Overview

The angiosperm life cycle, in numerous ways, complies with the an easy life cycle pattern for land plants (embryophytes), with changes characteristic of the seed plant habit (read an ext here). As in other seed plants, the microgametophyte (male, or sperm-producing gametophyte) is highly simplified and called a pollen grain.The megagametophyte (female, or egg-producing, gametophyte) develops within the ovule (immature seed). The pollen grain have to be released and also transported come the ovule-bearing structure before fertilization can occur. However, angiosperm pollen grains, megagametophytes, and fertilization different from those of various other seed tree in several important ways.

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Angiosperm pollen is developed in the anther, a floral structure that is typically made up of 2 pairs of fused microsporangia (microspore-producing sporangia, additionally called pollen sacs). Pollen grains of angiosperms are the many simplified in every one of seed plants. At maturity, every pollen grain has three cells: one tube cell (the cabinet that creates the pollen tube) and two sperm.


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Flowers reflecting ovaries and also anthers. In angiosperms, the ovules that contain the megagametophytes (female or egg-producing gametophytes) are enclosed in an ovary. The pollen grains are produced in microsporangia (pollen sacs) that are component 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.


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The pollen seed of angiosperms cannot land directly on the ovules due to the fact that the ovules room enclosed in a floral structure called an ovary (the ovary is shown in the figure at the peak of the page). Thus, the pollen seed land on a dedicated surface (the stigma), wherein they germinate. Angiosperms are siphonogamous (Greek, siphongamia = tube union), meaning that their sperm are not motile (i.e., absence flagella and cannot swim) and also are delivered to the egg by way of a pollen tube. The pollen pipe must prosper from the pollen serial on the stigma, down the style, and also into the ovary come make contact with an ovule.


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Lily flower and germinated pollen. Left: Close-up the lily (Lilium) flower showing open anthers and the stigma, the surface on i m sorry pollen will land. Right: Scanning electron micrograph (SEM image) the germinated lily (Lilium) pollen grains showing pollen tubes farming out of apertures (thin areas) in the pollen walls. Credit: Closeup that stamen and also stigma the Lilium (Subhrajyoti07, Wikimedia Commons, CC BY-SA 4.0); SEM micrograph the lily pollen pipe (Neutr0nics, Wikimedia Commons, CC BY-SA 3.0). Pictures modified native originals.


Unlike gymnosperm ovules, angiosperm ovules frequently have a double integument, or two unique integuments that surround the nucellus (megasporangium, the megaspore-producing sporangium) in which the megagametophyte develops. Angiosperm ovules are regularly folded over on themselves. The ovule is attached come the inner ovary wall by a stalk dubbed a funiculus or funicle.

The megagametophytes (female or egg-producing gametophytes) the angiosperms are also 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 they may have as few as 4 cells. One egg cell is current per megagametophyte, and also there is no archegonium. (The archegonium is a multicellular framework that surrounding the egg cell in many other tree groups).


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Lily ovules. Left: Cross ar of the ovary of a lily (Lilium) ovary, with 6 ovules; the ovules are indicated by the arrowheads. Right: Longitudinal ar of a solitary ovule v embryo sac (megagametophyte). Credit: Lilium ovary and 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 developed, pollination has occurred, and also the pollen tube has grown into the ovary come make call with the ovule, fertilization (fusion the egg and sperm) deserve to occur. Typically, the pollen pipe reaches the embryo sac via the micropyle (Greek, mikros pyle = little opening), or opening, in the integuments the the ovule. There, it discharges its sperm right into the embryo sac.


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One that the vital features the distinguishes angiosperms native all other seed plants is double fertilization through endosperm formation. Double fertilization occurs in siphonogamous seed tree (i.e., seed plants with nonmotile sperm) as soon as both sperm in a pollen tube unite with frameworks in a megagametophyte. In angiosperms, one sperm unites v the egg to form a diploid zygote, the an initial cell of a new sporophyte. The other sperm unites through the cell core or nuclei in the large central cell of the embryo sac to kind a primary endosperm nucleus. This cell nucleus is the first nucleus the the endosperm (Greek, endon sperma = in ~ seed), a form of food tissue unique to the seed of angiosperms.

After fertilization, the ovule becomes a seed. The seed is a structure containing a young, diploid sporophyte embryo and, typically, stored food for the embryo. In angiosperms, the food in the seed might be save in the form of endosperm, it might be stored in the cotyledon(s) (seed leaves) that the embryo, or it might be save on computer in both structures. The particle is safeguarded by a seed coat, which establishes from the integuments of the ovule.


Longitudinal sections of seeds through sporophyte embryos. Left: bean (Phaseolus) separation lengthwise to show the parts of the embryo, including the two food-storing cotyledons, the hypocotyl-root axis (sporophyte embryo axis listed below the cotyledons), and also the very first foliage leaves. No endosperm is apparent. Right: Corn (Zea mays, a monocot) embryo with one cotyledon and also conspicuous endosperm. Credits: Phaseolus particle (Bruce Krichoff, via flickr, CC by 2.0); Zea kernel (Jon Houseman and Matthew Ford, Wikimedia Commons, CC BY-SA 4.0). Pictures modified from originals.


Generalized angiosperm life cycle. Summary diagram for the life bicycle of one angiosperm. The diploid (2n), multicell sporophyte bear flowers. Embryo bag (megagametophytes) construct in the ovules, which are found in the ovary. Pollen seed (microgametophytes) develop in the pollen bag (microsporangia) that the anther. One megaspore mother cell wake up in every ovule; it experience meiosis, frequently giving increase to one practical megaspore. Numerous microspore mommy cells happen in the pollen sacs; each undergoes meiosis to produce four microspores. Megaspores build into embryo sacs, microspores into pollen grains. The pollen grains are released native the anther come land top top a stigma throughout pollination. The pollen serial germinates, creating a pollen tube the delivers the sperm to the egg in the ovule throughout fertilization. The sporophyte embryo develops in the seed. Top top germination the the seed, the sporophyte resumes growth. Credit: diagram by E.J. Hermsen (DEAL).


Pollen serial (microgametophyte) development

In flowering plants, pollen is created in the anthers. Anthers space structures commonly made increase of four fused pollen sacs, or microsporangia. Microspore mother cells (also referred to as pollen mommy cells) differentiate within the pollen bag (microsporangia) of the anthers. Every microspore mommy cell experience meiosis to create four haploid microspores. Each microspore then divides to produce its very own pollen grain. Many pollen seed may thus be produced in a solitary anther.

Each microspore divides double to produce the mature, three-celled pollen grain. The first department produces a tube cabinet (cell that will elongate to type the pollen tube) and also a generative cell. The generative cell climate divides to produce two sperm. Adhering to pollination, the pollen grain germinates and also the pipe cell starts elongating to form the pollen tube.


Pollen serial development. Development the the pollen serial from diploid (2n) microspore mother cell come 3-celled microgametophyte. Each micropore mother cell in a pollen sac undergoes meiosis to produce four haploid (n) microspores. Every microspore divides once to develop a 2-celled pollen grain. The generative cell divides again come yield 2 sperm. The tube cell will form the pollen tube the delivers the sperm adhering to pollination. Credit: diagram by E.J. Hermsen (DEAL), modified after ~ Foster & Gifford (1974).



Embryo sac (megagametophyte) development

In angiosperms, the megagametophyte (female or egg-producing gametophyte) is likewise called an embryo sac. The embryo sac develops within an ovule, i beg your pardon is contained within the ovary the a flower. In most angiosperms, the maturation embryo sac is a seven-celled, eight-nucleate structure. This kind of embryo sac is also called the Polygonum-type, after ~ the genus Polygonum (knotweed or smartweed). Many angiosperms (about 70%) are thought to have this form of embryo sac. The ax "Polygonum-type" does no refer just to the final configuration that the embryo sac, but additionally to its succession of development.

An necessary note around the diagrams in this section: The standard method to display angiosperm ovules is through the micropylar end (the finish of the ovule with the micropyle or opened in the integuments) at the bottom. This an option makes sense structurally since the micropyle faces the base of the ovule in plenty of angiosperms (an ovule orientation well-known as anatropous). However, when the angiosperm embryo sac and also its breakthrough are shown in isolation (without the bordering nucellus and also integuments), they are often portrayed with the micropyle at the top of the diagram.

This convention deserve to be confound when discussing embryo sac development, as it method whole-ovule and embryo-sac-only diagrams space rotated 180 levels from each other. Thus, I have actually made the selection to orient the diagrams listed below so the the micropyle is always at the top of the diagram. Remember: No issue the orientation the the ovule or the sequence of embryo sac development, the egg cabinet (and connected synergids) is always at the micropylar finish of the ovule.

Development that the Polygonum-type embryo sac

Embryo sac advancement begins v the production of megaspores. A large megaspore mother cell differentiates in ~ the occurring ovule. This megaspore mother cell experience meiosis to create four haploid megaspores. One megaspore (the one furthest indigenous the micropyle) is functional, when the rest degenerate. Thus, the Polygonum-type embryo sac exhibits monosporic (Greek, monos = one) development, meaning that it develops from a single spore.

The practical megaspore undergoes a series of free-nuclear divisions, or divisions of the nucleus there is no partitioning that the cytoplasm into separate cells. One cell nucleus divides mitotically to develop two nuclei, two nuclei divide to produce four, and also four divide to produce eight.


Development of the Polygonum-type embryo sac. A megaspore mother cell differentiates within the nucellus of one ovule. It experience meiosis to yield a tetrad (group of four) the megaspores. 3 of these degenerate, and the megaspore furthest from the micropyle is functional. This megaspore experience free-nuclear divisions, or divisions of the nucleus without the partitioning the cells. The megaspore cell nucleus divides when to create two nuclei. Every of this nuclei divides again to produce four nuclei. Each of these nuclei divides again to develop eight nuclei. The final step in development of the embryo sac will be partitioning of the cell (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 into seven different cells. At the end of the ovule close to the micropyle are an egg cell and two synergids. At the opposite end of the embryo sac are three cells dubbed antipodals. Finally, the huge central cell includes two polar nuclei. In the Polygonum-type embryo sac, every the nuclei space haploid, or have actually one collection of chromosomes.


Ovule v Polygonum-type embryo sac. Idealized diagram of one ovule with a Polygonum-type embryo sac, showing the 7 cells and eight nuclei surrounding by a thin nucellus (megasporangium) and double integuments. Keep in mind that the antipodals may failure and the polar nuclei may fuse to type a diploid nucleus before fertilization. The nucellus may additionally break down, so might not be observed in mature ovules. Credit: E.J. Hermsen (DEAL).


Double fertilization of the Polygonum-type embryo sac

As stated above, angiosperms have double fertilization. One sperm fuses with the egg to do a diploid zygote, the solitary cell that through division, growth, and advancement will ultimately yield a sporophyte embryo. The various other sperm fuses with the polar nuclei to kind a primary endosperm nucleus. In the Polygonum-type embryo sac, the major endosperm cell core is triploid, an interpretation that it has three set of chromosomes. One collection comes indigenous the sperm and two sets come native the polar nuclei. The major endosperm cell core will start 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 of the ovule: the antipodals, synergids, nucellus, and also integuments? The antipodals degenerate, either prior to or adhering to fertilization. They have no clear function in the embryo sac. The synergids room thought come play a duty in fertilization, back these additionally degenerate. Typically, the nucellus degenerates before maturation that the seed. The integument or integuments come to be the seed coat.


Polygonum-type embryo sac after double fertilization. Idealized diagram showing the Polygonum-type embryo sac following double fertilization. During double fertilization, the egg and also a sperm hold together to kind a diploid zygote. The various other sperm unites v the polar nuclei to form a triploid primary endosperm nucleus. The antipodals degenerate. The synergids beat a duty in fertilization, but also degenerate. Credit: diagram by E.J. Hermsen (DEAL).


Although the Polygonum-type embryo sac is the most common form of embryo sac in angiosperms, it is not believed to be the genealogical type. Rather, research studies on Amborella and various other basal angiosperms in the Austrobaileyales and Nymphaeales have burned light on the beginning of the Polygonum-type embryo sac and also the advancement of other species of embryo bag in angiosperms.

Ancestral organization of the angiosperm embryo sac

The Nuphar/Schisandra-type of embryo sac is believed to be the ancestral type of embryo sac for crown-group angiosperms, or every living angiosperms and their many recent typical ancestor (see here, here, here, and also here). This kind of embryo sac is found in members of Austrobaileyales and Nymphaeales, two orders the basal or ANA-grade angiosperms. Nuphar (Nymphaeales) is a type of water lily and Schisandra (Austrobaileyales) is a form of shrub native to the southeastern unified States, Mexico, and eastern to southeastern Asia.


Nuphar and Schisandra flowers. Left: Yellow pond-lily (Nuphar lutea, Nymphaeales). Right: bay 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, public Domain). Pictures modified indigenous originals.


TheNuphar/Schisandra-type embryo sac begins breakthrough in a manner comparable to thePolygonum-type embryo sac. It creates from a single megaspore that undergoes 2 sets that free-nuclear divisions, to produce a 4-nucleate stage. Cellular partitioning then results in a 4-celled, 4-nucleate embryo sac. The embryo sac has actually an egg, 2 synergids, and a polar cell core in a central cell. No antipodals are formed. Double fertilization in this kind of embryo sac returns a diploid zygote and also a diploid main endosperm cell nucleus (one collection of chromosomes indigenous a sperm + one set from the polar cell core = 2 sets the chromosomes in the primary endosperm nucleus).


The Nuphar/Schisandra-type embryo sac. Development of the Nuphar/Schisandra-type embryo sac is comparable to advance of the Polygonum-type embryo sac. Differences include the nuclei are situated only in ~ the micropylar end and partitioning of cell happens ~ the 4-nucleate stage. The mature embryo sac has an egg cell, two synergids, and also a solitary polar nucleus. Credit: diagram by E.J. Hermsen (DEAL), modified ~ Friedman & Ryerson (2009) and other documents on megagametogenesis in ANA-grade angiosperms (see references).


Modularity the the embryo sac

Research on breakthrough of the megagametophyte in ANA-grade angiosperms suggests that the 4 cells the the Nuphar/Schisandra-type embryo sac may act together a module or simple building block ~ above which other varieties of angiosperm embryo sacs room based (see here, here, here, and also here). Thus, the Polygonum-type embryo sac may have evolved from the Nuphar/Schisandra-type via a duplication of the straightforward 4-celled, 4-nucleate unit that egg cell + 2 synergids + 1 polar nucleus in the central cell. The antipodals plus among the polar nuclei in the central cell the the Polygonum-type embryo sac have the right to thus be viewed merely as a copy of this an easy unit in ~ the opposite finish of the embryo sac.

Interestingly, Amborella, the basalmost living angiosperm, walk not have a four-celled, four-nucleate embryo sac. Rather, it has an eight-celled, nine-nucleate embryo sac, which looks like a Polygonum-type embryo sac with one additional synergid. It is thought that the Amborella-type embryo sac developed independently indigenous the Nuphar/Schisandra-type embryo sac and also not from the Polygonum-type embryo sac (see here).


Comparison that embryo sacs. Left: Amborella-type embryo sac with an egg (E), three synergids (S), three antipodals (A), and also two polar nuclei (black dots). Center: Polygonum-type embryo sac with an egg, two synergids, three antipodals, and two polar nuclei. Right: Nuphar/Schisandra-type through an egg, 2 synergids, and also one polar nucleus. Every embryo sacs are oriented with the micropylar finish up. Credit: diagram by E.J. Hermsen (DEAL), modified after Friedman & Ryerson (2009) and also other files on megagametogenesis in ANA-grade angiosperms (see references).


Hypothesized development of embryo bag mapped unto a phylogeny. Evolution that angiosperm embryo bag under the modular theory, as mapped depend a simplified tree the angiosperm relationships. The Nuphar/Schisandra-type (4-celled, 4-nucleate) is a synapomorphy for crown-group angiosperms. The Amborella-type (8-celled, 9-nucleate) progressed only in the Amborella lineage. The Polygonum-type (7-celled, 8-nucleate) is a synapomorphy because that the clade consisting of all living angiosperms over the ANA-grade (i.e., Amborella, Nymphaeales, and also Austrobaileyales). Credit: chart by E.J. Hermsen (DEAL), modified after Friedman & Ryerson (2009) and other files on megagametogenesis in ANA-grade angiosperms (see references).


Other embryo sacs

In addition to Polygonum-, Nuphar/Schisandra-, and Amborella-type embryo sacs, other types of embryo sacs have evolved in other groups of angiosperms. We will certainly not testimonial them all 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 arises from one megaspore, and the other three megaspores degenerate. Examples: The three species of embryo sacs detailed above (Polygonum, Nuphar/Schisandra, and Amborella) space monosporic.Bisporic: In this type of development, a single embryo sac occurs from two of the 4 megaspore nuclei developed by meiosis of the megaspore mother cell. The other two nuclei degenerate.

Some sport in angiosperm embryo sac development. chart (after Maheshwari 1950) showing major variations in the advancement of the embryo sac (megagametophyte or mrs gametophyte) the angiosperms. The micropyle is a the peak of the image in every diagrams. Megasporogenesis is the procedure that produce a tetrad (group that four) megaspores native the megaspore mother cell (MMC) through department by meiosis. FMS = sensible megaspore. Megagametogenesis is the development of the woman gametophyte (embryo sac) with mitotic departments of the nuclei, cabinet partitioning, and differentiation (specialization) of cells. In the tires embryo sac, the egg cell is the big pink cell at the apex; the synergids room the greenish cells connected with the egg. Credit: number 2 from Schmid et al. (2015) Frontiers in plant Science (CC by 4.0).


Selected referrals & enhancement reading

Note: complimentary full message is made accessible by the publisher for items significant with a environment-friendly asterisk.

Journal articles

* Friedman, W.E., and K.C. Ryerson. 2009. Reconstructing the genealogical female gametophyte of angiosperms: insights from Amborella and also other ancient lineages of flower plants. American journal of Botany 96: 129-143. Https://doi.org/10.3732/ajb.0800311

* Friedman, W.E., and also J.H. Williams. 2003. Modularity of the angiosperm mrs gametophyte and its bearing top top the early evolution the endosperm in flower plants. Evolution 57: 216-230. Https://doi.org/10.1111/j.0014-3820.2003.tb00257.x

* Friedman, W.E., and J.H. Williams. 2004. Developmental evolution of the sexual process in old flowering plant lineages. The Plant cabinet 16: S119-S132. Https://doi.org/10.1105/tpc.017277

Friedman, W.E., W.N. Gallup, and also J.H. Williams. 2003. Female gametophyte advancement in Kadsura: ramifications for Schisandraceae, Austrobaileyales, and also the beforehand evolution of flowering plants. International newspaper of plant Sciences 164: S293-S305. Https://doi.org/10.1086/376877

* Madrid, E.N., and also W.E. Friedman. 2009. The developmental basis of an evolutionary diversification of mrs gametophyte structure in Piper and also Piperaceae. Annals of Botany 103: 869-884. Https://doi.org/10.1093/aob/mcp011

* Madrid, E.N., and W.E. Friedman. 2010. Woman gametophyte and early seed advance in Peperomia (Piperaceae). American journal 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 also D.D. Sokoloff. 2008. Comparative ovule and also megagametophyte development in Hydatellaceae and water lilies expose a mosaic the features amongst early angiosperms. Annals of Botany 101: 941-956. Https://doi.org/10.1093/aob/mcn032

* Schmid, M.W., A. Schmidt, U. Grossniklaus. 2015. The female gametophyte: an emerging model for cell type-specific systems biology in plant development. Frontiers in tree Science. Https://doi.org/10.3389/fpls.2015.00907

Tobe, H., Y. Kimoto, and also N. Prakash. 2007. Breakthrough and structure of the mrs gametophyte in Austrobaileya scandens (Austrobaileyaceae). Journal of tree Research 120: 431-436. Https://doi.org/10.1007/s10265-007-0085-0

Williams, J.H., and also W.E. Friedman. 2002. To know of diploid endosperm in very early angiosperm lineage. Nature 415: 522-526. Https://doi.org/10.1038/415522a

* Williams, J.H., and also W.E. Friedman. 2004.The four-celled woman gametophyte of Illicium (Illiciaceae; Austrobaileyales): ramifications for expertise the origin and early evolution of monocots, eumagnoliids, and also eudicots. American journal of Botany 91: 332-351. Https://doi.org/10.3732/ajb.91.3.332

Books & textbooks

Bergen, J.Y., and also O.W. Caldwell. 1914. Advent to Botany. Ginn and also Company, Boston. Review online at the internet Archive.

Esau, K. 1977. Anatomy of particle plants, second ed. Man Wiley & Sons, brand-new York.

Evert R.F., and also S.E. Eichhorn. 2013. Crow Biology that Plants, 8th ed. W.H. Freeman and also Co., brand-new York, brand-new York.

Foster, A.S., and also E.M. Gifford. 1974. To compare Morphology of Vascular Plants, 2nd ed. W.H. Freeman and also Co., mountain Francisco.

Maheshwari, P. 1950. An arrival to the Embryology of Angiosperms. McGraw-Hill publication Company, Inc., new York, Toronto, London. Check out online in ~ the internet Archive.

Simpson, M.G. 2010. Tree Systematics, second ed. Academic Press, Burlington, Massachusetts.

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Usage that text and images created for DEAL: Text top top this web page was created by Elizabeth J. Hermsen. Initial written content developed by E.J. Hermsen because that the Digital Encyclopedia of ancient Life that shows up on this web page is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 worldwide License. Original images and diagrams developed by E.J. Hermsen are likewise licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 international License.