Double fertilization

Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms). This process involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm). It begins when a pollen grain adheres to the stigma of the carpel, the female reproductive structure of a flower. The pollen grain then takes in moisture and begins to germinate, forming a pollen tube that extends down toward the ovary through the style. The tip of the pollen tube then enters the ovary and penetrates through the micropyle opening in the ovule. The pollen tube proceeds to release the two sperm in the megagametophyte.

The cells of an unfertilized ovule are 8 in number and arranged in the form of 3+2+3 (from top to bottom) i.e. 3 antipodal cells, 2 polar central cells, 2 synergids & 1 egg cell. One sperm fertilizes the egg cell and the other sperm combines with the two polar nuclei of the large central cell of the megagametophyte. The haploid sperm and haploid egg combine to form a diploid zygote,the process being called syngamy, while the other sperm and the two haploid polar nuclei of the large central cell of the megagametophyte form a triploid nucleus (triple fusion). Some plants may form polyploid nuclei. The large cell of the gametophyte will then develop into the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds and may function to disperse them.[1]

The two central cell maternal nuclei (polar nuclei) that contribute to the endosperm, arise by mitosis from the same single meiotic product that gave rise to the egg. The maternal contribution to the genetic constitution of the triploid endosperm is double that of the embryo.

In a study conducted in 2008 of the plant Arabidopsis thaliana, the migration of male nuclei inside the female gamete, in fusion with the female nuclei, has been documented for the first time using in vivo imaging. Some of the genes involved in the migration and fusion process have also been determined.[2]

Evidence of double fertilization in Gnetales, which are non-flowering seed plants, has been reported.[3]

Mature flower diagram
The parts of a flower
Double Fertilization
Double fertilization
Double fertilization in arabidopsis 2
Double fertilization in Arabidopsis

Brief history

Double fertilization was discovered more than a century ago by Sergei Nawaschin and Grignard in Kiev,[4] Russian Empire, and Léon Guignard in France. Each made the discovery independently of the other.[5] Lilium martagon and Fritillaria tenella were used in the first observations of double fertilization, which were made using the classical light microscope. Due to the limitations of the light microscope, there were many unanswered questions regarding the process of double fertilization. However, with the development of the electron microscope, many of the questions were answered. Most notably, the observations made by the group of W. Jensen showed that the male gametes did not have any cell walls and that the plasma membrane of the gametes is close to the plasma membrane of the cell that surrounds them inside the pollen grain.[6]

Double Fertilization in Gymnosperms

A far more rudimentary form of double fertilization occurs in the sexual reproduction of an order of gymnosperms commonly known as Gnetales.[3] Specifically, this event has been documented in both Ephedra and Gnetum, a subset of Gnetophytes.[7] In Ephedra nevadensis, a single binucleate sperm cell is deposited into the egg cell. Following the initial fertilization event, the second sperm nucleus is diverted to fertilize an additional egg nucleus found in the egg cytoplasm. In most other seed plants, this second 'ventral canal nucleus' is normally found to be functionally useless.[8] In Gnetum gnemon, numerous free egg nuclei exist in female cytoplasm inside the female gametophyte. Succeeding the penetration of the mature female gametophyte by the pollen tube, female cytoplasm and free nuclei move to surround the pollen tube. Released from the binucleate sperm cell are two sperm nuclei which then join with free egg nuclei to produce two viable zygotes, a homologous characteristic between families Ephedra and Gnetum.[9] In both families, the second fertilization event produces an additional diploid embryo. This supernumerary embryo is later aborted, leading to the synthesis of only one mature embryo.[10] The additional fertilization product in Ephedra does not nourish the primary embryo, as the female gametophyte is responsible for nutrient provision.[9] The more primitive process of double fertilization in gymnosperms results in two diploid nuclei enclosed in the same egg cell. This differs from the angiosperm condition, which results in the separation of the egg cell and endosperm.[11] Comparative molecular research on the genome of G. gnemon has revealed that gnetophytes are more closely related to conifers than they are to angiosperms.[12][13][14] The rejection of the anthophyte hypothesis, which identifies gnetales and angiosperms are sister taxa, leads to speculation that the process of double fertilization is a product of convergent evolution and arose independently among gnetophytes and angiosperms.[15]

In vitro double fertilization

In vitro double fertilization is often used to study the molecular interactions as well as other aspects of gamete fusion in flowering plants. One of the major obstacles in developing an in vitro double fertilization between male and female gametes is the confinement of the sperm in the pollen tube and the egg in the embryo sac. A controlled fusion of the egg and sperm has already been achieved with poppy plants.[16] Pollen germination, pollen tube entry, and double fertilization processes have all been observed to proceed normally. In fact, this technique has already been used to obtain seeds in various flowering plants and was named “test-tube fertilization”.[17]

Related structures and functions

Megagametophyte

The female gametophyte, the megagametophyte, that participates in double fertilization in angiosperms which is haploid is called the embryo sac. This develops within an ovule, enclosed by the ovary at the base of a carpel. Surrounding the megagametophyte are (one or) two integuments, which form an opening called the micropyle. The megagametophyte, which is usually haploid, originates from the (usually diploid) megaspore mother cell, also called the megasporocyte. The next sequence of events varies, depending on the particular species, but in most species, the following events occur. The megasporocyte undergoes a meiotic cell division, producing four haploid megaspores. Only one of the four resulting megaspores survives. This megaspore undergoes three rounds of mitotic division, resulting in seven cells with eight haploid nuclei (the central cell has two nuclei, called the polar nuclei). The lower end of the embryo sac consists of the haploid egg cell positioned in the middle of two other haploid cells, called synergids. The synergids function in the attraction and guidance of the pollen tube to the megagametophyte through the micropyle. At the upper end of the megagametophyte are three antipodal cells.

Microgametophyte

The male gametophytes, or microgametophytes, that participate in double fertilization are contained within pollen grains. They develop within the microsporangia, or pollen sacs, of the anthers on the stamens. Each microsporangium contains diploid microspore mother cells, or microsporocytes. Each microsporocyte undergoes meiosis, forming four haploid microspores, each of which can eventually develop into a pollen grain. A microspore undergoes mitosis and cytokinesis in order to produce two separate cells, the generative cell and the tube cell. These two cells in addition to the spore wall make up an immature pollen grain. As the male gametophyte matures, the generative cell passes into the tube cell, and the generative cell undergoes mitosis, producing two sperm cells. Once the pollen grain has matured, the anthers break open, releasing the pollen. The pollen is carried to the pistil of another flower, by wind or animal pollinators, and deposited on the stigma. As the pollen grain germinates, the tube cell produces the pollen tube, which elongates and extends down the long style of the carpel and into the ovary, where its sperm cells are released in the megagametophyte. Double fertilization proceeds from here.[18]

See also

References

  1. ^ Berger, F. (January 2008). "Double-fertilization, from myths to reality". Sexual Plant Reproduction. 21 (1): 3–5. doi:10.1007/s00497-007-0066-4.
  2. ^ Berger, F.; Hamamura, Y. & Ingouff, M. & Higashiyama, T. (August 2008). "Double fertilization – Caught In The Act". Trends in Plant Science. 13 (8): 437–443. doi:10.1016/j.tplants.2008.05.011. PMID 18650119.CS1 maint: Multiple names: authors list (link)
  3. ^ a b V. Raghavan (September 2003). "Some reflections on double fertilization, from its discovery to the present". New Phytologist. 159 (3): 565–583. doi:10.1046/j.1469-8137.2003.00846.x.
  4. ^ Kordium EL (2008). "[Double fertilization in flowering plants: 1898-2008]". Tsitol. Genet. (in Russian). 42 (3): 12–26. PMID 18822860.
  5. ^ Jensen, W. A. (February 1998). "Double Fertilization: A Personal View". Sexual Plant Reproduction. 11 (1): 1–5. doi:10.1007/s004970050113.
  6. ^ Dumas, C. & Rogowsky, P. (August 2008). "Fertilization and Early Seed Formation". Comptes Rendus Biologies. 331 (10): 715–725. doi:10.1016/j.crvi.2008.07.013. PMID 18926485.
  7. ^ Carmichael, J. S.; Friedman, W. E. (1995-12-01). "Double Fertilization in Gnetum gnemon: The Relationship between the Cell Cycle and Sexual Reproduction". The Plant Cell. 7 (12): 1975–1988. doi:10.1105/tpc.7.12.1975. ISSN 1040-4651. PMC 161055. PMID 12242365.
  8. ^ Friedman, William E. (1990). "Sexual Reproduction in Ephedra nevadensis (Ephedraceae): Further Evidence of Double Fertilization in a Nonflowering Seed Plant". American Journal of Botany. 77 (12): 1582–1598. doi:10.2307/2444491. JSTOR 2444491.
  9. ^ a b Carmichael, Jeffrey S.; Friedman, William E. (1996). "Double Fertilization in Gnetum gnemon (Gnetaceae): Its Bearing on the Evolution of Sexual Reproduction within the Gnetales and the Anthophyte Clade". American Journal of Botany. 83 (6): 767–780. doi:10.2307/2445854. JSTOR 2445854.
  10. ^ Friedman, W. E. (1995-04-25). "Organismal duplication, inclusive fitness theory, and altruism: understanding the evolution of endosperm and the angiosperm reproductive syndrome". Proceedings of the National Academy of Sciences. 92 (9): 3913–3917. doi:10.1073/pnas.92.9.3913. ISSN 0027-8424. PMC 42072. PMID 11607532.
  11. ^ Friedman, William E. (1994). "The Evolution of Embryogeny in Seed Plants and the Developmental Origin and Early History of Endosperm". American Journal of Botany. 81 (11): 1468–1486. doi:10.2307/2445320. JSTOR 2445320.
  12. ^ Bowe, L. Michelle; Coat, Gwénaële; dePamphilis, Claude W. (2000-04-11). "Phylogeny of seed plants based on all three genomic compartments: Extant gymnosperms are monophyletic and Gnetales' closest relatives are conifers". Proceedings of the National Academy of Sciences. 97 (8): 4092–4097. doi:10.1073/pnas.97.8.4092. ISSN 0027-8424. PMC 18159. PMID 10760278.
  13. ^ Winter, Kai-Uwe; Becker, Annette; Münster, Thomas; Kim, Jan T.; Saedler, Heinz; Theissen, Günter (1999-06-22). "MADS-box genes reveal that gnetophytes are more closely related to conifers than to flowering plants". Proceedings of the National Academy of Sciences. 96 (13): 7342–7347. doi:10.1073/pnas.96.13.7342. ISSN 0027-8424. PMC 22087. PMID 10377416.
  14. ^ Magallon, S.; Sanderson, M. J. (2002-12-01). "Relationships among seed plants inferred from highly conserved genes: sorting conflicting phylogenetic signals among ancient lineages". American Journal of Botany. 89 (12): 1991–2006. doi:10.3732/ajb.89.12.1991. ISSN 1537-2197. PMID 21665628.
  15. ^ Chaw, Shu-Miaw; Parkinson, Christopher L.; Cheng, Yuchang; Vincent, Thomas M.; Palmer, Jeffrey D. (2000-04-11). "Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from conifers". Proceedings of the National Academy of Sciences. 97 (8): 4086–4091. doi:10.1073/pnas.97.8.4086. ISSN 0027-8424. PMC 18157. PMID 10760277.
  16. ^ Zenkteler, M. (1990). "In vitro fertilization and wide hybridization in higher plants". Crit Rev Plant Sci. 9 (3): 267–279. doi:10.1080/07352689009382290.
  17. ^ Raghavan, V. (2005). Double fertilization: embryo and endosperm development in flowering plants (illustrated ed.). Birkhäuser. pp. 17–19. ISBN 978-3-540-27791-0.
  18. ^ Campbell N.A; Reece J.B (2005). Biology (7 ed.). San Francisco, CA: Pearson Education, Inc. pp. 774–777. ISBN 978-0-8053-7171-0.
Alternation of generations

Alternation of generations (also known as metagenesis) is the type of life cycle that occurs in those plants and algae in the Archaeplastida and the Heterokontophyta that have distinct haploid sexual and diploid asexual stages. In these groups, a multicellular gametophyte, which is haploid with n chromosomes, alternates with a multicellular sporophyte, which is diploid with 2n chromosomes, made up of n pairs. A mature sporophyte produces spores by meiosis, a process which reduces the number of chromosomes to half, from 2n to n.

The haploid spores germinate and grow into a haploid gametophyte. At maturity, the gametophyte produces gametes by mitosis, which does not alter the number of chromosomes. Two gametes (originating from different organisms of the same species or from the same organism) fuse to produce a zygote, which develops into a diploid sporophyte. This cycle, from gametophyte to gametophyte (or equally from sporophyte to sporophyte), is the way in which all land plants and many algae undergo sexual reproduction.

The relationship between the sporophyte and gametophyte varies among different groups of plants. In those algae which have alternation of generations, the sporophyte and gametophyte are separate independent organisms, which may or may not have a similar appearance. In liverworts, mosses and hornworts, the sporophyte is less well developed than the gametophyte and is largely dependent on it. Although moss and hornwort sporophytes can photosynthesise, they require additional photosynthate from the gametophyte to sustain growth and spore development and depend on it for supply of water, mineral nutrients and nitrogen. By contrast, in all modern vascular plants the gametophyte is less well developed than the sporophyte, although their Devonian ancestors had gametophytes and sporophytes of approximately equivalent complexity. In ferns the gametophyte is a small flattened autotrophic prothallus on which the young sporophyte is briefly dependent for its nutrition. In flowering plants, the reduction of the gametophyte is much more extreme; it consists of just a few cells which grow entirely inside the sporophyte.

Animals develop differently. They directly produce haploid gametes. No haploid spores capable of dividing are produced, so they do not have a haploid gametophyte alternating with a diploid sporophyte. (Some insects have a sex-determining system whereby haploid males are produced from unfertilized eggs; however the females are diploid.)

Life cycles of plants and algae with alternating haploid and diploid multicellular stages are referred to as diplohaplontic (the equivalent terms haplodiplontic, diplobiontic or dibiontic are also in use). Life cycles, such as those of animals, in which there is only a diploid multicellular stage are referred to as diplontic. Life cycles in which there is only a haploid multicellular stage are referred to as haplontic.

Author citation (botany)

In botanical nomenclature, author citation refers to citing the person or group of people who validly published a botanical name, i.e. who first published the name while fulfilling the formal requirements as specified by the International Code of Nomenclature for algae, fungi, and plants (ICN). In cases where a species is no longer in its original generic placement (i.e. a new combination of genus and specific epithet), both the author(s) of the original genus placement and those of the new combination are given (the former in parentheses).

In botany, it is customary (though not obligatory) to abbreviate author names according to a recognised list of standard abbreviations.

There are differences between the botanical code and the normal practice in zoology. In zoology, the publication year is given following the author name(s) and the authorship of a new combination is normally omitted. A small number of more specialized practices also vary between the recommendations of the botanical and zoological codes.

Endosperm

The endosperm is a tissue produced inside the seeds of most of the flowering plants following fertilization. It is triploid in most species. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This can make endosperm a source of nutrition in animal diet. For example, wheat endosperm is ground into flour for bread (the rest of the grain is included as well in whole wheat flour), while barley endosperm is the main source of sugars for beer production. Other examples of endosperm that forms the bulk of the edible portion are coconut "meat" and coconut "water", and corn. Some plants, such as orchids, lack endosperm in their seeds.

Fruit

In botany, a fruit is the seed-bearing structure in flowering plants (also known as angiosperms) formed from the ovary after flowering.

Fruits are the means by which angiosperms disseminate seeds. Edible fruits, in particular, have propagated with the movements of humans and animals in a symbiotic relationship as a means for seed dispersal and nutrition; in fact, humans and many animals have become dependent on fruits as a source of food. Accordingly, fruits account for a substantial fraction of the world's agricultural output, and some (such as the apple and the pomegranate) have acquired extensive cultural and symbolic meanings.

In common language usage, "fruit" normally means the fleshy seed-associated structures of a plant that are sweet or sour, and edible in the raw state, such as apples, bananas, grapes, lemons, oranges, and strawberries. On the other hand, in botanical usage, "fruit" includes many structures that are not commonly called "fruits", such as bean pods, corn kernels, tomatoes, and wheat grains. The section of a fungus that produces spores is also called a fruiting body.

Grex (horticulture)

The term grex (pl. greges or grexes; abbreviation gx), derived from the Latin noun grex, gregis meaning 'flock', has been coined to expand botanical nomenclature to describe hybrids of orchids, based solely on their parentage. Grex names are one of the three categories of plant names governed by the International Code of Nomenclature for Cultivated Plants; within a grex the cultivar group category can be used to refer to plants by their shared characteristics (rather than by their parentage), and individual orchid plants can be selected (and propagated) and named as cultivars.

Guranda Gvaladze

Guranda Gvaladze (In Georgian გურანდა ღვალაძე, born June 23, 1932, in Tbilisi, Georgia) ia a notable Georgian botanist, one of the founders of Plant Embryology in Georgia, Academician of the Abkhazian Regional Academy of Sciences (1997), Doctor of Biological Sciences (1974), Professor (1991). Her father Evgen Gvaladze (1900-1937) was a notable Lawyer and Publicist, one of the leaders of the National-Liberation Movement of Georgia of 1921-1937.

She graduated the Biological Faculty of the Tbilisi State University (1956). In 1956-1959 she was a Post-Graduate Student of the Institute of Botany of the Georgian Academy of Sciences. 1959-1983 Research Fellow (1959-1966) and Senior Research Fellow (1966-1983) of the Institute of Botany. 1983-1990 Head of the Department of Cultural Plants, 1990-2003 Head of the Department of Plant Reproduction at the Ketskhoveli Institute of Botany. 2003-2010 Chief Research Fellow of this Department. Since 2010 to present Gvaladze is the Emeritus Scientist of the institute of Botany. In 1962 She received the PhD degree in Biology, in 1974 the degree of Doctor of Biological Sciences (Full Doctor). In 1991 She received the scientific title of Professor.

Gvaladze is author of more than 180 scientific-research publications (among them 2 monographs and 1 manual) in the fields of Embryology of Flowering Plants, Double Fertilization, Apomixis, Ultrastructural research of Embryo Sac, etc. She is author of the Hypothesis about the stimulatory role of the Chalazal Polar Nucleus of the Central Cell of Angiosperm as compared to the Embryo in the preferential development of the Endosperm (1973-1974). Gvaladze is author of the 1st Manual "Reproduction of Plants" in Georgian (2008).

In the 1970s-1990s Gvaladze was active participant of the International Symposiums on Plant Embryology in France, India, Slovakia, Poland, Russia, etc. In 1984 She was a main organizer of such conference in Telavi (Georgia), in 1990 one of the organizers of the IX International Symposium on Plant Embryology (St.Petersburg, Russia).

Gvaladze is a member of the Board of the Georgian Botanical Society (since 1981), member of the International Association of Sexual Plant Reproduction Research (IASPRR, The Netherlands-Canada, since 1990. Gvaladze is one of the founding members of IASPRR), member of the International Organization for Plant Information (IOPI, USA, since 1999), member of EuroScience - the European Association for Promotion of Science and Technology (France, since 1998), Academician of the Abkhazian Regional Academy of Sciences (Tbilisi, since 1997), member of the Editorial Board of the "Bulletin of the Georgian Botanical Society". In 1983-2010 Gvaladze was a member of the Scientific Council of the Ketskhoveli Institute of Botany. In 1998 She was one of the founders of the Georgian National Section of EuroScience. In 1990 She received the International S.Navashin Medal "for outstanding contribution in Plant Embryology".

Gynoecium

Gynoecium (, from Ancient Greek γυνή, gyne, meaning woman, and οἶκος, oikos, meaning house) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) pistils and is typically surrounded by the pollen-producing reproductive organs, the stamens, collectively called the androecium. The gynoecium is often referred to as the "female" portion of the flower, although rather than directly producing female gametes (i.e. egg cells), the gynoecium produces megaspores, each of which develops into a female gametophyte which then produces egg cells.

The term gynoecium is also used by botanists to refer to a cluster of archegonia and any associated modified leaves or stems present on a gametophyte shoot in mosses, liverworts, and hornworts. The corresponding terms for the male parts of those plants are clusters of antheridia within the androecium.

Flowers that bear a gynoecium but no stamens are called pistillate or carpellate. Flowers lacking a gynoecium are called staminate.

The gynoecium is often referred to as female because it gives rise to female (egg-producing) gametophytes; however, strictly speaking sporophytes do not have a sex, only gametophytes do.Gynoecium development and arrangement is important in systematic research and identification of angiosperms, but can be the most challenging of the floral parts to interpret.

International Association for Plant Taxonomy

The International Association for Plant Taxonomy (IAPT) promotes an understanding of plant biodiversity, facilitates international communication of research between botanists, and oversees matters of uniformity and stability in plant names. The IAPT was founded on July 18, 1950 at the Seventh International Botanical Congress in Stockholm, Sweden. Currently, the IAPT headquarters is located in Bratislava, Slovakia. Its current president, since 2017, is Patrick S. Herendeen (Chicago Botanic Garden); vice-president is Gonzalo Nieto Feliner (Real Jardín Botánico, Madrid); and secretary-general is Karol Marhold (Plant Science and Biodiversity Centre, Slovak Academy of Sciences, Bratislava).

Both the taxonomic journal Taxon and the series Regnum Vegetabile are published by the IAPT. The latter series includes the International Code of Nomenclature for algae, fungi, and plants, Index Nominum Genericorum, and Index Herbariorum.

International Code of Nomenclature for Cultivated Plants

The International Code of Nomenclature for Cultivated Plants (ICNCP), also known as the Cultivated Plant Code, is a guide to the rules and regulations for naming cultigens, plants whose origin or selection is primarily due to intentional human activity. Cultigens under the purview of the ICNCP include cultivars, Groups (cultivar groups), and grexes. All organisms traditionally considered to be plants (including algae and fungi) are included. Taxa that receive a name under the ICNCP will also be included within taxa named under the International Code of Nomenclature for algae, fungi, and plants, for example, a cultivar is a member of a species.

Léon Guignard

Jean Louis Léon Guignard (13 April 1852 in Mont-sous-Vaudrey – 7 March 1928 in Paris) was a French pharmacist and botanist.

In 1882 he received his doctorate of sciences in Paris, and afterwards served as a professor of botany at the Faculty of Sciences of Lyon. In 1887 he succeeded Gaspard Adolphe Chatin as chair of botany at the Ecole Supérieure de Pharmacie in Paris. From 1900 to 1910 he was dean to the faculty of pharmacy.In 1899 he was named president of the Académie des sciences. He was also a member of the Société de biologie (from 1888), the Académie de Médecine (from 1897) and the Académie d'Agriculture (from 1915). In 1920 he was chosen a commander in the Legion d'Honneur.

Along with Russian biologist Sergei Navashin, he is credited as the co-discoverer of double fertilization in flowering plants. The two men made their discoveries independent of each other, Navashin in 1898 and Guignard in 1899. He also introduced a new method for detecting the presence of hydrocyanic acid in plants. In addition, he conducted significant research on the origin and structure of integuments for a large number of seeds.

Megaspore

Megaspores, also called macrospores, are a type of spore that is present in heterosporous plants. These plants have two spore types, megaspores and microspores. Generally speaking, the megaspore, or large spore, germinates into a female gametophyte, which produces egg cells. These are fertilized by sperm produced by the male gametophyte developing from the microspore. Heterosporous plants include the following:

seed plants (gymnosperms and flowering plants)

water ferns (Salviniales)

spikemosses (Selaginellaceae)

Ovule

In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: The integument, forming its outer layer, the nucellus (or remnant of the megasporangium), and the female gametophyte (formed from a haploid megaspore) in its center. The female gametophyte — specifically termed a megagametophyte— is also called the embryo sac in angiosperms. The megagametophyte produces an egg cell for the purpose of fertilization.

Plant reproduction

Plant reproduction is the production of new offspring in plants, which can be accomplished by sexual or asexual reproduction. Sexual reproduction produces offspring by the fusion of gametes, resulting in offspring genetically different from the parent or parents. Asexual reproduction produces new individuals without the fusion of gametes, genetically identical to the parent plants and each other, except when mutations occur. In seed plants, the offspring can be packaged in a protective seed, which is used as an agent of dispersal.

Pollen tube

A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain—either from the stigma (in flowering plants) to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms. In maize, this single cell can grow longer than 12 inches (30 cm) to traverse the length of the pistil.

Pollen tubes were first discovered by Giovanni Battista Amici in the 19th century.

They are used as a model for understanding plant cell behavior. Research is ongoing to comprehend how the pollen tube responds to extracellular guidance signals to achieve fertilization.

Pollination

Pollination is the transfer of pollen from a male part of a plant to a female part of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents are animals such as insects, birds, and bats; water; wind; and even plants themselves, when self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species it can produce hybrid offspring in nature and in plant breeding work.

In angiosperms, after the pollen grain has landed on the stigma, it develops a pollen tube which grows down the style until it reaches an ovary. Sperm cells from the pollen grain then move along the pollen tube, enter an ovum cell through the micropyle and fertilise it, resulting in the production of a seed.

A successful angiosperm pollen grain (gametophyte) containing the male gametes is transported to the stigma, where it germinates and its pollen tube grows down the style to the ovary. Its two gametes travel down the tube to where the gametophyte(s) containing the female gametes are held within the carpel. One nucleus fuses with the polar bodies to produce the endosperm tissues, and the other with the ovule to produce the embryo Hence the term: "double fertilization".

In gymnosperms, the ovule is not contained in a carpel, but exposed on the surface of a dedicated support organ, such as the scale of a cone, so that the penetration of carpel tissue is unnecessary. Details of the process vary according to the division of gymnosperms in question. Two main modes of fertilization are found in gymnosperms. Cycads and Ginkgo have motile sperm that swim directly to the egg inside the ovule, whereas conifers and gnetophytes have sperm that are unable to swim but are conveyed to the egg along a pollen tube.

The study of pollination brings together many disciplines, such as botany, horticulture, entomology, and ecology. The pollination process as an interaction between flower and pollen vector was first addressed in the 18th century by Christian Konrad Sprengel. It is important in horticulture and agriculture, because fruiting is dependent on fertilization: the result of pollination. The study of pollination by insects is known as anthecology.

Pteridophyte

A pteridophyte is a vascular plant (with xylem and phloem) that reproduces using spores. Because pteridophytes produce neither flowers nor seeds, they are also referred to as "cryptogams", meaning that their means of reproduction is hidden. The pteridophytes include the ferns, horsetails, and the lycophytes (clubmosses, spikemosses, and quillworts). These are not a monophyletic group because ferns and horsetails are more closely related to seed plants than to the lycophytes. Therefore, "Pteridophyta" is no longer a widely accepted taxon, although the term pteridophyte remains in common parlance, as do pteridology and pteridologist as a science and its practitioner, to indicate lycophytes and ferns as an informal grouping, such as the International Association of Pteridologists and the Pteridophyte Phylogeny Group.

Raceme

A raceme ( or ) is an unbranched, indeterminate type of inflorescence bearing pedicellate flowers (flowers having short floral stalks called pedicels) along its axis. In botany, an axis means a shoot, in this case one bearing the flowers. In indeterminate inflorescence-like racemes, the oldest flowers are borne towards the base and new flowers are produced as the shoot grows, with no predetermined growth limit. A plant that flowers on a showy raceme may have this reflected in its scientific name, e.g. Cimicifuga racemosa. A compound raceme, also called a panicle, has a branching main axis. Examples of racemes occur on mustard (genus Brassica) and radish (genus Raphanus) plants.

Seed

A seed is an embryonic plant enclosed in a protective outer covering. The formation of the seed is part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosperm plants.

Seeds are the product of the ripened ovule, after fertilization by pollen and some growth within the mother plant. The embryo is developed from the zygote and the seed coat from the integuments of the ovule.

Seeds have been an important development in the reproduction and success of gymnosperm and angiosperm plants, relative to more primitive plants such as ferns, mosses and liverworts, which do not have seeds and use water-dependent means to propagate themselves. Seed plants now dominate biological niches on land, from forests to grasslands both in hot and cold climates.

The term "seed" also has a general meaning that antedates the above – anything that can be sown, e.g. "seed" potatoes, "seeds" of corn or sunflower "seeds". In the case of sunflower and corn "seeds", what is sown is the seed enclosed in a shell or husk, whereas the potato is a tuber.

Many structures commonly referred to as "seeds" are actually dry fruits. Plants producing berries are called baccate. Sunflower seeds are sometimes sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed. Different groups of plants have other modifications, the so-called stone fruits (such as the peach) have a hardened fruit layer (the endocarp) fused to and surrounding the actual seed. Nuts are the one-seeded, hard-shelled fruit of some plants with an indehiscent seed, such as an acorn or hazelnut.

Sergei Navashin

Sergei Gavrilovich Navashin (Russian: Серге́й Гаврилович Навашин); (14 December 1857 – 10 December 1930) was a Russian biologist. He discovered double fertilization in plants in 1898.

Subdisciplines
Plant groups
Plant morphology
(glossary)
Plant growth and habit
Reproduction
Plant taxonomy
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