Plant reproductive morphology is the study of the physical form and structure (themorphology) of those parts of plants directly or indirectly concerned withsexual reproduction.
Among all living organisms,flowers, which are the reproductive structures offlowering plants (angiosperms), are the most varied physically and show a correspondingly great diversity in methods of reproduction.[1] Plants that are not flowering plants (green algae,mosses,liverworts,hornworts,ferns andgymnosperms such asconifers) also have complex interplays between morphological adaptation and environmental factors in their sexual reproduction.
The breeding system, or how thesperm from one plant fertilizes theovum of another, depends on the reproductive morphology, and is the single most important determinant of the genetic structure of nonclonal plant populations.
Christian Konrad Sprengel (1793) studied the reproduction of flowering plants and for the first time it was understood that thepollination process involved bothbiotic andabiotic interactions.Charles Darwin's theories ofnatural selection utilized this work to build histheory of evolution, which includes analysis of thecoevolution of flowers and theirinsectpollinators.
Plants have complex lifecycles involving analternation of generations. One generation, thesporophyte, producesspores which then grow to become the next generation, thegametophyte. These producegametes, theeggs andsperm, which then unite and grow to become sporophytes, completing the cycle.
Spores may be identical (isospores) or come in different sizes (microspores andmegaspores), but strictly speaking, spores and sporophytes are neither male nor female because they do not producegametes. The alternate generation, gametophytes, can bemonoicous (bisexual), where an individual can produce both eggs and sperm, ordioicous (unisexual), where one produces only eggs and another produces only sperm.
In thebryophytes (liverworts,mosses, andhornworts), the sexual gametophyte is the dominant generation. Inferns andseed plants (includingcycads,conifers,flowering plants, etc.) the sporophyte is the dominant generation; the obvious visible plant, whether a small herb or a large tree, 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 flowering plants is often described using sexual terms (e.g. "female" or "male")based on the sexuality of the gametophyte it produces. For example, a sporophyte 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 gametophytes of both sexes.[2][page needed]
In angiosperms the flower is the characteristic sexual reproductive structure, which varies enormously across the group. The bisexual flower (termed"perfect" botanically), ofRanunculus glaberrimus in the figure provides an example of the common structures. Acalyx of outersepals and acorolla of innerpetals form theperianth, the non-sexual part of the flower. Next inwards grow numerousstamens that producepollen grains, each grain producing a tiny male gametophyte from a microspore. Stamens collectively form the androecium. Finally in the middle there arecarpels, which at maturity contain one or moreovules, and within each ovule is a tiny female gametophyte produced from a megaspore.[3] Carpels also have astigma which receives pollen and astyle which connects the stigma to the ovary and enables the pollen to grow into the ovary for the female gametophyte to achieve fertilization. Carpels collectively form the gynoecium.
In other flowering plants, two or more carpels and their styles and stigmas may be fused together to varying degrees in the same flower. This entire structure may be called apistil.
A flower with functioning stamens and carpels is described as "bisexual" or "hermaphroditic". A unisexual flower is one in which either the stamens or the carpels are missing,vestigial or otherwise sterile. Staminate unisexual flowers have only functional stamens and are thus male, and carpellate or pistillate unisexual flowers have only functional carpels and are thus female.
If only bisexual flowers are found on plants of a species, it is described as homoecious,[4] the most common angiosperm arrangement.[5] If both staminate and carpellate unisexual flowers are always found on the same plant, the species is described asmonoecious. If each plant has either only staminate or carpellate flowers, the species is described asdioecious. A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species.[6]
Members of the birch family (Betulaceae) are examples of monoecious plants with unisexual flowers. A mature alder tree (Alnus species) produces longcatkins containing only male flowers, each with four stamens and a minute perianth, and separate, short catkins of female flowers, each without a perianth.[7] (See the illustration ofAlnus glutinosa.)
Most hollies (members of the genusIlex) are dioecious. Each plant produces either functionally male flowers or functionally female flowers. InIlex aquifolium (see the illustration), the common European holly, both kinds of flower have four sepals and four white petals; male flowers have four stamens, female flowers usually have four non-functional reduced stamens and a four-celled ovary.[8] Since only female plants are able to set fruit and produce berries, this has consequences for gardeners.Amborella represents the first known group of flowering plants to separate from their common ancestor. It too is dioecious; at any one time, each plant produces either flowers with functional stamens but no carpels, or flowers with a few non-functional stamens and a number of fully functional carpels. However,Amborella plants may change their "sex" over time. In one study, five cuttings from a male plant produced only male flowers when they first flowered, but at their second flowering three switched to producing female flowers.[9]
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.[10]
A species such asFraxinus 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.[7] 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 calledhomogamous), or may have mixtures of two or more sexual forms (heterogamous).[11] Thus goatsbeards (Tragopogon species) have heads of bisexual florets, like other members of the tribe Cichorieae,[12] whereas marigolds (Calendula species) generally have heads with the outer florets bisexual and the inner florets staminate (male).[13]
LikeAmborella, 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.[14] 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]
The complexity of the morphology of flowers and its variation within populations has led to a rich terminology.
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Outcrossing, cross-fertilization or allogamy, in which offspring are formed by the fusion of thegametes of two different plants, is the most common mode of reproduction amonghigher 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.[29]
The primary mechanism used by flowering plants to ensure outcrossing involves a genetic mechanism known asself-incompatibility. Various aspects of floral morphology promote allogamy. In plants with bisexual flowers, the anthers and carpels may mature at different times, plants beingprotandrous (with the anthers maturing first) or protogynous (with the carpels mature first).[citation needed] Monoecious species, with unisexual flowers on the same plant, may produce male and female flowers at different times.[citation needed]
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".[6] 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.[6] 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.[6] 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.[6]
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