The flowers are male, having stamens but no carpels.


The flowers are female, having carpels but no stamens.


Bisexual is where every flower on the plant has both male (stamens) and female (carpels) structures. Alternative names for bisexual are: androgynous, hermaphroditic, monoclinous and synoecious. Recent studies in the likely most recent ancestral flower of angiosperms suggests that the most recent ancestral flower was bisexual, and that unisexual flowers evolved independently many times afterwards. (The ancestral flower itself is at the moment as elusive as ever).


Diclinous is a synonym for unisexual.
Unisexual plants can be either Monoecious or Dioecious, see below.


Many plants are Monoecious, that is have separate male and female flowers where both occur on the same plant. Monoecious plants can pollinate each other and (probably) themselves too.


Much fewer plants are Dioecious where separate male and female flowers are on separate plants.

This has reproduction implications: both male and female plants need to be nearby in order for fertilization of female flowers by male flowers to occur. If fertilization does not occur, a female plant will not produce fruit or berries (but a male plant will always produce pollen). In dioecious plants it is generally that the male flowers last much longer than the female flowers which turn to fruit soon after being fertilised. The male flowers just keep on going ready to fertilise other female plants of the species.

Some dioecious trees can switch sex. For instance about 2% of male Quindio Wax Palm Trees (Ceroxylon quinduense) in Columbia suddenly switched to having female flowers. But no one knows why they do this - perhaps by switching to female they can produce seeds which helps the palm trees colonise new areas faster(?). Maple Trees (which are also dioecious) can also change sex.


Incapable of reproducing by sexual means. Barren also means similar. The anthers or stigmas are not functional on this species. They may be absent altogether.

Many ferns (all?) have plants with fertile fronds and others with sterile fronds. Your Author has not tagged any fern with the sterile logo because there are others of the same species which are fertile - they just come as fertile and sterile plants. The same is true for many Horsetails. Some (all?) Stonecrop plants also have separate fertile and sterile plants. There are other plants which have both sterile and fertile flowers on the same plant, such as Guelder-rose and Hydrangea, where the larger sterile petals act as a flag to signal that nectar is available on the smaller but more numerous fertile flowers. Also many plants from the dead-nettle family, such as Black Horehound and Hedge Woundwort, have flower spikes with both fertile and empty (sterile) sepal tubes. Grape Hyacinth too has both fertile flowers and sterile flowers in the same flowering spike. The same is true of many rayed-daisy type flowers such as Greater Knapweed, where the ray florets are sterile and the inner disc florets are fertile. Lords and Ladies also has both sterile and fertile parts of the flower - but in this case they are usually well hidden by the spathe. Because all these plants do have some fertile flowers - your Author has not listed them as sterile.

Sterility is unrelated to whether or not the plant produces nectar, which is there in plants with it to attract pollinating insects. A sterile plant may or might not produce nectar. If a sterile plant does produce pollen then the sterility is due to some other abnormality, usually manifesting as chromosomal problems such as polyploidy. The presence of sterile flowers on some plants (which also have fertile flowers) often attracts a lot of bees because those flowers usually produce a lot of nectar.

Double-flowered plants are often sterile if the all the stamens are replaced by petals. This can happen when both copies of the double-flowered gene AGAMOUS are damaged. This gene codes for proteins responsible for the production of stamens and carpels. If both copies of this gene are either deleted or damaged, then the where stamens would normally be produced petals replace them and where carpels should have been produced the sepals are instead replaced by a new flower, which is also genetically damaged and that produces another petal where a stamen should have been produced, plus yet another new flower instead of a sepal, ad-infinitum... or until all the available room has been used up. [It looks to your Author like it produces petals starting from the outside and progressing towards the centre - rather than the other way around where it shouldn't run out of room]. The result is a recursive pattern of sepal-petal-petal units. Because neither stamens nor carpels are formed the flower is sexually sterile. Most horticultural Roses are double-flowered and therefore sterile. They are useless to bees foraging for nectar.


Alternative names for polygamous are androgynomonoecious, polygamomonoecious and trimonoecious.
Having both bisexual & (male &/OR female) flowers on the same plant. i.e. bearing both unisexual and hermaphrodite flowers on the same plant.


Having either bisexual OR male OR female flowers on separate plants.


Having a mixture of bisexual/hermaphrodite flowers on some plants and only female flowers on other plants. These might include male plants, sterile plants (male sterile or female sterile) and hermaphrodite plants - and a little-known fourth class of partially male sterile plants where temperature influences sexual expression and the degree of male sterility but not in the former 3 classes. Examples of gynodioecious flowers include Buck's-horn Plantain, Bladder Campion, Devil's-bit Scabious and Meadow Saxifrage


Having bisexual and male flowers on separate plants.


Having both bisexual and male flowers on the same plant. examples include Hellebores such as Stinking Hellebore.


Having both bisexual/hermaphrodite and female flowers on the same plant. Examples include Catchfly and many plants belonging to the Asteraceae family (Dandelion & Daisy).





Heterostylous - having two forms of bisexual flower: Pin and Thrum, either of which can only fertilise the opposite form. That is, they are Distylous. Pin forms have a long style and short stamens, whereas thrum forms have long stamens and a short style. Because the pin and thrum forms are determined by genes, the same plant has either all pin flowers or all thrum flowers (the Author thinks). See Primrose for a much fuller explanation and for information on further forms.

Almost all heterostylous plants have a definite number of stamens, usually either 2, 4, 5, 6, 10, or 12 (usually missing out 8 and most uneven numbers except 5). In Some species the thrum anthers are larger than the pin anthers - such as those in species of Lithospermum (Gromwells) or Amsinckia (Fiddlenecks) which are both in the Boraginaceae family. In some Primula species pin stigmas are hemispherical whilst thrum stigmas are a little flattened. In most distylous species the thrum pollen is larger than the pin form, but some fewer species show no dimorphism in pollen size.

In species of Lythrum the thrum pollen turns a dark colour when subjected to iodine but the pin form remains the same colour - which is probably because the thrum pollen contains starch whereas the pin form doesn't.

The following genera are distylous:

In the Boraginaceae family:
Amsinckia, Anchusa, Echioides, Lithospermum and Pulmonaria

In the Clusiaceae family:

In the Linaceae family:

In the Menyanthaceae family:
Menyanthes, Nymphoides

In the Oleaceae family:
Forsythia, Jasminium

In the Plumbaginaceae family:
Armeria, Ceratostygma, Limonium

In the Polygonaceae family:

In the Pontederiaceae family:
Pontederia, Eichornia

In the Primulaceae family:
Hottonia, Primula

In the Rubiaceae family:
Asperula, Nertera

[This list may be incomplete]


Purple Loosestrife takes heterostyly to the third level where there are three types of flower morphs. It is thus Tristylous. Each flower morph has two types of stamens (but only one stigma), all three sets at differing heights. The two types of stamens are in sets of (nominally) six each. The first morphological type where the style is short and the two stamens are medium and long; the second morph has a medium length style and long and short stamens; the third morph has a long style with medium and short stamens. Pollen transferred from flowers of the same morph will not result in fertilization because the individual morphs are self-incompatible. The three flower morphs are adapted to pollination by different insects. Generally, pollen from the stamen nearest to the stigma will pollinate the stigma.

Pale Pink-sorrel is similar to Purple Loosestrife in that they have 5 each of stamens, stamens and styles in any of the three possible height permutations (as for Purple Loosestrife).

A list of families/genera follows which are tristylous, some of which are also distylous.

FAMILY GENUS distylous tristylous
[This list may be incomplete]


Occasionally, Primroses can be found which are neither pin nor thrum in form, but instead both stigma and anthers appear at the top of the opening. This is the homostylous form. If both are at the top of the opening it is called long-homostylous. This type of arrangement is fully self-fertile, able to pollinate itself (unlike either pin or thrum forms). Because this form is self-fertile, wherever one is found, there will be many others produced as offspring from the one. They are particularly abundant around Somerset. Japanese Cowslip (Primula japonica) (which have tiered whorls of flowers up the stem) are wholly heterostylous. [Notes - this homostylous form has no icon as it will only ever apply to those few species of flowers which exhibit pin/thrum forms - apart from the Japanese Cowslip flower just mentioned where they are all homostylous]

A protandrous flower is one in which the stamens shed pollen before the stigma is receptive. Examples being the flowers of Jacob's Ladder (Polemonium caeruleum) and Foxglove (Digitalis purpurea). A protandrous flower is first functionally male and afterwards functionally female, but not both at the same time. This is only one of the ways in which some plants try to protect themselves from self-fertilisation.

But in the case of Foxglove (Digitalis purpurea), where the flowers open from the bottom upwards in the long spike and there is a time when the fully opened females at the bottom are concurent with the freshly-opening males nearer the top. Pollinators often start from the bottom with the female flowers then work their way up to the males nearer the top, thus they have not pollinated any flowers on the plant. But when they move on to the next Foxglove plant they pollinate the female flowers at the bottom with pollen from a differing Foxglove, thus helping cross-pollination and hindering self-pollination.

A protogynous flower is the opposite of a protandrous flower. The stigma is receptive to pollen before the anthers open and release their pollen, during the interval the stigma becomes unresponsive to pollen. Thus self-fertilisation is again thwarted. Examples of flowers exhibiting protogyny include the flowers of Common Figwort (Scrophularia nodosa), Greater Plantain (Plantago major), Christmas-Rose (Helleborus niger), Horse-Chestnut (Aesculus hippocastanum), Magnolia, Hortonia and Daphnandra. Other good examples are Meadow Crane's-bill (Geranium pratense), Red Campion (Silene dioica), Honeysuckle (Lonicera periclymenum) and Pink (Dianthus plumarius). There are many more, including Grass-of-Parnassus (Parnassia palustris) where the stamens take up a definite position in space as time progresses.

In Greater Plantain (Plantago major) the (female) stigmas are exposed first starting at the bottom and the stamens only start appearing after the stigmas have withered. Thus stamens are at the bottom and stigmas above them further up the long this inflorescence. This strategy largely prevents self-pollination.

Lords and Ladies (Arum maculatum) is also protogynous but with a slightly different strategy. At first both male and female flowers deep within it are sterile. But then one day the female sexual organs deep within the spadix become active and they release an odour attracting the tiny Diptera flies which then become trapped by the expanding but still sterile male flowers in a ring above the female flowers at the bottom. On the second day the male flowers become active and allow the trapped flies to escape, and as they do so they unintentionally take sticky pollen grains with them. When they visit another Lords-and-Ladies flower they fertilise it with the pollen.

Dichogamy / Dichogamous
Both Protogyny and Protandra are examples of Dichogamy where the stamens and stigmas of bisexual flowers ripen at differing times in order to prevent self-fertilisation.

Homogamy / Homogamous
Definition a) : Homogamy is the antonym of dichogamy and is where both male and female organs of the plant mature at the same time. Therefore it is capable of self-fertilisation - providing all other factors are favourable.
Definition b) : in the case of the Asteraceae family Homogamous heads consist of just one type of floret - either all ray florets (as in Dandelion (Taraxacum officinale) or all disc florets (as in Pineappleweed (Matricaria discoidea) or Tansy (Tanacetum vulgare).

Geitonogamy / Geitonogamous
Geitonogamy is a type of self-pollination where the flowers on a plant are fertilised by the pollen from another flower on the same plant.

Heterogamy / Heterogamous
Heterogamous flowerheads are where the heads comprise two differing sexes of florets, examples being the ray (which are either unisexual or neuter) and disc florets (which are usually usually bisexual) belonging to the Asteraceae (Dandelion & Daisy family). Alternatively, it can mean bearing two (or more) sorts of flowers in one cluster of flowers.

Plant sexuality is not always so straightforward, there are many half-way houses and permutations on the above.  Plant Reproductive Morphology

Autogamy / Autogamous
Autogamy is self-fertilisation by the sperm in the pollen from a stamen on a flower go and fertilise the carpels of a flower on the same plant. It can fertilise either the same flower (in which case it occurred autogamously) or it can fertilise a separate flower on the same plant (in which case it occurred geitnogamously). The opposite of Autogamy is Alogamy.

Alogamy / Alogamous
The opposite of Autogamy. Alogamy is where the pollen from one plant fertilises the flower on another plant (either of the same species or of a differing species - in the latter case this would be a case of cross-hybridisation).

Agamospermy / Agamospermous
This is where viable seed is produced by a flower without fertilisation having taken place.

Agamospecies and Hybrid Swarms
A species of plant in which reproduction occurs almost exclusively without fertilisation having taken place. Such plants exhibit agamospermy. Examples include Goldilocks Buttercup (Ranunculus auricomus), the Sea-Lavender with the binomial name Limonium binoversum and also the numerous (400?) Brambles (Rubus) species, the c.250 Hawkweeds (Hieracium), the c.250 Dandelions (Taraxacum), the 20 or so Whitebeams (Sorbus), the 13 or so Lady's Mantles (Alchemilla), the 9 Sea-Lavenders (Limonium) and others besides. In fact most apomictic microspecies (which have hybrid swarms) are agamospecies. These apomictic species produce viable seed without fertilisation (either self-fertilisation or cross-fertilisation) by others and the progeny they produce are identical to themselves and the seeds that they in turn produce are also identical - unless of course some mutation occurred, in which case a new microspecies is formed - another taxon to add to the number of species in that hybrid swarm - but this happens relatively infrequently.
See Apomicts


  • 'EOR' or '⊕' is the Logical 'Exclusive OR' (sometimes spelled 'EXOR' or 'XOR') being 'either one or the other', but not both.
  • '&' is the Logical 'AND' meaning both at the same time.

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