Plants have fascinating reproductive systems, and the way that their reproduction systems function can be a matter of great interest. Unlike humans, plants can reproduce in slightly different ways. If you have looked into this matter, then you must have come across the term monoecious plants.
Now this word sounds complex, but we’re going to explain everything about them in a digestible way with examples of monoecious plants. We will talk in detail about this type of plant. Furthermore, we will be addressing some common confusion regarding this topic, so keep reading to learn everything you need to know about monoecious plants.
What are Monoecious Plants?
You may not have known this, but most plants have flowers that have both the male and female reproductive parts on them. This means that the flowers have stamens, which are the male reproductive part, and carpels, the female reproductive part. 90% of plants have flowers like this, and they are said to be bisexual.
Plants that have both the male and female parts on the same flower can also be called hermaphrodites.
What about the other 10%? Those plant species are called unisexual. This is because in these plants, the flower doesn’t have both the reproductive parts on the plant. In unisexual plants, there is also another division. They are divided into two categories: (1) Monoecious plants and (2) dioecious plants.
While we will discuss dioecious plants, our primary focus is going to be monoecious plants. Firstly, let’s look at the meaning of the term monoecious. Most of you probably know that mono represents one, so this term means “one house” To make things clearer, monoecious plants have both the male and female reproductive parts on the same plant, BUT they are on different flowers.
So, a plant that has separate male and female unisexual flowers is called monoecious. Monoecious plants can also be referred to as ‘perfect’ plants because they have both male and female reproductive parts on the same plant.
Can Monoecious Plants Self Pollinate?
You must already know that pollination is the process when the stigma of a plant receives pollen transferred from another. This is how plants fertilize flowers and reproduce. So, self-pollination means when a plant can pollinate and fertilize itself without needing pollen from another plant.
Since monoecious plants have both male and female flowers on the same plant, they do have the ability to self-pollinate. Such plants can successfully fertilize themselves.
Self-pollination can happen in many ways. Pollen can be transferred by a bee from the anther to the stigma of the plant. When a bee sits on the anther it unintentionally picks up pollen, and then when it goes and sits on the flower’s stigma, the flower may be fertilized. Pollination can be a result of the environment as well. For example, wind can blow the pollen from the anther to the stigma very easily.
Just because monoecious plants can self-pollinate, it does not mean that they don’t cross-pollinate. They cross-pollinate as well through the same ways they self-pollinate. Cross-pollination is the transfer of pollen to/from another plant.
Some Common Monoecious Plants Examples:
Although the percentage of monoecious plants is small, there are actually many of them which are very common plants. So, here is our list of some of the common monoecious plant examples and a brief description for each of them:
Birch
Scientific Name: Betula pendula (Silver birch)
Scientific Classifications:
Kingdom: Plantae
Clade: Angiosperms
Clade: Rosids
Order: Fagales
Family: Betulaceae
Subfamily: Betuloideae
Genus: Betula L.
Birch is a medium-sized tree that is commonly seen around residential areas because they look gorgeous. This tree has bark with unique silver color and black details. It also has a lot of thin leaves which makes it look elegant.
It has monoecious flowers, which stay in groups of 3 on the scales of the tree’s catkins. The flowers have a calyx which contains the anther, that produces the pollen that is then transferred to the stigma.
Oak
Scientific Classifications:
Kingdom: Plantae
Clade: Tracheophytes
Clade: Eudicots
Order: Fagales
Family: Fagaceae
Subfamily: Quercoideae
Genus: Quercus L.
Oaks are very popular hardwood trees that have broad leaves, making them great for improving aesthetics. They have very small monoecious flowers. The female flowers are very small in size, so they cannot be easily noticed by the naked eye. The male part of the plant is the catkin which hangs down.
What mostly happens is that the wind blows the pollen from the drooping catkins to the stigma of the flowers.
Spruce
Scientific Classifications:
Kingdom: Plantae
(unranked): Gymnosperms
Division: Pinophyta
Class: Pinopsida
Order: Pinales
Family: Pinaceae
Genus: Picea
There are many variations of Spruce trees: white spruce, Norway spruce, Colorado spruce, etc. All spruce trees are monoecious, so they all have both separate male and female parts. They have male pollen cones and female seed-producing cones.
Pollination usually happens around June as the cones start to develop only in May.
These cones have an oblong cylindrical shape; they aren’t too big and have a reddish color.
Pine
Scientific Classifications:
Kingdom: Plantae
(unranked): Gymnosperms
Division: Pinophyta
Class: Pinopsida
Order: Pinales
Family: Pinaceae
Genus: Pinus L.
Most pine trees are monoecious, so they reproduce sexually. Pine trees also fall into the gymnosperm plant category. This means that their seeds are bare, and not inside an ovary.
Both male and female flowers of this tree turn into cones as they mature, but the male cones tend to be softer. Pollen grains are released from the microspores in the male yellow flowers. Then the wind takes them to the megaspores of the female.
Squash
Scientific Classifications:
Kingdom: Plantae
Clade: Angiosperms
Clade: Rosids
Order: Cucurbitales
Family: Cucurbitaceae
Genus: Cucurbita L.
If you have had a squash plant before then, you might have noticed that only about 50% of the flowers that bloom give you squash fruit. Well, this is because one half is male, and the other half is female.
However, not all squash plants are monoecious. They are all unisexual, but a large portion of them is dioecious.
Oil palm
Scientific name: Attalea speciosa
Classifications:
Kingdom: Plantae
Clade: Angiosperms
Clade: Monocots
Order: Arecales
Family: Arecaceae
Genus: Attalea
Species: A. speciosa
Like all other plants in this list, the oil palm plant is also monoecious. They produce clusters of male and female flowers by alternate cycles. This plant belongs to a family known as Arecoideae. One unique thing about them is that these trees have no branches.
One very important thing to note about oil palm plants is that they cross-pollinate more often than self-pollinating because male and female flowers are produced in alternate cycles. As a result, at the same time, both large amounts of female and male flowers aren’t present in the tree.
Walnuts
Scientific Name: Juglans regia
Scientific Classifications:
Kingdom: Plantae
Clade: Angiosperms
Clade: Eudicots
Order: Fagales
Family: Juglandaceae
Genus: Juglans
Species: J. Regia
There is a wide variety of walnut trees, and all of them are monoecious. Walnut trees self-pollinate for the most part, but some species of walnut trees cross-pollinate more. Producers will get the best quality nuts if you plant the same type of walnut trees together.
Cucumber
Scientific Name: Cucumis sativus
Scientific Classifications:
Kingdom: Plantae
Clade: Tracheophytes
Clade: Eudicots
Order: Cucurbitales
Family: Cucurbitaceae
Genus: Cucumis
Species: C. sativus
Known as Magnolia acuminata, the cucumber tree is monoecious. Self-pollination is not very common amongst these trees as the anthers of the male flowers begin to produce pollen before female flowers grow enough to become receptive.
Since the timing for the female and male flowers on this tree is not in sync, they primarily reproduce by cross-pollination.
Corn
Scientific Name: Zea Mays
Scientific Classifications:
Kingdom: Plantae
Clade: Angiosperms
Clade: Commelinids
Order: Poales
Family: Poaceae
Genus: Zea
Species: Z. mays
Corn has to be the most common example of monoecious plants. The male part of the corn is called the tassel, and the female part is called the ear. Since corn plants are usually planted together in corn fields and with such close proximity to each other, they are almost entirely cross-pollinated.
If you look at the numbers, you will see that even less than 5% of corn plants are produced from self-pollination. It’s also one of the plants that can be genetically modified. The ear that is at the top is generally only grown completely, and you may see another ear at the node below the uppermost node in some cases.
How Do Monoecious Plants Differ from Dioecious Plants?
Since both Monoecious and Dioecious plants are unisexual plants, people confuse them quite often. The main similarity between these two types of plants is that one flower does not have both male and female parts.
The difference between them is quite large: One monoecious plant has both separate male and female flowers, whereas dioicous plants have either just male or female parts. Therefore, dioecious plants can only cross-pollinate and cannot self-pollinate, while monoecious plants can do both.
This also means that female dioecious plants will need to have plants with male parts for them to be able to reproduce through cross-pollination. So monoecious plants do not have a set gender, whereas dioecious plants do.
You may wonder, how do you tell the gender of dioecious plants? If you buy it from a store, they will tell you the gender, and if not, you can find it easily. If you see that there is a stamen with a lot of pollen grains, then this means that the dioecious plant is male.
Are All Plants Either Monoecious or Dioecious Plants?
All plants are definitely not either monoecious or dioecious. As we have said in our first section, there are bisexual plants. We have already said that most plant species in the world are actually not monoecious or dioecious.
These plants, as we have mentioned, are hermaphrodites, meaning that they have both male and female parts on the same flower. Bisexual plants, as well as monoecious plants, are often called ‘perfect’ plants because they can usually reproduce without the help of another plant.
Can Monoecious Plants Avoid Self-Pollination?
There are mechanisms that monoecious plants can develop to avoid self-pollination. The most common structural mechanism that plants develop to avoid self-pollination is that they let the pollen be released before the stigma becomes receptive. Like Oil palm plants, some monoecious plants also produce male and female flowers in alternate cycles to avoid fertilizing themselves.
By varying the length of the flowers, the plant may also prevent self-pollination. This mostly prevents self-pollination by insects because it becomes extremely difficult for bees to transfer pollen grains from long style to short style flowers and vice versa. Primroses often use this structural technique to prevent self-fertilization.
There is also a chemical prevention method that is dependent on the genetics of the plant. Some plants produce a few chemicals which cause abnormal growth of the pollen tube, which makes the flower unsuitable for fertilization.
How do Monoecious Plants Pollinate?
If you have read the rest of the article, then you should be able to tell the answer to this question to some extent. Monoecious plants have both male and female parts, which means that they can self-pollinate. It’s important to remember that cross-pollination also occurs in these plants.
Pollination primarily happens in two ways. The first way is through insect pollination, where usually bees randomly collect up pollen grains when they sit on the pollen-loaded stamen of a male flower. Then, they drop the pollen on the stigma of a female flower.
The wind is a large contributor to pollination. It blows the pollen grains from the stamen/anther into the air, and then it carries it to the stigma, so the female flower is then fertilized.
The Evolution of Monoecious Plants
Firstly, more plants were bisexual with both male and female reproductive parts. Through andromonoecy, flowers of both sexes were produced from bisexual/ hermaphroditic flowers.
The evolution of monoecious plants is widely caused by genes of male and female sterility. Monoecious plants could be thought of as a middle ground between dioecious and hermaphrodite flowers. Diversifying selection of floral gender ratios has also made some dioecious plants evolve to monoecy.
However, for the most part, monoecy is considered to be a step from hermaphroditic plants to dioecious plants. Many consider that monoecious and dioecious flowers are related because they both are unisexual.
Conclusion
Now that we have come to the end of the article, you should ask yourself whether you truly understand now what monoecious plants are.
The variation amongst these monoecious plants and their different mechanisms through which they deal with the environment is very interesting. We would encourage you to be as enthusiastic as possible so you can absorb all the information that you read!
Resources for the Article:
https://www.fs.fed.us/wildflowers/pollinators/What_is_Pollination/
https://passel2.unl.edu/view/lesson/bf87afd5be26/4
https://www.examplesof.net/2018/05/example-of-monoecious-plants.html
https://en.wikipedia.org/wiki/Birch
https://www.gardenguides.com/12444600-are-there-male-female-oak-trees.html
https://campus.albion.edu/stowell-arboretum/white-spruce/
https://www.treevitalize.net/how-do-pine-trees-reproduce/
https://academic.oup.com/aob/article/108/8/1529/160299
https://www.thespruce.com/walnut-tree-species-3269725
https://naturewalk.yale.edu/trees/magnoliaceae/magnolia-acuminata/cucumber-tree-94
https://www.krugerseed.com/en-us/agronomy-library/corn-pollination-and-fertilization.html
https://www.britannica.com/science/pollination/Mechanisms-that-prevent-self-pollination