Monoecious, dioecious & hermaphroditic plants: explanation and examples
The sheer variety of flowers on plants is almost incomprehensible. Therefore, a classification exists designating plants as monoecious, dioecious or as hermaphrodites.
Botanist lingo can sometimes be a bit confusing. One often hears the terms “monoecious”, “dioecious”, and “hermaphrodite” in connection with pollination and fruit bearing. These words describe the distribution of male and female floral organs on plants. But what exactly is the meaning of monoecious, dioecious and hermaphrodite plants, and what can we as gardeners derive from them? This article provides clarity and valuable tips for your garden.
By the way: Regardless of the domesticity of plants, they can be pollinated by wind or water as well as by insects. Different types of pollination occur in all types.
What are monoecious plants?
Monoecious plants bear both purely male and purely female flowers. Male flowers contain stamens that carry pollen. Female flowers can be recognised by the carpels, the so-called pistil, onto whose stigma the pollen is transferred. Monoecious plants can therefore be distinguished by the fact that they produce two different types of flowers.
Tip: The monoecious nature of plants is also called monoecy in technical language, which comes from ancient Greek and loosely translates to something like “single” and “house”. This is meant to signify that the male and female are in the same house. So, monoecy means that the flowers of both sexes are on the same plant.
However, the male and female flowers of monoecious plants do not always appear at the same time, on the same branch, or even at every age of the plant. This is because even if a plant develops both types of flowers, inbreeding must usually be avoided in order to produce healthy offspring. Therefore, many monoecious plants develop the flowers of one sex first, followed by the flowers of the other sex after a period of time (dichogamy).
Alternatively, the flower types can grow spatially separated from each other, such as by being located in different places on the plant. As a rule, pollen-producing male flowers are found higher up, so that their pollen is blown far away from their own female flower.
Sometimes, plants only develop both types of flowers when they are old: developing fruit then becomes the task of the fully developed, adult plants while pollen is donated by younger specimens in the area.
Last but not least, self-fertilisation and inbreeding can also be prevented by a genetic barrier: so-called sterility genes block the ingrowth of the pollen tube into the ovary of the female flower.
Examples of monoecious plants
Monoecious plants include many of our wild and cultivated plants, as well as some exotic plants:
- Pine (Pinus)
- Larch (Larix)
- Hazel (Corylus avellana)
- Walnut (Juglans regia)
- Alder (Alnus)
- Hornbeam (Carpinus betulus)
- Birch (Betula)
- Most conifers (coniferous trees)
- Sweet chestnuts (Castanea sativa)
- Watermelon (Citrullus lanatus)
- Maize (Zea mays)
- Cucumber (Cucumis sativus)
- Pumpkin (Cucurbita spec.)
- Courgette (Cucurbita pepo convar. giromontiina)
Some plants that were originally dioecious were chosen for monoecy, that is, selectively bred. This has the advantage that growers do not have to care for male plants that do not yield – only for fruiting specimens. The common grape vine (Vitis vinifera) and true hemp (Cannabis sativa) are two examples of cultivated monoecious plants.
Monoecious plants still produce flowers, even if fertilisation is not possible. Therefore, if you just want to enjoy the ornamental value of beautiful flowers, there is nothing special to consider when planting monoecious plants. However, if you want to harvest fruit from a plant that is monoecious, then there are things that you need to find out beforehand. What matters is whether it is self-fertile or needs pollen from a foreign, genetically different individual to form healthy seeds.
Monoecious nuts: Chestnuts, hazelnuts and walnuts, for example, grow dichogamously – male and female flowers do not appear at the same time. If there is no suitable pollen donor in the neighbourhood, fruit yield may be low or absent altogether as a result.
Single-seeded vegetables: Cucurbits such as cucumber, zucchini and squash are capable of self-pollination. However, cross-pollination can improve fruit set. Corn is a cross-pollinator, so unless corn is also being grown nearby, two different varieties should be grown. In principle, a high diversity of varieties often has a positive effect on yield.
What are hermaphrodite plants?
Whereas purely female and purely male flowers are formed in the case of separately sexed monoecious plants, plants that are hermaphrodites have only one type of flower – namely, hermaphroditic. These contain both the male and female floral organs: pistils and stamens.
However, the fact that both organs are so close to each other does not mean that hermaphrodite flowers can self-fertilise. In fact, there are self-fertile and non-self-fertile hermaphroditic plants.
If the hermaphroditic individual is self-fertile, pollination often occurs when the flower is closed – or when an insect moves around the flower collecting nectar. Pollen is then transferred to its own stigma and fertilises it. In many cases, however, inbreeding is avoided even in hermaphroditic plants: for example, through the sterility genes already mentioned, sometimes also in combination with so-called heterostyly. This is very beautiful to observe in primroses: there are flowers with short and those with long pistils, which promotes cross-fertilisation and inhibits self-fertilisation.
But self-fertilisation is also quite possible: this is called autogamy. Autogamous, that is, self-fruiting, plants can be found in many gardens. For example, tomatoes (Solanum lycopersicum), strawberries (Fragaria x ananassa), and most cherry varieties (Prunus spec.) are self-fertile, so they do not require a pollination partner.
Tip: The vast majority of plants, about 95%, are monoecious or even hermaphroditic. By definition, plants that are hermaphrodites are also monoecious: male and female flower organs are, after all, on the same plant.
Examples of hermaphrodite plants
The majority of our native plants and crops form hermaphroditic flowers:
- Apple trees (Malus spec.)
- Pear trees (Pyrus spec.)
- Cherry trees (Prunus spec.)
- Roses (Rosa)
- Olive trees (Olea europaea)
- Most vegetable plants
- Many ornamental plants, for example daisies (Bellis perennis)
Even with hermaphroditic plants, it is important to know whether or not they can successfully self-fertilise if fruits and seeds are to be produced. If it is not the fruit of a vegetable, but another part of the plant that is desired, harvested or eaten, then there is of course no need to look for a partner for fertilisation.
Examples of self-fertile hermaphroditic plants:
- Tomatoes (Solanum lycopersicum)
- Aubergine (Solanum melangena)
- Bell pepper (Capsicum annuum)
- Many beans (Phaseolus spec.)
- Peas (Pisum sativum)
- Lamb’s lettuce (Valerianella locusta)
- Most strawberries (Fragaria x ananassa)
- All sour cherries and few sweet cherry varieties: e.g. ‘Stella’ and ‘Lapins’
Examples of cross-pollinated hermaphroditic crops:
- Most sweet cherries
- Parsnips (Pastinaca sativa)
- Leeks (Allium porrum)
- Cabbages (Brassicacaea)
Often the flowering window is not very long. For cross-pollination to be successful, the flowering times of the selected varieties must naturally overlap. Therefore, pollination tables are available for many plants in order to show which varieties are suitable pollinators.
Tip – Promote pollination: Whether plants are monoecious, dioecious or hermaphroditic, in the vast majority of cases, cross-pollination results in higher quality of fruit and often more yield. Because pollination is very often done by insects, both amateur gardeners and professional fruit and vegetable growers encourage pollination by attracting insects.
By the way: Categorising plants as “monoecious,” “dioecious,” and “hermaphroditic” is simply an attempt by botanists to identify a system in the variability of nature. But as always, exceptions confirm the rule – no botanical system fits reality like a glove. Mixed forms often occur. An example is the horse chestnut (Aesculus hippocastanum), which can produce hermaphroditic flowers as well as purely male and purely female flowers. There are other plants that are capable of producing hermaphroditic flowers and male or female flowers.
What are dioecious plants?
Only about 5% of plants are dioecious – that is, with male and female flowers on different specimens. This means that there are purely male and purely female plants. Dioecy has the advantage that self-pollination and inbreeding are completely avoided. An individual can only fertilise or be fertilised by another. This guarantees regular genetic mixing. In dioecious plant species, fruiting can only ever be observed in the females, but the males show abundant pollen production.
Tip: The dioecious nature of plants is also referred to in technical terminology as dioecy, which comes from ancient Greek and loosely translates to something like “twice” and “house”. The sexes are therefore in “two houses” or, in other words, on two different plants.
Examples of dioecious plants
Dioecious garden plants:
- Goat willow (Salix caprea)
- Holly (Ilex spec.)
- Ginkgo (Ginkgo biloba) – only males are usually planted
- Seaberry (Hippophae rhamnoides)
- Kiwi (Actinidia deliciosa) – monoecious varieties also exist
- Light carnation (Silence dioica)
- Shrubby cinquefoil (Potentialla futicosa) – only females are usually planted
- Hops (Humulus lupulus)
- Yew (Taxus baccata)
- Skimmia (Skimmia japonica)
- Spinach (Spinacia oleracea)
- Some watermelon varieties
- Asparagus (Asparagus officinalis)
If harvesting or admiring the fruit is the goal of planting, then both females and males must be planted for fruit to occur in dioecious plants. Often, a few male plants offer sufficient pollen production to fertilise many females.
For some plants, fruiting is normally desired: who would want to go without the red berries of the yew, holly or skimmia, not to mention tasty seaberries or kiwi? With other plants, however, it is better to avoid fertilisation altogether. Ginkgo females produce very unpleasant smelling fruits – therefore, as a rule, only male ginkgos are planted.
In some of the plants presented, only a few varieties have been bred to be self-fertile or at least monoecious in order to simplify cultivation.