The angiosperm life cycle, in countless ways, complies with the simple life bike pattern for land plants (embryophytes), with changes characteristic of the seed plant habit (read an ext here). As in various other seed plants, the microgametophyte (male, or sperm-producing gametophyte) is very simplified and called a pollen grain.The megagametophyte (female, or egg-producing, gametophyte) establishes within the ovule (immature seed). The pollen grain need to be released and transported to the ovule-bearing structure before fertilization have the right to occur. However, angiosperm pollen grains, megagametophytes, and also fertilization differ from those of other seed tree in several essential ways.
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Angiosperm pollen is produced in the anther, a floral structure that is frequently made up of 2 pairs of fused microsporangia (microspore-producing sporangia, likewise called pollen sacs). Pollen grains of angiosperms space the most simplified in every one of seed plants. In ~ maturity, every pollen grain includes three cells: one tube cell (the cabinet that forms the pollen tube) and also two sperm.
Flowers showing ovaries and anthers. In angiosperms, the ovules the contain the megagametophytes (female or egg-producing gametophytes) are enclosed in an ovary. The pollen grains are developed in microsporangia (pollen sacs) that are part of the anther. Credit: Hypogynous flower, perigynous flower, and epigynous flower (Drawings by Ivy Livingston, BIODIDAC, CC BY-NC 4.0). Drawings modified native originals.
The pollen seed of angiosperms can not land directly on the ovules because the ovules are enclosed in a floral structure called an ovary (the ovary is shown in the number at the peak of the page). Thus, the pollen seed land top top a specialized surface (the stigma), where they germinate. Angiosperms space siphonogamous (Greek, siphon + gamia = pipe union), an interpretation that their sperm are not motile (i.e., lack flagella and cannot swim) and are delivered to the egg by method of a pollen tube. The pollen pipe must grow from the pollen serial on the stigma, under the style, and also into the ovary come make contact with an ovule.
Lily flower and germinated pollen. Left: Close-up the lily (Lilium) flower showing open up anthers and also the stigma, the surface ar on i beg your pardon pollen will land. Right: Scanning electron micrograph (SEM image) that germinated lily (Lilium) pollen grains mirroring pollen tubes cultivation out the apertures (thin areas) in the pollen walls. Credit: Closeup the stamen and also stigma the Lilium (Subhrajyoti07, Wikimedia Commons, CC BY-SA 4.0); SEM micrograph of lily pollen tube (Neutr0nics, Wikimedia Commons, CC BY-SA 3.0). Photos modified native originals.
Unlike gymnosperm ovules, angiosperm ovules frequently have a double integument, or two distinct integuments that surround the nucellus (megasporangium, the megaspore-producing sporangium) in i m sorry the megagametophyte develops. Angiosperm ovules are often folded over on themselves. The ovule is attached come the inner ovary wall by a stalk called a funiculus or funicle.
The megagametophytes (female or egg-producing gametophytes) of angiosperms are additionally called embryo sacs. The megagametophytes are very simplified contrasted to those of various other seed plants. Many angiosperm megagametophytes are comprised of just seven cells, although castle may have as few as four cells. One egg cabinet is current per megagametophyte, and there is no archegonium. (The archegonium is a multicellular framework that surrounds the egg cabinet in numerous other tree groups).
Lily ovules. Left: Cross section of the ovary the a lily (Lilium) ovary, with 6 ovules; the ovules are shown by the arrowheads. Right: Longitudinal ar of a solitary ovule v embryo sac (megagametophyte). Credit: Lilium ovary and also Lilium embryo 8 nuclei (Jon Houseman & Matthew Ford, Wikimedia Commons, CC BY-SA 4.0). Photos modified because that DEAL.
Once the embryo sac has actually developed, pollination has occurred, and also the pollen tube has actually grown into the ovary to make contact with the ovule, fertilization (fusion the egg and sperm) have the right to occur. Typically, the pollen pipe reaches the embryo sac via the micropyle (Greek, mikros + pyle = little opening), or opening, in the integuments of the ovule. There, it discharges that is sperm right into the embryo sac.
One of the crucial features that distinguishes angiosperms native all various other seed plants is double fertilization with endosperm formation. Double fertilization wake up in siphonogamous seed tree (i.e., seeds plants through nonmotile sperm) when both sperm in a pollen tube unite with structures in a megagametophyte. In angiosperms, one sperm unites with the egg to kind a diploid zygote, the first cell the a new sporophyte. The other sperm unites with the cell nucleus or nuclei in the large central cell the the embryo sac to form a primary endosperm nucleus. This nucleus is the an initial nucleus the the endosperm (Greek, endon + sperma = within seed), a type of food tissue distinctive to the seed of angiosperms.
After fertilization, the ovule i do not care a seed. The seed is a structure containing a young, diploid sporophyte embryo and, typically, save on computer food for the embryo. In angiosperms, the food in the seed may be save on computer in the form of endosperm, it might be save on computer in the cotyledon(s) (seed leaves) of the embryo, or it might be save on computer in both structures. The particle is safeguarded by a seed coat, which develops from the integuments that the ovule.
Longitudinal sections of seeds with sporophyte embryos. Left: p (Phaseolus) separation lengthwise to display the parts of the embryo, including the 2 food-storing cotyledons, the hypocotyl-root axis (sporophyte embryo axis listed below the cotyledons), and the an initial foliage leaves. No endosperm is apparent. Right: Corn (Zea mays, a monocot) embryo through one cotyledon and conspicuous endosperm. Credits: Phaseolus seeds (Bruce Krichoff, via flickr, CC through 2.0); Zea kernel (Jon Houseman and also Matthew Ford, Wikimedia Commons, CC BY-SA 4.0). Pictures modified from originals.
Generalized angiosperm life cycle. Summary diagram because that the life bike of an angiosperm. The diploid (2n), multicellular sporophyte bear flowers. Embryo sacs (megagametophytes) build in the ovules, which are discovered in the ovary. Pollen grains (microgametophytes) build in the pollen bag (microsporangia) of the anther. One megaspore mother cell wake up in each ovule; it experience meiosis, typically giving climb to one useful megaspore. Plenty of microspore mother cells take place in the pollen sacs; every undergoes meiosis to create four microspores. Megaspores build into embryo sacs, microspores right into pollen grains. The pollen grains room released from the anther come land ~ above a stigma during pollination. The pollen serial germinates, forming a pollen tube that delivers the sperm come the egg in the ovule throughout fertilization. The sporophyte embryo develops in the seed. Upon germination of the seed, the sporophyte resumes growth. Credit: diagram by E.J. Hermsen (DEAL).
Pollen serial (microgametophyte) development
In flower plants, pollen is developed in the anthers. Anthers are structures frequently made increase of four fused pollen sacs, or microsporangia. Microspore mom cells (also referred to as pollen mom cells) differentiate within the pollen sacs (microsporangia) the the anthers. Every microspore mom cell experience meiosis to develop four haploid microspores. Each microspore then divides to produce its own pollen grain. Countless pollen grains may for this reason be produced in a solitary anther.
Each microspore divides double to develop the mature, three-celled pollen grain. The first division produces a tube cabinet (cell that will certainly elongate to type the pollen tube) and a generative cell. The generative cell then divides to produce two sperm. Following pollination, the pollen serial germinates and also the tube cell begins elongating to form the pollen tube.
Pollen serial development. Development the the pollen serial from diploid (2n) microspore mommy cell come 3-celled microgametophyte. Each micropore mom cell in a pollen sac undergoes meiosis to create four haploid (n) microspores. Each microspore divides once to create a 2-celled pollen grain. The generative cabinet divides again come yield two sperm. The pipe cell will kind the pollen tube the delivers the sperm following pollination. Credit: chart by E.J. Hermsen (DEAL), modified ~ Foster & Gifford (1974).
Embryo sac (megagametophyte) development
In angiosperms, the megagametophyte (female or egg-producing gametophyte) is also called an embryo sac. The embryo sac establishes within an ovule, i m sorry is included within the ovary that a flower. In most angiosperms, the maturation embryo sac is a seven-celled, eight-nucleate structure. This kind of embryo sac is additionally called the Polygonum-type, after the genus Polygonum (knotweed or smartweed). Most angiosperms (about 70%) room thought to have this form of embryo sac. The ax "Polygonum-type" does not refer just to the final configuration of the embryo sac, but also to its succession of development.
An vital note around the diagrams in this section: The standard means to display angiosperm ovules is v the micropylar end (the end of the ovule v the micropyle or opened in the integuments) at the bottom. This choice makes sense structurally since the micropyle faces the base of the ovule in countless angiosperms (an ovule orientation recognized as anatropous). However, once the angiosperm embryo sac and its breakthrough are presented in isolation (without the neighboring nucellus and also integuments), they room often depicted with the micropyle in ~ the optimal of the diagram.
This convention deserve to be confuse when discussing embryo sac development, as it method whole-ovule and also embryo-sac-only diagrams space rotated 180 levels from each other. Thus, I have actually made the selection to orient the diagrams below so that the micropyle is constantly at the top of the diagram. Remember: No matter the orientation that the ovule or the succession of embryo sac development, the egg cell (and associated synergids) is constantly at the micropylar end of the ovule.Development the the Polygonum-type embryo sac
Embryo sac advance begins v the production of megaspores. A large megaspore mommy cell differentiates within the arising ovule. This megaspore mommy cell experience meiosis to develop four haploid megaspores. One megaspore (the one furthest from the micropyle) is functional, while the remainder degenerate. Thus, the Polygonum-type embryo sac exhibits monosporic (Greek, monos = one) development, definition that it establishes from a single spore.
The functional megaspore experience a series of free-nuclear divisions, or departments of the nucleus without partitioning the the cytoplasm into separate cells. One cell nucleus divides mitotically to create two nuclei, 2 nuclei divide to create four, and also four division to produce eight.
Development of the Polygonum-type embryo sac. A megaspore mother cell differentiates in ~ the nucellus of an ovule. It experience meiosis to productivity a tetrad (group of four) the megaspores. Three of this degenerate, and the megaspore furthest from the micropyle is functional. This megaspore undergoes free-nuclear divisions, or departments of the nucleus there is no the partitioning the cells. The megaspore nucleus divides when to create two nuclei. Each of these nuclei divides again to develop four nuclei. Each of these nuclei divides again to create eight nuclei. The last step in development of the embryo sac will be partitioning of the cells (shown below). The diagram is oriented so the the micropyle is at the peak of the figure. Credit: diagram by E.J. Hermsen (DEAL).
Finally, the cytoplasm of the embryo sac is partitioned right into seven different cells. At the end of the ovule close to the micropyle room an egg cell and two synergids. At the opposite finish of the embryo sac space three cells referred to as antipodals. Finally, the big central cell consists of two polar nuclei. In the Polygonum-type embryo sac, all the nuclei room haploid, or have actually one collection of chromosomes.
Ovule with Polygonum-type embryo sac. Idealized diagram of one ovule with a Polygonum-type embryo sac, mirroring the seven cells and eight nuclei surrounded by a slim nucellus (megasporangium) and twin integuments. Keep in mind that the antipodals may breakdown and the polar nuclei might fuse to kind a diploid nucleus prior to fertilization. The nucellus may also break down, so may not be observed in mature ovules. Credit: E.J. Hermsen (DEAL).
Double fertilization that the Polygonum-type embryo sac
As discussed above, angiosperms have dual fertilization. One sperm fuses v the egg to do a diploid zygote, the solitary cell that v division, growth, and development will eventually yield a sporophyte embryo. The other sperm fuses with the polar nuclei to form a primary endosperm nucleus. In the Polygonum-type embryo sac, the major endosperm cell nucleus is triploid, meaning that it has actually three set of chromosomes. One collection comes native the sperm and two set come indigenous the polar nuclei. The main endosperm nucleus will begin dividing to type the endosperm, the food for the young sporophyte within angiosperm seeds.
What wake up to the various other cells and also structures the the ovule: the antipodals, synergids, nucellus, and also integuments? The antipodals degenerate, either before or following fertilization. They have actually no clear duty in the embryo sac. The synergids space thought come play a duty in fertilization, back these likewise degenerate. Typically, the nucellus degenerates before maturation of the seed. The integument or integuments become the seed coat.
Polygonum-type embryo sac after twin fertilization. Idealized diagram showing the Polygonum-type embryo sac following double fertilization. During double fertilization, the egg and a sperm hold together to form a diploid zygote. The various other sperm unites with the polar nuclei to kind a triploid main endosperm nucleus. The antipodals degenerate. The synergids play a role in fertilization, but also degenerate. Credit: chart by E.J. Hermsen (DEAL).
Although the Polygonum-type embryo sac is the many common form of embryo sac in angiosperms, that is not believed to be the genealogical type. Rather, research studies on Amborella and other basal angiosperms in the Austrobaileyales and Nymphaeales have melted light ~ above the beginning of the Polygonum-type embryo sac and also the advancement of other species of embryo bag in angiosperms.Ancestral company of the angiosperm embryo sac
The Nuphar/Schisandra-type the embryo sac is thought to be the ancestral kind of embryo sac because that crown-group angiosperms, or every living angiosperms and their many recent usual ancestor (see here, here, here, and here). This form of embryo sac is uncovered in members of Austrobaileyales and Nymphaeales, two orders of basal or ANA-grade angiosperms. Nuphar (Nymphaeales) is a kind of water lily and Schisandra (Austrobaileyales) is a form of shrub aboriginal to the southeastern united States, Mexico, and eastern come southeastern Asia.
Nuphar and Schisandra flowers. Left: Yellow pond-lily (Nuphar lutea, Nymphaeales). Right: just starvine (Schisandra glabra, Austrobaileyales). Credits: Yellow water-lily (Randi Hausken, via Wikimedia Commons, CC BY-SA 2.0); Schisandra glabra (Helen Lowe Metzman/USGS bee Inventory and Monitoring, via flickr, windy Domain). Pictures modified from originals.
TheNuphar/Schisandra-type embryo sac begins development in a manner comparable to thePolygonum-type embryo sac. It establishes from a solitary megaspore that undergoes two sets of free-nuclear divisions, to create a 4-nucleate stage. Moving partitioning then results in a 4-celled, 4-nucleate embryo sac. The embryo sac has actually an egg, two synergids, and also a polar cell nucleus in a central cell. No antipodals room formed. Twin fertilization in this kind of embryo sac returns a diploid zygote and a diploid main endosperm nucleus (one set of chromosomes from a sperm + one collection from the polar nucleus = two sets that chromosomes in the major endosperm nucleus).
The Nuphar/Schisandra-type embryo sac. Development that the Nuphar/Schisandra-type embryo sac is comparable to advance of the Polygonum-type embryo sac. Differences include the nuclei are located only in ~ the micropylar end and also partitioning of cell happens after ~ the 4-nucleate stage. The tires embryo sac has an egg cell, 2 synergids, and a single polar nucleus. Credit: diagram by E.J. Hermsen (DEAL), modified after ~ Friedman & Ryerson (2009) and also other documents on megagametogenesis in ANA-grade angiosperms (see references).
Modularity that the embryo sac
Research on breakthrough of the megagametophyte in ANA-grade angiosperms says that the 4 cells the the Nuphar/Schisandra-type embryo sac might act as a module or an easy building block ~ above which other species of angiosperm embryo sacs are based (see here, here, here, and here). Thus, the Polygonum-type embryo sac may have evolved from the Nuphar/Schisandra-type via a duplication that the an easy 4-celled, 4-nucleate unit that egg cabinet + 2 synergids + 1 polar cell core in the central cell. The antipodals plus among the polar nuclei in the central cell of the Polygonum-type embryo sac can thus be viewed just as a doubling of this straightforward unit in ~ the opposite end of the embryo sac.
Interestingly, Amborella, the basalmost living angiosperm, go not have actually a four-celled, four-nucleate embryo sac. Rather, it has actually an eight-celled, nine-nucleate embryo sac, which looks like a Polygonum-type embryo sac with one extr synergid. That is assumed that the Amborella-type embryo sac advanced independently native the Nuphar/Schisandra-type embryo sac and also not indigenous the Polygonum-type embryo sac (see here).
Comparison of embryo sacs. Left: Amborella-type embryo sac through an egg (E), 3 synergids (S), 3 antipodals (A), and two polar nuclei (black dots). Center: Polygonum-type embryo sac with an egg, 2 synergids, three antipodals, and also two polar nuclei. Right: Nuphar/Schisandra-type with an egg, 2 synergids, and one polar nucleus. Every embryo sacs are oriented through the micropylar end up. Credit: diagram by E.J. Hermsen (DEAL), modified ~ Friedman & Ryerson (2009) and also other files on megagametogenesis in ANA-grade angiosperms (see references).
Hypothesized evolution of embryo sacs mapped depend a phylogeny. Evolution that angiosperm embryo bag under the modular theory, together mapped unto a simplified tree of angiosperm relationships. The Nuphar/Schisandra-type (4-celled, 4-nucleate) is a synapomorphy because that crown-group angiosperms. The Amborella-type (8-celled, 9-nucleate) evolved only in the Amborella lineage. The Polygonum-type (7-celled, 8-nucleate) is a synapomorphy because that the clade consisting of all life angiosperms over the ANA-grade (i.e., Amborella, Nymphaeales, and also Austrobaileyales). Credit: diagram by E.J. Hermsen (DEAL), modified after Friedman & Ryerson (2009) and also other files on megagametogenesis in ANA-grade angiosperms (see references).
Other embryo sacs
In enhancement to Polygonum-, Nuphar/Schisandra-, and Amborella-type embryo sacs, other varieties of embryo bag have evolved in other groups of angiosperms. We will certainly not evaluation them every here. In a general sense, angiosperm embryo sacs loss into one of the adhering to categories:Monosporic: In this kind of development, the embryo sac occurs from one megaspore, and also the various other three megaspores degenerate. Examples: The three varieties of embryo bag detailed above (Polygonum, Nuphar/Schisandra, and Amborella) space monosporic.Bisporic: In this kind of development, a solitary embryo sac arises from 2 of the 4 megaspore nuclei produced by meiosis the the megaspore mommy cell. The other two nuclei degenerate.
Some sports in angiosperm embryo sac development. diagram (after Maheshwari 1950) showing major variations in the advance of the embryo sac (megagametophyte or mrs gametophyte) of angiosperms. The micropyle is a the top of the picture in every diagrams. Megasporogenesis is the procedure that to produce a tetrad (group the four) megaspores native the megaspore mom cell (MMC) through division by meiosis. FMS = sensible megaspore. Megagametogenesis is the breakthrough of the woman gametophyte (embryo sac) v mitotic departments of the nuclei, cabinet partitioning, and also differentiation (specialization) of cells. In the tires embryo sac, the egg cabinet is the big pink cell at the apex; the synergids are the greenish cells connected with the egg. Credit: number 2 indigenous Schmid et al. (2015) Frontiers in plant Science (CC by 4.0).
Selected referrals & addition reading
Note: complimentary full message is made easily accessible by the publisher because that items significant with a eco-friendly asterisk.Journal articles
* Friedman, W.E., and K.C. Ryerson. 2009. Reconstructing the ancestral female gametophyte that angiosperms: insights from Amborella and other old lineages of flower plants. American newspaper of Botany 96: 129-143. Https://doi.org/10.3732/ajb.0800311
* Friedman, W.E., and J.H. Williams. 2003. Modularity that the angiosperm mrs gametophyte and its bearing ~ above the at an early stage evolution of endosperm in flower plants. Evolution 57: 216-230. Https://doi.org/10.1111/j.0014-3820.2003.tb00257.x
* Friedman, W.E., and also J.H. Williams. 2004. Developmental evolution of the sexual process in old flowering tree lineages. The Plant cell 16: S119-S132. Https://doi.org/10.1105/tpc.017277
Friedman, W.E., W.N. Gallup, and J.H. Williams. 2003. Mrs gametophyte development in Kadsura: implications for Schisandraceae, Austrobaileyales, and also the early evolution of flowering plants. International journal of tree Sciences 164: S293-S305. Https://doi.org/10.1086/376877
* Madrid, E.N., and W.E. Friedman. 2009. The developmental basis of an evolution diversification of mrs gametophyte framework in Piper and Piperaceae. Annals the Botany 103: 869-884. Https://doi.org/10.1093/aob/mcp011
* Madrid, E.N., and W.E. Friedman. 2010. Mrs gametophyte and also early seed advance in Peperomia (Piperaceae). American newspaper of Botany 97: 1-14. Https://doi.org/10.3732/ajb.0800423
* Rudall, P.J., M.V. Remizowa, A.S. Beer, E. Bradshaw, D.W. Stevenson, T.D. MacFarlane, R.E. Tuckett, S.R. Yadav, and D.D. Sokoloff. 2008. Compare ovule and also megagametophyte advance in Hydatellaceae and also water lilies disclose a mosaic that features among early angiosperms. Annals of Botany 101: 941-956. Https://doi.org/10.1093/aob/mcn032
* Schmid, M.W., A. Schmidt, U. Grossniklaus. 2015. The mrs gametophyte: an arising model for cell type-specific solution biology in tree development. Frontiers in plant Science. Https://doi.org/10.3389/fpls.2015.00907
Tobe, H., Y. Kimoto, and N. Prakash. 2007. Advancement and framework of the female gametophyte in Austrobaileya scandens (Austrobaileyaceae). Journal of plant Research 120: 431-436. Https://doi.org/10.1007/s10265-007-0085-0
Williams, J.H., and W.E. Friedman. 2002. To know of diploid endosperm in an early angiosperm lineage. Nature 415: 522-526. Https://doi.org/10.1038/415522a
* Williams, J.H., and W.E. Friedman. 2004.The four-celled mrs gametophyte of Illicium (Illiciaceae; Austrobaileyales): ramifications for knowledge the origin and also early development of monocots, eumagnoliids, and eudicots. American newspaper of Botany 91: 332-351. Https://doi.org/10.3732/ajb.91.3.332Books & textbooks
Bergen, J.Y., and also O.W. Caldwell. 1914. Arrival to Botany. Ginn and Company, Boston. Read online in ~ the web Archive.
Esau, K. 1977. Anatomy of particle plants, 2nd ed. Man Wiley & Sons, brand-new York.
Evert R.F., and S.E. Eichhorn. 2013. Crow Biology that Plants, 8 hours ed. W.H. Freeman and Co., brand-new York, brand-new York.
Foster, A.S., and also E.M. Gifford. 1974. Compare Morphology that Vascular Plants, 2nd ed. W.H. Freeman and also Co., mountain Francisco.
Maheshwari, P. 1950. An advent to the Embryology the Angiosperms. McGraw-Hill publication Company, Inc., brand-new York, Toronto, London. Check out online at the internet Archive.
Simpson, M.G. 2010. Tree Systematics, second ed. Scholastic Press, Burlington, Massachusetts.
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