Plant reproductive morphology
Plant reproductive morphology is the study of the physical form and structure (the morphology) of those parts of plants directly or indirectly concerned with sexual reproduction.
Among all living organisms,
Use of sexual terminology
Plants have complex lifecycles involving
In the
, etc.) the sporophyte is the dominant generation; the obvious visible plant, whether a small herb or a large tree, is the sporophyte, and the gametophyte is very small. In bryophytes and ferns, the gametophytes are independent, free-living plants, while in seed plants, each female megagametophyte, and the megaspore that gives rise to it, is hidden within the sporophyte and is entirely dependent on it for nutrition. Each male gametophyte typically consists of two to four cells enclosed within the protective wall of a pollen grain.The sporophyte of a flowering plant is often described using sexual terms (e.g. "female" or "male") based on the sexuality of the gametophyte it gives rise to. For example, a sporophyte that produces spores that give rise only to male gametophytes may be described as "male", even though the sporophyte itself is asexual, producing only spores. Similarly, flowers produced by the sporophyte may be described as "unisexual" or "bisexual", meaning that they give rise to either one sex of gametophyte or both sexes of the gametophyte.[2][page needed]
Flowering plants
Basic flower morphology
The
Each carpel in Ranunculus species is an
Variations
A perfect flower has both stamens and carpels, and is described as "bisexual" or "hermaphroditic". A unisexual flower is one in which either the stamens or the carpels are missing,
Members of the birch family (
Most hollies (members of the genus
In extreme cases, almost all of the parts present in a complete flower may be missing, so long as at least one carpel or one stamen is present. This situation is reached in the female flowers of duckweeds (Lemna), which consist of a single carpel, and in the male flowers of spurges (Euphorbia) which consist of a single stamen.[11]
A species such as Fraxinus excelsior, the common ash of Europe, demonstrates one possible kind of variation. Ash flowers are wind-pollinated and lack petals and sepals. Structurally, the flowers may be bisexual, consisting of two stamens and an ovary, or may be male (staminate), lacking a functional ovary, or female (carpellate), lacking functional stamens. Different forms may occur on the same tree, or on different trees.[8] The Asteraceae (sunflower family), with close to 22,000 species worldwide, have highly modified inflorescences made up of flowers (florets) collected together into tightly packed heads. Heads may have florets of one sexual morphology – all bisexual, all carpellate or all staminate (when they are called homogamous), or may have mixtures of two or more sexual forms (heterogamous).[12] Thus goatsbeards (Tragopogon species) have heads of bisexual florets, like other members of the tribe Cichorieae,[13] whereas marigolds (Calendula species) generally have heads with the outer florets bisexual and the inner florets staminate (male).[14]
Like Amborella, some plants undergo sex-switching. For example, Arisaema triphyllum (Jack-in-the-pulpit) expresses sexual differences at different stages of growth: smaller plants produce all or mostly male flowers; as plants grow larger over the years the male flowers are replaced by more female flowers on the same plant. Arisaema triphyllum thus covers a multitude of sexual conditions in its lifetime: nonsexual juvenile plants, young plants that are all male, larger plants with a mix of both male and female flowers, and large plants that have mostly female flowers.[15] Other plant populations have plants that produce more male flowers early in the year and as plants bloom later in the growing season they produce more female flowers.[citation needed]
Terminology
The complexity of the morphology of flowers and its variation within populations has led to a rich terminology.
- Androdioecious: having male flowers on some plants, bisexual ones on others.[6]
- Androecious: having only male flowers (the male of a dioecious population); producing pollen but no seed.[16]
- Androgynous: see bisexual.[6]
- Androgynomonoecious: having male, female, and bisexual flowers on the same plant, also called trimonoecious.[16]
- Andromonoecious: having both bisexual and male flowers on the same plant.[6]
- Bisexual: each flower of each individual has both male and female structures, i.e. it combines both sexes in one structure.carpels. Other terms used for this condition are androgynous, hermaphroditic, monoclinous and synoecious.
- Dichogamous: having sexes developing at different times; producing pollen when the stigmas are not receptive,[6] either protandrous or protogynous. This promotes outcrossing by limiting self-pollination.[17] Some dichogamous plants have bisexual flowers, others have unisexual flowers.
- Diclinous: see Unisexual.[6]
- Dioecious: having either only male or only female flowers.[6] No individual plant of the population produces both pollen and ovules.[18] (From the Greek for "two households". See also the Wiktionary entry for dioecious.)
- Gynodioecious: having hermaphrodite flowers and female flowers on separate plants.[19]
- Gynoecious: having only female flowers (the female of a dioecious population); producing seed but not pollen.[20]
- Gynomonoecious: having both bisexual and female flowers on the same plant.[6]
- Hermaphroditic: see bisexual.[6]
- Homogamous: male and female sexes reach maturity in synchrony; producing mature pollens when stigma is receptive.
- Imperfect: (of flowers) having some parts that are normally present not developed,[21] e.g. lacking stamens. See also Unisexual.
- Monoclinous: see bisexual.[6]
- Monoecious: In the commoner narrow sense of the term, it refers to plants with unisexual flowers which occur on the same individual.[2] In the broad sense of the term, it also includes plants with bisexual flowers.[6] Individuals bearing separate flowers of both sexes at the same time are called simultaneously or synchronously monoecious and individuals that bear flowers of one sex at one time are called consecutively monoecious.[22] (From the Greek monos "single" + oikia "house". See also the Wiktionary entry for monoecious.)
- Perfect: (of flowers) see bisexual.[6]
- Polygamodioecious: mostly dioecious, but with either a few flowers of the opposite sex or a few bisexual flowers on the same plant.[2]
- Polygamomonoecious: see polygamous.[6] Or, mostly monoecious, but also partly polygamous.[2]
- Polygamous: having male, female, and bisexual flowers on the same plant.[6] Also called polygamomonoecious or trimonoecious.[23] Or, with bisexual and at least one of male and female flowers on the same plant.[2]
- Protandrous: (of dichogamous plants) having male parts of flowers developed before female parts, e.g. having flowers that function first as male and then change to female or producing pollen before the stigmas of the same plant are receptive.[6] (Protoandrous is also used.)
- Protogynous: (of dichogamous plants) having female parts of flowers developed before male parts, e.g. having flowers that function first as female and then change to male or producing pollen after the stigmas of the same plant are receptive.[6]
- Subandroecious: having mostly male flowers, with a few female or bisexual flowers.[24]
- Subdioecious: having some individuals in otherwise dioecious populations with flowers that are not clearly male or female. The population produces normally male or female plants with unisexual flowers, but some plants may have bisexual flowers, some both male and female flowers, and others some combination thereof, such as female and bisexual flowers. The condition is thought to represent a transition between bisexuality and dioecy.[25][26]
- Subgynoecious: having mostly female flowers, with a few male or bisexual flowers.[citation needed]
- Synoecious: see bisexual.[6]
- Trimonoecious: see polygamous[6] and androgynomonoecious.[16]
- Trioecious: with male, female and bisexual flowers on different plants.[27]
- Unisexual: having either functionally male or functionally female flowers.[6] This condition is also called diclinous, incomplete or imperfect.
Outcrossing
This section needs additional citations for verification. (June 2021) |
Outcrossing, cross-fertilization or allogamy, in which offspring are formed by the fusion of the gametes of two different plants, is the most common mode of reproduction among higher plants. About 55% of higher plant species reproduce in this way. An additional 7% are partially cross-fertilizing and partially self-fertilizing (autogamy). About 15% produce gametes but are principally self-fertilizing with significant out-crossing lacking. Only about 8% of higher plant species reproduce exclusively by non-sexual means. These include plants that reproduce vegetatively by runners or bulbils, or which produce seeds without embryo fertilization (apomixis). The selective advantage of outcrossing appears to be the masking of deleterious recessive mutations.[28]
The primary mechanism used by flowering plants to ensure outcrossing involves a genetic mechanism known as
Dioecy, the condition of having unisexual flowers on different plants, necessarily results in outcrossing, and probably evolved for this purpose. However, "dioecy has proven difficult to explain simply as an outbreeding mechanism in plants that lack self-incompatibility".[7] Resource-allocation constraints may be important in the evolution of dioecy, for example, with wind-pollination, separate male flowers arranged in a catkin that vibrates in the wind may provide better pollen dispersal.[7] In climbing plants, rapid upward growth may be essential, and resource allocation to fruit production may be incompatible with rapid growth, thus giving an advantage to delayed production of female flowers.[7] Dioecy has evolved separately in many different lineages, and monoecy in the plant lineage correlates with the evolution of dioecy, suggesting that dioecy can evolve more readily from plants that already produce separate male and female flowers.[7]
See also
- Apomixis
- Vegetative reproduction
- Botany
- Evolution of sexual reproduction
- Flower
- Evolutionary history of plants: Flowers
- Flower: Development
- Meiosis
References
Citations
- S2CID 7424193.
- ^ a b c d e Hickey, M. & King, C. (2001). The Cambridge Illustrated Glossary of Botanical Terms. Cambridge University Press.
- ^ Sporne 1974, pp. 14–15.
- ^ Whittemore, Alan T. "Ranunculus". Flora of North America. Retrieved 2013-03-04 – via www.eFloras.org.
- ^ Sporne 1974, pp. 125–127.
- ^ ISBN 978-1-84246-422-9.
- ^ JSTOR 2445418.
- ^ a b Stace 2010, pp. 292–296.
- ^ Stace 2010, p. 669.
- S2CID 84793812.
- ^ Sporne 1974, pp. 15–16.
- ^ Barkley, Theodore M.; Brouillet, Luc & Strother, John L. "Asteraceae". Flora of North America. Retrieved 2013-03-04 – via www.eFloras.org.
- ^ Barkley, Theodore M.; Brouillet, Luc & Strother, John L. "Chichorieae". Flora of North America. Retrieved 2013-03-04 – via www.eFloras.org.
- ^ Strother, John L. "Calendula". Flora of North America. Retrieved 2013-03-04 – via www.eFloras.org.
- JSTOR 2484467.
- ^ ISBN 9780470650028.
- .
- ^ Baskauf, Steve (2002). "Sexual systems in angiosperms". Archived from the original on 2018-07-03. Retrieved 2013-02-27.
- ^ "Gynodioecious". Dictionary of Botany. Retrieved 2013-04-10.
- ISBN 978-90-5782-147-9.
- ^ Cook 1968, p. 131.
- ISBN 978-81-906757-4-1.
- ISBN 3-540-64597-7. p. 4
- S2CID 25414438. Retrieved 30 December 2020.
- PMID 10860914.
- PMID 21665739.
- ISBN 978-1-84246-604-9.
- ISBN 978-0-12-092860-6.
Sources
- Cook, J. Gordon (1968). ABC of plant terms. Watford, UK: Merrow. OCLC 474319451.
- Sporne, K.R. (1974). The Morphology of Angiosperms. London: Hutchinson. ISBN 978-0-09-120611-6.
- ISBN 978-0-521-70772-5.
Further reading
- Darwin, Charles (1877). The Different Forms of Flowers on Plants of the Same Species. London: J. Murray.
- Linnaeus, Carl (1735). Systema Naturae.
- Sattler, Rolf (1973). Organogenesis of Flowers: a Photographic Text-Atlas. University of Toronto Press. ISBN 978-0-8020-1864-9.
External links
- Media related to Plant reproductive morphology at Wikimedia Commons
- Images of sexual systems in flowering plants at bioimages.vanderbilt.edu