Difference between revisions of "Flowering plant"
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[[Monocotyledon|Liliopsida]] - Monocots<br> | [[Monocotyledon|Liliopsida]] - Monocots<br> | ||
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| − | The '''flowering plants''' | + | The '''flowering plants''' or '''angiosperms''' are the most widespread group of [[Embryophytes|land plants]]. The flowering plants and the [[gymnosperms]] comprise the two extant groups of [[Spermatophyte|seed plants]]. The flowering plants are distinguished from other seed plants by a series of [[Apomorphy#Definitions|apomorphies]], or derived [[character (biology)|characteristics]]. |
==Angiosperm derived characteristics== | ==Angiosperm derived characteristics== | ||
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* [[Flower]]s | * [[Flower]]s | ||
| − | The flowers of flowering plants are the most remarkable feature | + | The flowers of flowering plants are the most remarkable feature distinguishing them from other seed plants. Flowers aided angiosperms by enabling a wider range of evolutionary relationships and broadening the [[ecological niche]]s open to them, allowing flowering plants to eventually dominate [[terrestrial ecoregion|terrestrial]] ecosystems. |
| − | * [[Stamen]]s with | + | * [[Stamen]]s with two pairs of pollen sacs |
| − | Stamens are much lighter than the corresponding | + | Stamens are much lighter than the corresponding organs of gymnosperms and have contributed to the diversification of angiosperms through time with [[adaptation]]s to specialized [[pollination]] syndromes, such as particular pollinators. Stamens have also become modified through time to prevent [[self-fertilization]], which has permitted further diversification, allowing angiosperms to eventually fill more niches. |
| − | * Reduced male | + | * Reduced male parts, three cells |
| − | The | + | The male [[gametophyte]] in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants. The smaller pollen decreases the time from pollination – the pollen grain reaching the female plant – to [[fertilization]] of the ovary; in gymnosperms fertilization can occur up to a year after pollination, while in angiosperms the fertilization begins very soon after pollination. The shorter time leads to angiosperm plants setting seeds sooner and faster than gymnosperms, which is a distinct evolutionary advantage. |
* Closed [[carpel]] enclosing the ovules (carpel or carpels and accessory parts may become the [[fruit]]) | * Closed [[carpel]] enclosing the ovules (carpel or carpels and accessory parts may become the [[fruit]]) | ||
| − | The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized the carpel and some surrounding tissues develop into a fruit, another opportunity for angiosperms to increase their domination of the terrestrial ecosystem with evolutionary adaptations to dispersal mechanisms. | + | The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized the carpel and some surrounding tissues develop into a fruit, another opportunity for angiosperms to increase their domination of the terrestrial ecosystem with evolutionary adaptations to [[biological dispersal|dispersal]] mechanisms. |
* Reduced female gametophyte, seven cells with eight nuclei | * Reduced female gametophyte, seven cells with eight nuclei | ||
| − | The reduced female gametophyte, like the | + | The reduced female gametophyte, like the reduced male gametophyte may be adaptations allowing for more rapid seed set, eventually leading to such flowering plant adaptations as annual herbaceous life cycles, allowing the flowering plants to fill even more niches. |
* [[Endosperm]] | * [[Endosperm]] | ||
| − | Endosperm formation generally 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 for the seedling when it first appears. | + | Endosperm formation generally 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 for the [[seedling]] when it first appears. |
| − | These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. The major exception to the dominance of terrestrial ecosystems by flowering plants is the coniferous forest. | + | These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. The major exception to the dominance of terrestrial ecosystems by flowering plants is the [[coniferous forest]]. |
| − | == | + | ==Evolution== |
| − | The | + | [[Image:flower086.JPG|thumb|280px|Pink [[Hyacinth]] tree in flower, at sunset {{ifdc|Image:Flower086.JPG|log=2008 January 13}}]] |
| + | Land plants have existed for about 425 million years. Early land plants [[plant sexuality|reproduced]] by [[spore]]s like their [[Aquatic plant|aquatic counterparts]]. [[marine biology|Marine organisms]] can easily scatter copies of themselves to float away and grow elsewhere. Land plants soon found it advantageous to protect their copies from drying out and other hazards by enclosing them in a case, the [[seed]]. Early seed bearing plants, like the [[ginkgo]], and [[conifer]]s (such as [[pine]]s and [[fir]]s), did not produce flowers. | ||
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| + | The earliest [[fossil]] of an angiosperm, or flowering plant, ''[[Archaefructus|Archaefructus liaoningensis]]'', is dated to about 125 million years BP[http://www.pbs.org/wgbh/nova/flower/anatomy.html]. Pollen, considered directly linked to flower development, has been found in the fossil record perhaps as long ago as 130 million years. | ||
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| + | While there is only hard evidence of such flowers existing about 130 million years ago, there is some circumstantial evidence that they may have existed 250 million years ago. A chemical used by plants to defend their flowers, [[oleanane]], has been detected in fossil plants that old, including [[gigantopterid]]s[http://www.sciencedaily.com/releases/2001/04/010403071438.htm], which evolved at that time and bear many of the traits of modern, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail, one of the earliest examples of [[petrified wood|petrification]]. | ||
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| + | The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of [[evolution]] that it was called an "abominable mystery" by [[Charles Darwin]].<ref>[http://www.pnas.org/cgi/content/full/101/7/1904 Darwin's abominable mystery: Insights from a supertree of the angiosperms]. Proceedings of the National Academy of Sciences of the United States of America. T. Jonathan Davies, Timothy G. Barraclough, Mark W. Chase, Pamela S. Soltis, Douglas E. Soltis, and Vincent Savolainen. Published (online) February 6, 2004.</ref> However the fossil record has grown since the time of Darwin, and recently discovered angiosperm fossils such as ''Archaefructus'', along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps. Several groups of extinct gymnosperms, particularly [[seed fern]]s, have been proposed as the [[most recent common ancestor|ancestors]] of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. Some older fossils, such as the upper [[Triassic]] ''[[Sanmiguelia]]'', have been suggested. Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late [[Triassic]] (245-202 million years ago). The relationship of the earlier [[gigantopterid]]s to flowering plants is still enigmatic. | ||
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| + | A close relationship between Angiosperms and [[Gnetophyte]]s, suggested on the basis of [[Morphology (biology)|morphological]] evidence, has been disputed on the basis of [[molecular biology|molecular evidence]] that suggest Gnetophytes are more closely related to other [[gymnosperm]]s. | ||
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| + | Recent [[DNA]] analysis ([[molecular systematics]]) [http://www.pbs.org/wgbh/nova/transcripts/3405_flower.html] [http://www.amjbot.org/cgi/content/full/91/6/997] show that [[Amborella|''Amborella trichopoda'']], found on the Pacific island of [[New Caledonia]], belongs to a [[sister group]] of the other flowering plants, and morphological studies [http://www.eurekalert.org/pub_releases/2006-05/uoca-spp051506.php] suggest that it has features which may have been characteristic of the earliest flowering plants. | ||
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| + | The great angiosperm [[Adaptive radiation|radiation]], when a great diversity of angiosperms appear in the fossil record, occurred in the mid-[[Cretaceous]] (approximately 100 million years ago). However, a study in 2007 estimated that the division of the five most recent (the genus ''[[Ceratophyllum]]'', the family [[Chloranthaceae]], the [[eudicot]]s, the [[magnoliid]]s, and the [[monocot]]s) of the eight main groups occurred around 140 million years ago.[http://www.pnas.org/cgi/content/abstract/104/49/19363?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=1&andorexacttitle=and&andorexacttitleabs=and&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT] | ||
| + | By the late Cretaceous, angiosperms appear to have become the predominant group of land plants, and many fossil plants recognizable as belonging to modern families (including [[beech]], [[oak]], [[maple]], and [[magnolia]]) appeared. | ||
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| + | [[Image:Syrphid fly on Grape hyacinth.jpg|thumb|200px|A [[Syrphid fly]] on a [[Grape hyacinth]]]] | ||
| + | It is generally assumed that the [[function (biology)|function]] of flowers, from the start, was to involve the mobile [[animal]]s in the [[reproduction]] process. Pollen can be scattered without bright [[color]]s and obvious shapes. Expending [[energy]] on these structures would appear to be a liability, unless they provide significant benefit. | ||
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| + | [[Island genetics]] provides one proposed explanation for the sudden, fully developed appearance of flowering plants. Island genetics is believed to be a common source of [[speciation]] in general, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Flowering plants may have evolved in an isolated setting like an [[island]] or island chain, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a [[wasp]], for example). Such a relationship, with a hypothetical wasp carrying pollen from one plant to another much the way [[fig wasp]]s do today, could result in both the plant(s) and their partners developing a high degree of [[specialization (biology)|specialization]]. Note that the wasp example is not incidental; [[bees]], which apparently evolved specifically due to mutualistic plant relationships, are descended from wasps. | ||
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| + | Animals are also involved in the distribution of seeds. [[Fruit]], which is formed by the enlargement flower parts, is frequently a seed disbursal tool which depends upon animals, who eat or otherwise disturb it, incidentally scattering the seeds it contains (see [[frugivory]]). While many such [[mutualism|mutualistic relationship]]s remain too fragile to survive [[competition (biology)|competition]] with mainland animals and spread, flowers proved to be an unusually effective means of production, spreading (whatever their actual origin) to become the dominant form of land plant life. | ||
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| + | Flowers are derived from [[leaf]] and [[stem]] components, arising from a combination of [[gene]]s normally responsible for forming new shoots.[http://unisci.com/stories/20012/0615015.htm] The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the [[ovary (plants)|ovary]] (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least "ovary inferior". | ||
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| + | Flower evolution continues to the present day; modern flowers have been so profoundly influenced by humans that some of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, perhaps already having symbiotic [[companion plant]] relationships with them, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection.[http://www.livescience.com/othernews/050526_flower_power.html] | ||
== Classification == | == Classification == | ||
| − | [[Image: | + | [[Image:Monocot vs dicot crop Pengo.jpg|150px|thumb|A monocot (left), and dicot]] |
| − | The botanical term "Angiosperm", from the [[ancient Greek]] ''αγγειον'' (receptacle) and ''σπερμα'' (seed), was coined in the form Angiospermae by [[Paul Hermann]] in [[1690]], as the name of that one of his primary divisions of the plant kingdom | + | The botanical term "Angiosperm", from the [[ancient Greek]] ''αγγειον'' (receptacle) and ''σπερμα'' (seed), was coined in the form Angiospermae by [[Paul Hermann]] in [[1690]], as the name of that one of his primary divisions of the plant [[kingdom (biology)|kingdom]]. This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or flowering plants with [[achene|achenial]] or schizo-carpic fruits, the whole fruit or each of its pieces being here regarded as a seed and naked. The term and its antonym were maintained by [[Carolus Linnaeus]] with the same sense, but with restricted application, in the names of the orders of his class [[Didynamia]]. Its use with any approach to its modern scope only became possible after [[1827]], when [[Robert Brown (botanist)|Robert Brown]] established the existence of truly naked ovules in the [[Cycadophyta|Cycadeae]] and [[Pinophyta|Coniferae]], and applied to them the name Gymnosperms. From that time onwards, so long as these Gymnosperms were, as was usual, reckoned as dicotyledonous flowering plants, the term Angiosperm was used antithetically by botanical writers, with varying scope, as a group-name for other dicotyledonous plants. |
| − | + | In [[1851]], [[Wilhelm Friedrich Benedikt Hofmeister|Hofmeister]] discovered the changes occurring in the embryo-sac of flowering plants, and determined the correct relationships of these to the [[Vascular plant|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. This is the sense in which the term is used today. | |
| − | In most | + | In most taxonomies, the flowering plants are treated as a coherent group. The most popular descriptive name has been Angiospermae (Angiosperms), with Anthophyta ("flowering plants") a second choice. These names are not linked to any rank. The [[Wettstein system]] and the [[Engler system]] use the name Angiospermae, at the assigned rank of subdivision. The [[Reveal system]] treated flowering plants as subdivision [[Magnoliophytina]] (Frohne & U. Jensen ex Reveal, Phytologia 79: 70 1996), but later split it to Magnoliopsida, Liliopsida and Rosopsida. The [[Takhtajan system]] and [[Cronquist system]] treat this group at the rank of [[division (biology)|division]], leading to the name Magnoliophyta (from the family name Magnoliaceae). The [[Dahlgren system]] and [[Thorne system (1992)]] treat this group at the rank of class, leading to the name Magnoliopsida. However, the [[APG system]], of 1998, and the [[APG II system]], of 2003<ref name=APG2>{{cite journal|journal=''Botanical Journal of the Linnean Society''|volume=141|pages=399-436|author=Angiosperm Phylogeny Group|year=2003|title=An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II.|url=http://www.blackwell-synergy.com/links/doi/10.1046/j.1095-8339.2003.t01-1-00158.x/full/}}</ref>, do not treat it as a formal taxon but rather treat it as a clade without a formal [[botanical name]] and use the name angiosperms for this clade. |
| − | + | ===Internal classification=== | |
| + | 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 general consensus about how the flowering plants should be arranged has recently begun to emerge, through the work of the [[Angiosperm Phylogeny Group]], who published an influential reclassification of the angiosperms in [[1998]]. An update incorporating more recent research was published as APG II<ref name=APG2/> in [[2003]]. | ||
| − | + | Traditionally, the flowering plants are divided into two groups, which in the Cronquist system are called ''Magnoliopsida'' (at the rank of class, formed from the family name ''Magnoliacae'') and ''Liliopsida'' (at the rank of class, formed from the family name ''[[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 English a member of either group may be called a ''[[dicotyledon]]'' (plural ''dicotyledons'') and ''[[monocotyledon]]'' (plural ''monocotyledons''), or abbreviated, as ''dicot'' (plural ''dicots'') and ''monocot'' (plural ''monocots''). These names derive from the observation that the dicots most often have two ''[[cotyledon]]s'', or embryonic leaves, within each seed. The monocots usually have only one, but the rule is not absolute either way. From a diagnostic point of view the number of cotyledons is neither a particularly handy nor reliable character. | |
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| − | + | Recent studies, as by the APG, show that the monocots form [[holophyletic]] or [[monophyletic]] group; this [[clade]] is given the name ''[[monocots]]''. However, the dicots are not (they are a [[paraphyletic]] group). Nevertheless, within the dicots a monophyletic group does exist, called the ''[[eudicots]]'' or ''[[tricolpates]]'', and including most of the dicots. The name ''tricolpates'' derives from a type of [[pollen]] found widely within this group. The name ''eudicots'' is formed combining ''dicot'' with the prefix ''eu-'' (from Greek, for "well," or "good," botanically indicating "true"), as the eudicots share the characters traditionally attributed to the dicots, such as flowers with four or five parts (four or five [[petal]]s, four or five [[sepal]]s). Separating this group of eudicots from the rest of the (former) dicots leaves a remainder, which sometimes are called informally ''[[palaeodicot]]s'' (Greek prefix ''"palaeo-"'' means "old"). As this remnant group is not monophyletic this is a term of convenience only. | |
| − | + | [[DNA]] studies identify 8 major groups among the flowering plants, [[Amborellales]], [[Nymphaeales]], [[Austrobaileyales]], [[Chloranthales]], [[Ceratophyllales]], [[magnoliids]], [[eudicots]] and [[monocots]]. The relationships between these groups are as yet unresolved. | |
== Flowering plant diversity == | == Flowering plant diversity == | ||
| − | The number of species of flowering plants is estimated to be in the range of 250,000 to 400,000. The number of families in APG (1998) was 462. In APG II (2003) it is not settled; at maximum it is 457, but within this number there are 55 optional segregates, so that the minimum number of families in this system is 402. | + | [[Image:Flores.gif|200px|thumb|right|Various flower colors and shapes]] |
| + | The number of [[species]] of flowering plants is estimated to be in the range of 250,000 to 400,000. | ||
| + | <ref>{{cite journal|author=Thorne, R. F.|url=http://www.ingentaconnect.com/content//iapt/tax/2002/00000051/00000003/art00009|title=How many species of seed plants are there?|journal=Taxon|volume=51|pages=511-522|year=2002}}></ref> | ||
| + | <ref>{{cite journal|author=Scotland, R. W. & Wortley, A. H.|url=http://www.ingentaconnect.com/content/iapt/tax/2003/00000052/00000001/art00011|title=How many species of seed plants are there?|journal=Taxon|volume=52|pages=101-104|year=2003}}</ref> | ||
| + | <ref>{{cite journal|author=Govaerts, R.url=http://www.ingentaconnect.com/content/iapt/tax/2003/00000052/00000003/art00016|title=How many species of seed plants are there? - a response|journal=Taxon|volume=52|issue=3|pages=583-584|year=2003}}</ref> | ||
| + | The number of [[family (biology)|families]] in APG (1998) was 462. In APG II<ref name=APG2/> (2003) it is not settled; at maximum it is 457, but within this number there are 55 optional segregates, so that the minimum number of families in this system is 402. | ||
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| + | 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 5 clades contain a little over 250 species in total, i.e. less than 0.1% of flowering plant diversity, divided among 9 families. | ||
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| + | The most diverse families of flowering plants, in their APG circumscriptions, in order of number of species, are: | ||
| + | <div style="-moz-column-count:2; column-count:2;"> | ||
| + | # [[Asteraceae]] or Compositae ([[daisy]] family): 23,600 species<ref name=APWeb>{{cite url|author=Stevens, P.F.|year=2001 onwards|url=http://www.mobot.org/MOBOT/Research/APweb/welcome.html|title=Angiosperm Phylogeny Website (at Missouri Botanical Garden)}}</ref> | ||
| + | # [[Orchidaceae]] ([[orchid]] family): 21,950 species<ref name=APWeb/> | ||
| + | # [[Fabaceae]] or Leguminosae ([[pea]] family): 19,400<ref name=APWeb/> | ||
| + | # [[Rubiaceae]] ([[madder]] family): 13,183<ref>{{cite url|title=Kew Scientist 30 (October2006)|url=http://www.kew.org/kewscientist/ks_30.pdf}}</ref> | ||
| + | # [[Poaceae]] or Gramineae ([[grass]] family): 10,035<ref name=APWeb/> | ||
| + | # [[Lamiaceae]] or Labiatae ([[mentha|mint]] family): 7,173<ref name=APWeb/> | ||
| + | # [[Euphorbiaceae]] ([[spurge]] family): 5,735<ref name=APWeb/> | ||
| + | # [[Cyperaceae]] ([[sedge]] family): 4,350<ref name=APWeb/> | ||
| + | # [[Malvaceae]] ([[mallow]] family): 4,225<ref name=APWeb/> | ||
| + | # [[Araceae]] ([[aroid]] family): 4,025<ref name=APWeb/> | ||
| + | </div> | ||
| + | In the list above (showing only the 10 largest families), the Orchidaceae, Poaceae, Cyperaceae and Araceae are monocot families; the others are dicot families. | ||
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| + | ==Vascular anatomy== | ||
| − | The | + | The amount and [[complexity]] of tissue-formation in flowering plants exceeds that of Gymnosperms. The [[vascular bundle]]s of the stem are arranged such that the [[xylem]] and [[phloem]] form concentric rings. |
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| − | In the | + | In the Dicotyledons, 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. | |
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== The flower, fruit, and seed == | == The flower, fruit, and seed == | ||
===Flowers=== | ===Flowers=== | ||
{{main|Flower|Plant sexuality}} | {{main|Flower|Plant sexuality}} | ||
| − | The characteristic feature of angiosperms is the flower | + | 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 [[seed]]s. The floral apparatus may arise terminally on a shoot or from the axil of a leaf. Occasionally, as in [[Violet (plant)|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]]. |
| − | The reproductive cells produced by flowers are of two kinds | + | The reproductive cells produced by flowers are of two kinds. Microspores which will divide to become [[pollen|pollen grains]], are the "male" cells and are borne in the [[stamen]]s (or microsporophylls). 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). |
| − | While the majority of flowers are | + | 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 [[sepal]]s and [[petal]]s (or [[tepal]]s 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 generally 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. |
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| + | While the majority of flowers are perfect or [[hermaphrodite]] (having both male and female 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 [[pollinator]]s cannot easily transfer pollen to the pistil (receptive part of the carpel). Homomorphic flowers may employ a biochemical (physiological) mechanism called [[Self-incompatibility in plants|self-incompatibility]] to discriminate between self- and non-self pollen grains. In other species, the male and female parts are morphologically separated, developing on different flowers. | ||
=== Fertilization and embryogenesis=== | === Fertilization and embryogenesis=== | ||
{{Main|Fertilization|Plant embryogenesis}} | {{Main|Fertilization|Plant embryogenesis}} | ||
| − | Double fertilization refers to a process | + | Double fertilization refers to a process in which two [[sperm]] cells fertilize two [[cell (biology)|cells]] in the [[plant ovary|ovary]]. The [[pollen]] grain adheres to the stigma of the [[carpel]] (female reproductive structure) and grows a [[pollen tube]] that penetrates the [[ovum]] through a tiny pore called a [[micropyle]]. Two sperm cells are released into the ovary through this tube. One of the two sperm cells fertilizes the egg cell, forming a [[diploid]] zygote or embryo, also called the [[ovule]]. The other sperm cell fuses with two haploid polar nuclei in the center of the embryo sac. The resulting cell is [[triploid]] (3n). This triploid cell divides through [[mitosis]] and forms the endosperm, a nutrient-rich [[tissue (biology)|tissue]] inside the fruit. When seed develops without fertilization, the process is known as [[apomixis]]. |
=== Fruit and seed === | === Fruit and seed === | ||
{{main|Seed|Fruit}} | {{main|Seed|Fruit}} | ||
| − | As the development of embryo and endosperm proceeds within the embryo-sac, | + | As the development of embryo and endosperm proceeds within the embryo-sac, the sac wall enlarges and combines with the [[nucellus]] (which is likewise enlarging) and the [[integument]] to form the ''seed-coat''. The ovary wall develops to form the [[fruit]] or [[pericarp]], whose form is closely associated with the manner of distribution of the seed. |
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| + | Frequently the influence of fertilization is felt beyond the [[ovary]], and other parts of the flower take part in the formation of the fruit, e.g. the floral receptacle in the [[apple]], [[strawberry]] and others. | ||
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| + | The character of the seed-coat bears a definite relation to that of the fruit. They protect the embryo and aid in dissemination; they may also directly promote germination. Among plants with indehiscent fruits, the fruit generally provides protection for of the embryo and secures dissemination. In this case, the seed-coat is only slightly developed. If the fruit is [[Dehiscence (botany)|dehiscent]] and the seed is exposed, the seed-coat is generally well developed, and must discharge the functions otherwise executed by the fruit. | ||
== Economic importance == | == Economic importance == | ||
| − | Agriculture is almost entirely dependent on angiosperms, either directly or indirectly through livestock feed. Of all the families | + | [[Image:Wheat-haHula-ISRAEL2.JPG|left|thumb|A mature wheat field in northern Israel.]] |
| + | [[Agriculture]] is almost entirely dependent on angiosperms, either directly or indirectly through [[livestock]] feed. Of all the families plants, the [[Poaceae]], or grass family, is by far the most important, providing the bulk of all feedstocks ([[rice]], corn ([[maize]]), [[wheat]], [[barley]], [[rye]], [[oat]]s, [[pearl millet]], [[sugar cane]], [[sorghum]]). The [[Fabaceae]], or legume family, comes in second place. Also of high importance are the [[Solanaceae]], or nightshade family ([[potato]]es, [[tomato]]es, and [[capsicum|pepper]]s, among others), the [[Cucurbitaceae]], or [[gourd]] family (also including [[pumpkin]]s and [[melon]]s), the [[Brassicaceae]], or [[mustard plant]] family (including [[rapeseed]] and [[cabbage]]), and the [[Apiaceae]], or [[parsley]] family. Many of our fruits come from the [[Rutaceae]], or rue family, and the [[Rosaceae]], or rose family (including [[apple]]s, [[pear]]s, [[cherry|cherries]], [[apricots]], [[plums]], etc). | ||
| − | In some parts of the world, certain single species assume paramount importance because of their variety of uses | + | In some parts of the world, certain single species assume paramount importance because of their variety of uses, for example the coconut (''[[coconut|Cocos nucifera]]'') on Pacific [[atoll]]s, and the olive (''[[olive|Olea europaea]]'') in the [[Mediterranean]]. |
| − | Flowering plants also provide economic resources in the form of [[wood]], [[paper]], fiber ([[cotton]], [[flax]], and [[hemp]], among others), medicines ([[digitalis]], [[camphor]]), decorative and landscaping plants, and | + | Flowering plants also provide economic resources in the form of [[wood]], [[paper]], fiber ([[cotton]], [[flax]], and [[hemp]], among others), medicines ([[digitalis]], [[camphor]]), decorative and landscaping plants, and many other uses. The main area in which they are surpassed by other plants is [[timber]] production. |
==See also== | ==See also== | ||
| + | {{Commons|Magnoliophyta}} | ||
| + | {{Wikispecies|Magnoliophyta}} | ||
| + | {{Wikibookspar|Dichotomous Key|Magnoliophyta}} | ||
* [[List of flowers]] | * [[List of flowers]] | ||
| − | == References | + | == References == |
| − | {{ | + | {{reflist}} |
| − | + | ||
| − | + | ==External links== | |
* [http://tolweb.org/tree?group=Angiosperms&contgroup=Spermatopsida Angiosperms] – Tree of Life Web Project | * [http://tolweb.org/tree?group=Angiosperms&contgroup=Spermatopsida Angiosperms] – Tree of Life Web Project | ||
*Cronquist, Arthur. (1981) ''An Integrated System of Classification of Flowering Plants''. Columbia Univ. Press, New York. | *Cronquist, Arthur. (1981) ''An Integrated System of Classification of Flowering Plants''. Columbia Univ. Press, New York. | ||
* Dilcher, D. 2000. Toward a new synthesis: Major evolutionary trends in the angiosperm fossil record. ''PNAS [Proceedings of the National Academy of Sciences of the United States of America]'' 97: 7030-7036 (available online [http://www.pnas.org/cgi/content/abstract/97/13/7030?ck=nck here]) | * Dilcher, D. 2000. Toward a new synthesis: Major evolutionary trends in the angiosperm fossil record. ''PNAS [Proceedings of the National Academy of Sciences of the United States of America]'' 97: 7030-7036 (available online [http://www.pnas.org/cgi/content/abstract/97/13/7030?ck=nck here]) | ||
| + | * {{cite book|author = Heywood, V. H., Brummitt, R. K., Culham, A. & Seberg, O. |title = Flowering Plant Families of the World|edition = | publisher = Firefly Books| year = 2007| location = Richmond Hill, Ontario, Canada|isbn = 1-55407-206-9}} | ||
* [http://www.news.harvard.edu/gazette/1999/12.16/angiosperms.html Oldest Known Flowering Plants Identified By Genes], William J. Cromie, Harvard Gazette, [[December 16]], [[1999]]. | * [http://www.news.harvard.edu/gazette/1999/12.16/angiosperms.html Oldest Known Flowering Plants Identified By Genes], William J. Cromie, Harvard Gazette, [[December 16]], [[1999]]. | ||
| − | * | + | * L. Watson and M.J. Dallwitz (1992 onwards). [http://www.biologie.uni-hamburg.de/b-online/delta/angio/ The families of flowering plants: descriptions, illustrations, identification, information retrieval.] |
| − | |||
* Simpson, M.G. ''Plant Systematics''. Elsevier Academic Press. 2006. | * Simpson, M.G. ''Plant Systematics''. Elsevier Academic Press. 2006. | ||
* Raven, P.H., R.F. Evert, S.E. Eichhorn. ''Biology of Plants'', 7th Edition. W.H. Freeman. 2004. | * Raven, P.H., R.F. Evert, S.E. Eichhorn. ''Biology of Plants'', 7th Edition. W.H. Freeman. 2004. | ||
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Revision as of 20:06, 24 January 2008
| Magnoliophyta (flowering plants)
Fossil range: Late Jurassic - Recent
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 Magnolia virginiana flower | ||||
| Plant Info | ||||
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| Scientific classification | ||||
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| Magnoliopsida - Dicots Liliopsida - Monocots |
The flowering plants or angiosperms are the most widespread group of land plants. The flowering plants and the gymnosperms comprise the two extant groups of seed plants. The flowering plants are distinguished from other seed plants by a series of apomorphies, or derived characteristics.
Angiosperm derived characteristics
The flowers of flowering plants are the most remarkable feature distinguishing them from other seed plants. Flowers aided angiosperms by enabling a wider range of evolutionary relationships and broadening the ecological niches open to them, allowing flowering plants to eventually dominate terrestrial ecosystems.
- 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 specialized pollination syndromes, such as particular pollinators. Stamens have also become modified through time to prevent self-fertilization, which has permitted further diversification, allowing angiosperms to eventually fill more niches.
- Reduced male parts, three cells
The male gametophyte in angiosperms is significantly reduced in size compared to those of gymnosperm seed plants. The smaller pollen decreases the time from pollination – the pollen grain reaching the female plant – to fertilization of the ovary; in gymnosperms fertilization can occur up to a year after pollination, while in angiosperms the fertilization begins very soon after pollination. The shorter time leads to angiosperm plants setting seeds sooner and faster than gymnosperms, which is a distinct evolutionary advantage.
The closed carpel of angiosperms also allows adaptations to specialized pollination syndromes and controls to prevent self-fertilization, thereby maintaining increased diversity. Once the ovary is fertilized the carpel and some surrounding tissues develop into a fruit, another opportunity for angiosperms to increase their domination of the terrestrial ecosystem with evolutionary adaptations to dispersal mechanisms.
- Reduced female gametophyte, seven cells with eight nuclei
The reduced female gametophyte, like the reduced male gametophyte may be adaptations allowing for more rapid seed set, eventually leading to such flowering plant adaptations as annual herbaceous life cycles, allowing the flowering plants to fill even more niches.
Endosperm formation generally 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 for the seedling when it first appears.
These distinguishing characteristics taken together have made the angiosperms the most diverse and numerous land plants and the most commercially important group to humans. The major exception to the dominance of terrestrial ecosystems by flowering plants is the coniferous forest.
Evolution
Land plants have existed for about 425 million years. Early land plants reproduced by spores like their aquatic counterparts. Marine organisms can easily scatter copies of themselves to float away and grow elsewhere. Land plants soon found it advantageous to protect their copies from drying out and other hazards by enclosing them in a case, the seed. Early seed bearing plants, like the ginkgo, and conifers (such as pines and firs), did not produce flowers.
The earliest fossil of an angiosperm, or flowering plant, Archaefructus liaoningensis, is dated to about 125 million years BP[1]. Pollen, considered directly linked to flower development, has been found in the fossil record perhaps as long ago as 130 million years.
While there is only hard evidence of such flowers existing about 130 million years ago, there is some circumstantial evidence that they may have existed 250 million years ago. A chemical used by plants to defend their flowers, oleanane, has been detected in fossil plants that old, including gigantopterids[2], which evolved at that time and bear many of the traits of modern, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail, one of the earliest examples of petrification.
The apparently sudden appearance of relatively modern flowers in the fossil record posed such a problem for the theory of evolution that it was called an "abominable mystery" by Charles Darwin.[1] However the fossil record has grown since the time of Darwin, and recently discovered angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps. Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. Some older fossils, such as the upper Triassic Sanmiguelia, have been suggested. Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late Triassic (245-202 million years ago). The relationship of the earlier gigantopterids to flowering plants is still enigmatic.
A close relationship between Angiosperms and Gnetophytes, suggested on the basis of morphological evidence, has been disputed on the basis of molecular evidence that suggest Gnetophytes are more closely related to other gymnosperms.
Recent DNA analysis (molecular systematics) [3] [4] show that Amborella trichopoda, found on the Pacific island of New Caledonia, belongs to a sister group of the other flowering plants, and morphological studies [5] suggest that it has features which may have been characteristic of the earliest flowering plants.
The great angiosperm radiation, when a great diversity of angiosperms appear in the fossil record, occurred in the mid-Cretaceous (approximately 100 million years ago). However, a study in 2007 estimated that the division of the five most recent (the genus Ceratophyllum, the family Chloranthaceae, the eudicots, the magnoliids, and the monocots) of the eight main groups occurred around 140 million years ago.[6] By the late Cretaceous, angiosperms appear to have become the predominant group of land plants, and many fossil plants recognizable as belonging to modern families (including beech, oak, maple, and magnolia) appeared.
It is generally assumed that the function of flowers, from the start, was to involve the mobile animals in the reproduction process. Pollen can be scattered without bright colors and obvious shapes. Expending energy on these structures would appear to be a liability, unless they provide significant benefit.
Island genetics provides one proposed explanation for the sudden, fully developed appearance of flowering plants. Island genetics is believed to be a common source of speciation in general, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Flowering plants may have evolved in an isolated setting like an island or island chain, where the plants bearing them were able to develop a highly specialized relationship with some specific animal (a wasp, for example). Such a relationship, with a hypothetical wasp carrying pollen from one plant to another much the way fig wasps do today, could result in both the plant(s) and their partners developing a high degree of specialization. Note that the wasp example is not incidental; bees, which apparently evolved specifically due to mutualistic plant relationships, are descended from wasps.
Animals are also involved in the distribution of seeds. Fruit, which is formed by the enlargement flower parts, is frequently a seed disbursal tool which depends upon animals, who eat or otherwise disturb it, incidentally scattering the seeds it contains (see frugivory). While many such mutualistic relationships remain too fragile to survive competition with mainland animals and spread, flowers proved to be an unusually effective means of production, spreading (whatever their actual origin) to become the dominant form of land plant life.
Flowers are derived from leaf and stem components, arising from a combination of genes normally responsible for forming new shoots.[7] The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the ovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least "ovary inferior".
Flower evolution continues to the present day; modern flowers have been so profoundly influenced by humans that some of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, perhaps already having symbiotic companion plant relationships with them, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection.[8]
Classification
The botanical term "Angiosperm", from the ancient Greek αγγειον (receptacle) and σπερμα (seed), was coined in the form Angiospermae by Paul Hermann in 1690, as the name of that one of his primary divisions of the plant kingdom. This included flowering plants possessing seeds enclosed in capsules, distinguished from his Gymnospermae, or 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 term and its antonym were maintained by Carolus Linnaeus with the same sense, but with restricted application, in the names of the orders of his class Didynamia. Its use with any approach to its modern scope only became possible after 1827, when Robert Brown established the existence of truly naked ovules in the Cycadeae and Coniferae, and applied to them the name Gymnosperms. From that time onwards, so long as these Gymnosperms were, as was usual, reckoned as dicotyledonous flowering plants, the term Angiosperm was used antithetically by botanical writers, with varying scope, as a group-name for other dicotyledonous plants.
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. 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 (Angiosperms), with Anthophyta ("flowering plants") a second choice. These names are not linked to any rank. The Wettstein system and the Engler system use the name Angiospermae, at the assigned rank of subdivision. The Reveal system treated flowering plants as subdivision Magnoliophytina (Frohne & U. Jensen ex Reveal, Phytologia 79: 70 1996), but later split it to Magnoliopsida, Liliopsida and Rosopsida. The Takhtajan system and Cronquist system treat this group at the rank of division, leading to the name Magnoliophyta (from the family name Magnoliaceae). The Dahlgren system and Thorne system (1992) treat this group at the rank of class, leading to the name Magnoliopsida. However, the APG system, of 1998, and the APG II system, of 2003[2], do not treat it as a formal taxon but rather treat it as a clade without a formal botanical name and use the name angiosperms for this clade.
Internal classification
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 general consensus about how the flowering plants should be arranged has recently begun to emerge, through the work of the Angiosperm Phylogeny Group, who published an influential reclassification of the angiosperms in 1998. An update incorporating more recent research was published as APG II[2] in 2003.
Traditionally, the flowering plants are divided into two groups, which in the Cronquist system are called Magnoliopsida (at the rank of class, formed from the family name Magnoliacae) and Liliopsida (at the rank of class, formed from the family name 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 English a member of either group may be called a dicotyledon (plural dicotyledons) and monocotyledon (plural monocotyledons), or abbreviated, as dicot (plural dicots) and monocot (plural monocots). These names derive from 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 diagnostic point of view the number of cotyledons is neither a particularly handy nor reliable character.
Recent studies, as by the APG, show that the monocots form holophyletic or monophyletic group; this clade is given the name monocots. However, the dicots are not (they are a paraphyletic group). Nevertheless, within the dicots a monophyletic group does exist, called the eudicots or tricolpates, and including most of the dicots. The name tricolpates derives from a type of pollen found widely within this group. The name eudicots is formed combining dicot with the prefix eu- (from Greek, for "well," or "good," botanically indicating "true"), as the eudicots share the characters traditionally attributed to the dicots, such as flowers with four or five parts (four or five petals, four or five sepals). Separating this group of eudicots from the rest of the (former) dicots leaves a remainder, which sometimes are called informally palaeodicots (Greek prefix "palaeo-" means "old"). As this remnant group is not monophyletic this is a term of convenience only.
DNA studies identify 8 major groups among the flowering plants, Amborellales, Nymphaeales, Austrobaileyales, Chloranthales, Ceratophyllales, magnoliids, eudicots and monocots. The relationships between these groups are as yet unresolved.
Flowering plant diversity
The number of species of flowering plants is estimated to be in the range of 250,000 to 400,000. [3] [4] [5] The number of families in APG (1998) was 462. In APG II[2] (2003) it is not settled; at maximum it is 457, but within this number there are 55 optional segregates, so that the minimum number of families in this system is 402.
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 5 clades contain a little over 250 species in total, i.e. less than 0.1% of flowering plant diversity, divided among 9 families.
The most diverse families of flowering plants, in their APG circumscriptions, in order of number of species, are:
- Asteraceae or Compositae (daisy family): 23,600 species[6]
- Orchidaceae (orchid family): 21,950 species[6]
- Fabaceae or Leguminosae (pea family): 19,400[6]
- Rubiaceae (madder family): 13,183[7]
- Poaceae or Gramineae (grass family): 10,035[6]
- Lamiaceae or Labiatae (mint family): 7,173[6]
- Euphorbiaceae (spurge family): 5,735[6]
- Cyperaceae (sedge family): 4,350[6]
- Malvaceae (mallow family): 4,225[6]
- Araceae (aroid family): 4,025[6]
In the list above (showing only the 10 largest families), the Orchidaceae, Poaceae, Cyperaceae and Araceae are monocot families; the others are dicot families.
Vascular anatomy
The amount and complexity of tissue-formation in flowering plants exceeds that of Gymnosperms. The vascular bundles of the stem are arranged such that the xylem and phloem form concentric rings.
In the Dicotyledons, 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.
The flower, fruit, and seed
Flowers
- Main article: Flower
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. The floral apparatus may arise terminally on a shoot or from the axil of a leaf. 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.
The reproductive cells produced by flowers are of two kinds. Microspores which will divide to become pollen grains, are the "male" cells and are borne in the stamens (or microsporophylls). 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).
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 generally 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.
While the majority of flowers are perfect or hermaphrodite (having both male and female 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. In other species, the male and female parts are morphologically separated, developing on different flowers.
Fertilization and embryogenesis
- Main article: Fertilization
Double fertilization refers to a process in which two sperm cells fertilize two cells in the ovary. The pollen grain adheres to the stigma of the carpel (female reproductive structure) and grows a pollen tube that penetrates the ovum through a tiny pore called a micropyle. Two sperm cells are released into the ovary through this tube. One of the two sperm cells fertilizes the egg cell, forming a diploid zygote or embryo, also called the ovule. The other sperm cell fuses with two haploid polar nuclei in the center of the embryo sac. The resulting cell is triploid (3n). This triploid cell divides through mitosis and forms the endosperm, a nutrient-rich tissue inside the fruit. When seed develops without fertilization, the process is known as apomixis.
Fruit and seed
- Main article: Seed
As the development of embryo and endosperm proceeds within the embryo-sac, the sac wall enlarges and combines with the nucellus (which is likewise enlarging) and the integument to form the seed-coat. The ovary wall develops to form the fruit or pericarp, whose form is closely associated with the manner of distribution of the seed.
Frequently the influence of fertilization is felt beyond the ovary, and other parts of the flower take part in the formation of the fruit, e.g. the floral receptacle in the apple, strawberry and others.
The character of the seed-coat bears a definite relation to that of the fruit. They protect the embryo and aid in dissemination; they may also directly promote germination. Among plants with indehiscent fruits, the fruit generally provides protection for of the embryo and secures dissemination. In this case, the seed-coat is only slightly developed. If the fruit is dehiscent and the seed is exposed, the seed-coat is generally well developed, and must discharge the functions otherwise executed by the fruit.
Economic importance
Agriculture is almost entirely dependent on angiosperms, either directly or indirectly through livestock feed. Of all the families plants, the Poaceae, or grass family, is by far the most important, providing the bulk of all feedstocks (rice, corn (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 (potatoes, tomatoes, and peppers, among others), the Cucurbitaceae, or gourd family (also including pumpkins and melons), the Brassicaceae, or mustard plant family (including rapeseed and cabbage), and the Apiaceae, or parsley family. Many of our fruits come from the Rutaceae, or rue family, and the Rosaceae, or rose family (including apples, pears, cherries, apricots, plums, etc).
In some parts of the world, certain single species assume paramount importance because of their variety of uses, for example the coconut (Cocos nucifera) on Pacific atolls, and the olive (Olea europaea) in the Mediterranean.
Flowering plants also provide economic resources in the form of wood, paper, fiber (cotton, flax, and hemp, among others), medicines (digitalis, camphor), decorative and landscaping plants, and many other uses. The main area in which they are surpassed by other plants is timber production.
See also
Template:Commons Template:Wikispecies Template:Wikibookspar
References
- ↑ Darwin's abominable mystery: Insights from a supertree of the angiosperms. Proceedings of the National Academy of Sciences of the United States of America. T. Jonathan Davies, Timothy G. Barraclough, Mark W. Chase, Pamela S. Soltis, Douglas E. Soltis, and Vincent Savolainen. Published (online) February 6, 2004.
- ↑ 2.0 2.1 2.2 Angiosperm Phylogeny Group (2003). "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG II.". Botanical Journal of the Linnean Society 141: 399-436. http://www.blackwell-synergy.com/links/doi/10.1046/j.1095-8339.2003.t01-1-00158.x/full/.
- ↑ Thorne, R. F. (2002). "How many species of seed plants are there?". Taxon 51: 511-522. http://www.ingentaconnect.com/content//iapt/tax/2002/00000051/00000003/art00009.>
- ↑ Scotland, R. W. & Wortley, A. H. (2003). "How many species of seed plants are there?". Taxon 52: 101-104. http://www.ingentaconnect.com/content/iapt/tax/2003/00000052/00000001/art00011.
- ↑ Govaerts, R.url=http://www.ingentaconnect.com/content/iapt/tax/2003/00000052/00000003/art00016+(2003).+"How many species of seed plants are there? - a response". Taxon 52 (3): 583-584.
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 Template:Cite url
- ↑ Template:Cite url
External links
- Angiosperms – Tree of Life Web Project
- Cronquist, Arthur. (1981) An Integrated System of Classification of Flowering Plants. Columbia Univ. Press, New York.
- Dilcher, D. 2000. Toward a new synthesis: Major evolutionary trends in the angiosperm fossil record. PNAS [Proceedings of the National Academy of Sciences of the United States of America] 97: 7030-7036 (available online here)
- Heywood, V. H., Brummitt, R. K., Culham, A. & Seberg, O. (2007). Flowering Plant Families of the World. Richmond Hill, Ontario, Canada: Firefly Books. ISBN 1-55407-206-9.
- Oldest Known Flowering Plants Identified By Genes, William J. Cromie, Harvard Gazette, December 16, 1999.
- L. Watson and M.J. Dallwitz (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval.
- Simpson, M.G. Plant Systematics. Elsevier Academic Press. 2006.
- Raven, P.H., R.F. Evert, S.E. Eichhorn. Biology of Plants, 7th Edition. W.H. Freeman. 2004.
