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 ofangiosperms, 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 involvingalternation of generations. One generation, thesporophyte, gives rise to the next generation, thegametophyte asexually viaspores. 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, the gametophyte, produces gametes,eggs and/orsperm. A gametophyte can bemonoicous (bisexual), producing both eggs and sperm, or dioicous (unisexual), either female (producing eggs) or male (producing 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, 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]
Theflower is the characteristic structure concerned with sexual reproduction in flowering plants (angiosperms). Flowers vary enormously in their structure (morphology). Aperfect flower, like that ofRanunculus glaberrimus shown in the figure, has acalyx of outersepals and acorolla of innerpetals and both male and femalesex organs. The sepals and petals together form theperianth. Next inwards there are numerousstamens, which producepollen grains, each containing a microscopic male gametophyte. Stamens may be called the "male" parts of a flower and 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.[3] Carpels may be called the "female" parts of a flower and collectively form the gynoecium.
Each carpel inRanunculus species is anachene that produces one ovule,[4] which when fertilized becomes a seed. If the carpel contains more than one seed, as inEranthis hyemalis, it is called afollicle. Two or more carpels may be fused together to varying degrees and the entire structure, including the fusedstyles and stigmas may be called apistil. The lower part of the pistil, where the ovules are produced, is called theovary. It may be divided into chambers (locules) corresponding to the separate carpels.[5]
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,vestigial or otherwise non-functional. Each flower is either staminate (having only functional stamens and thus male), or carpellate or pistillate (having only functional carpels and thus female). If separate staminate and carpellate flowers are always found on the same plant, the species is described asmonoecious. If separate staminate and carpellate flowers are always found on different plants, the species is described asdioecious.[6] A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species.[7]
Members of the birch family (Betulaceae) are examples of monoecious plants with unisexual flowers. A mature alder tree (Alnus species) produces long catkins containing only male flowers, each with four stamens and a minute perianth, and separate stalked groups of female flowers, each without a perianth.[8] (See the illustration ofAlnus serrulata.)
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.[9] 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.[10]
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 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.[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 calledhomogamous), 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]
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.[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]
The complexity of the morphology of flowers and its variation within populations has led to a rich terminology.
 | This sectionneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources in this section. Unsourced material may be challenged and removed.(June 2021) (Learn how and when to remove this message) |
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.[28]
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".[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]
Media related toPlant reproductive morphology at Wikimedia Commons| Subdisciplines | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant groups | |||||||||||
| Plant anatomy |
| ||||||||||
| Plant physiology Materials | |||||||||||
| Plant growth and habit | |||||||||||
| Reproduction | |||||||||||
| Plant taxonomy | |||||||||||
| Practice | |||||||||||
| |||||||||||
