Editing Plant reproductive morphology (section)

===Variations===
{{see also|Flower#Floral function}}
[[File:Homoecy Monoecy Dioecy en.svg|thumb|459x459px|The basic cases of sexuality in flowering plants.]]

[[File:Alnus_glutinosa_flowers_and_fruit.jpg|thumb|upright=1.3|''[[Alnus glutinosa]]'', the common or European alder,  has unisexual flowers and is [[monoecious]]. The male-flower catkins are hanging down on the left, the much smaller female flowers are above and last season's fruit on the right.]]

[[File:Hollyflowers.jpg|upright=1.3|thumb|''[[Ilex aquifolium]]'' has unisexual flowers and is [[dioecious]]: (above and top right) a 'shoot' with flowers from a male plant, showing robust [[stamen]]s with [[pollen]], and a female-flower [[Stigma (botany)|stigma]], reduced and sterile; and (below and bottom right) a shoot with flowers from a female plant, showing a robust stigma and male-flower stamens ([[staminode]]s), reduced, sterile, with no pollen.]]

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<ref name="Stevens-2001"/>, the most common angiosperm arrangement.<ref name="Simpson-2010">{{Cite book|last=Simpson |first=Michael G.|url=https://www.google.com.au/books/edition/Plant_Systematics/dj8KRImgyf4C?gbpv=1|title=Plant Systematics|date=2010|publisher=Academic Press|isbn=978-0-12-374380-0|edition=2nd|pages=163|
language=en}}</ref> If both staminate and carpellate unisexual flowers are always found on the same plant, the species is described as [[monoecious]]. If each plant has either only staminate or carpellate flowers, the species is described as [[dioecious]]. A 1995 study found that about 6% of angiosperm species are dioecious, and that 7% of genera contain some dioecious species.<ref name="Renner-1995"/>

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, short catkins of female flowers, each without a perianth.{{sfn|Stace|2010|pp=292–296}} (See the illustration of ''[[Alnus glutinosa]]''.)

Most hollies (members of the genus ''[[Ilex]]'') are dioecious. Each plant produces either functionally male flowers or functionally female flowers. In ''[[Ilex 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.{{sfn|Stace|2010|p=669}} 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.<ref name="Buzgo-2004"/>

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.{{sfn|Sporne|1974|pp=15–16}}

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.{{sfn|Stace|2010|pp=292–296}} 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 [[homogamy (biology)|homogamous]]), or may have mixtures of two or more sexual forms (heterogamous).<ref name="Barkley"/> Thus goatsbeards (''[[Tragopogon]]'' species) have heads of bisexual florets, like other members of the tribe Cichorieae,<ref name="Barkley-2"/> whereas marigolds (''[[Calendula]]'' species) generally have heads with the outer florets bisexual and the inner florets staminate (male).<ref name="Strother"/>

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.<ref name="Ewing-1982"/> 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|date=March 2013}}