertilization of the ovule and development of fruit containing seeds.[14] The floral apparatus may arise terminally on a shoot or from the axil of a leaf (where the petiole attaches to the stem).[citation needed] Occasionally, as in violets, a flower arises singly in the axil of an ordinary foliage-leaf. More typically, the flower-bearing portion of the plant is sharply distinguished from the foliage-bearing or vegetative portion, and forms a more or less elaborate branch-system called an inflorescence.
There are two kinds of reproductive cells produced by flowers. Microspores, which will divide to become pollen grains, are the "male" cells and are borne in the stamens (or microsporophylls).[15] The "female" cells called megaspores, which will divide to become the egg cell (megagametogenesis), are contained in the ovule and enclosed in the carpel (or megasporophyll).[15]
The flower may consist only of these parts, as in willow, where each flower comprises only a few stamens or two carpels. Usually, other structures are present and serve to protect the sporophylls and to form an envelope attractive to 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 colored, and is more delicate in structure. It functions to attract insect or bird pollinators. Attraction is effected by color, scent, and nectar, which may be secreted in some part of the flower. The characteristics that attract pollinators account for the popularity of flowers and flowering plants among humans.[citation needed]
While the majority of flowers are perfect or hermaphrodite (having both pollen and ovule producing parts in the same flower structure), flowering plants have developed numerous morphological and physiological mechanisms to reduce or prevent self-fertilization. Heteromorphic flowers have short carpels and long stamens, or vice versa, so animal pollinators cannot easily transfer pollen to the pistil (receptive part of the carpel). Homomorphic flowers may employ a biochemical (physiological) mechanism called self-incompatibility to discriminate between self and non-self pollen grains. Alternatively, in dioecious species, the male and female parts are morphologically separated, developing on different individual flowers.[16]
Taxonomy
History of classification
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 but he used this term to refer to a group of plants which form only a subset of what today are known as angiosperms. Hermannn's Angiospermae including only flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, which were flowering plants with achenial or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked. The terms Angiospermae and Gymnospermae were used by Carl Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia.
The terms angiosperms and gymnosperm fundamentally changed in meaning in 1827 when Robert Brown established the existence of truly naked ovules in the Cycadeae and Coniferae.[17] The term gymnosperm was from then on applied to seed plants with naked ovules, and the term angiosperm to seed plants with enclosed ovules. However, for many years after Brown's discovery, the primary division of the seed plants was seen as between monocots and dicots, with gymnosperms as a small subset of the dicots.[18]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2b/NSRW_Auxanometer.png/170px-NSRW_Auxanometer.png)
In 1851, Hofmeister discovered the changes occurring in the embryo-sac of flowering plants, and determined the correct relationships of these to the Cryptogamia. This fixed the position of Gymnosperms as a class distinct from Dicotyledons, and the term Angiosperm then gradually came to be accepted as the suitable designation for the whole of the flowering plants other than Gymnosperms, including the classes of Dicotyledons and Monocotyledons.[18] This is the sense in which the term is used today.
In most taxonomies, the flowering plants are treated as a coherent group. The most popular descriptive name has been Angiospermae, with Anthophyta (lit. 'flower-plants') a second choice (both unranked). The Wettstein system and Engler system treated them as a subdivision (Angiospermae). The Reveal system also treated them as a subdivision (Magnoliophytina),[19] but later split it to Magnoliopsida, Liliopsida, and Rosopsida. The Takhtajan system and Cronquist system treat them as a division (Magnoliophyta).[citation needed] The Dahlgren system and Thorne system (1992) treat them as a class (Magnoliopsida). The APG system of 1998, and the later 2003[20] and 2009[21] revisions, treat 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 a subclass (Magnoliidae).[22]
The internal classification of this group has undergone considerable revision. The Cronquist system, proposed by Arthur Cronquist in 1968 and published in its full form in 1981, is still widely used but is no longer believed to accurately reflect phylogeny. A consensus about how the flowering plants should be arranged has recently begun to emerge through the work of the Angiosperm Phylogeny Group (APG), which published an influential reclassification of the angiosperms in 1998. Updates incorporating more recent research were published as the APG II system in 2003,[20] the APG III system in 2009,[21][23] and the APG IV system in 2016.
Traditionally, the flowering plants are divided into two groups,
to which the Cronquist system ascribes the classes Magnoliopsida (from "Magnoliaceae" and Liliopsida (from "Liliaceae"). Other descriptive names allowed by Article 16 of the ICBN include Dicotyledones or Dicotyledoneae, and Monocotyledones or Monocotyledoneae, which have a long history of use. In plain English, their members may be called "dicotyledons" ("dicots") and "monocotyledons" ("monocots"). The Latin behind these names refers the observation that the dicots most often have two cotyledons, or embryonic leaves, within each seed. The monocots usually have only one, but the rule is not absolute either way. From a broad diagnostic point of view, the number of cotyledons is neither a particularly handy, nor a reliable character.[citation needed]
Recent studies, as by the APG, show that the monocots form a monophyletic group (a clade) but that the dicots are paraphyletic. Nevertheless, the majority of dicot species fall into a clade, the eudicots or tricolpates, and most of the remaining fall into another major clade, the magnoliids, containing about 9,000 species. The rest include a paraphyletic grouping of early branching taxa known collectively as the basal angiosperms, plus the families Ceratophyllaceae and Chloranthaceae.[citation needed]
Modern classification
![]() | This article needs to be updated.(September 2021) |
There are eight groups of living angiosperms:
- Basal angiosperms (ANA: Amborella, Nymphaeales, Austrobaileyales)
- Amborella, a single species of shrub from New Caledonia;
- Nymphaeales, about 80 species,[24] water lilies and Hydatellaceae;
- Austrobaileyales, about 100 species[24] of woody plants from various parts of the world
- Core angiosperms (Mesangiospermae)[22]
- Chloranthales, 77 known species[25] of aromatic plants with toothed leaves;
- Magnoliids, about 9,000 species,[24] characterised by trimerous flowers, pollen with one pore, and usually branching-veined leaves—for example magnolias, bay laurel, and black pepper;
- Monocots, about 70,000 species,[24] characterised by trimerous flowers, a single cotyledon, pollen with one pore, and usually parallel-veined leaves—for example grasses, orchids, and palms;
- Ceratophyllum, about 6 species[24] of aquatic plants, perhaps most familiar as aquarium plants;
- Eudicots, about 175,000 species,[24] characterised by 4- or 5-merous flowers, pollen with three pores, and usually branching-veined leaves—for example sunflowers, petunia, buttercup, apples, and oaks.
The exact relationships among these eight groups is not yet clear, although there is agreement that the first three groups to diverge from the ancestral angiosperm were Amborellales, Nymphaeales, and Austrobaileyales (basal angiosperms)[26] Of the remaining five groups (core angiosperms), the relationships among the three broadest groups remains unclear (magnoliids, monocots, and eudicots). Zeng and colleagues (Fig. 1) describe four competing schemes.[27][clarification needed]The eudicots and monocots are the largest and most diversified, with ~ 75% and 20% of angiosperm species, respectively. Some analyses make the magnoliids the first to diverge, others the monocots.[28] Ceratophyllum seems to group with the eudicots rather than with the monocots. The APG IV retained the overall higher order relationship described in APG III.[21]
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Detailed Cladogram of the Angiosperm Phylogeny Group (APG) IV classification.[1] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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