Flowers use different traits to attract pollinators. Some flowers are reddish and fleshy with a smell like rotten meat. This is because they are pollinated by flies, like blowflies. These traits like color, shape, odor, and size, are used by flowers to attract pollinators. In this article, we will describe the different types of pollination for flowers, the traits that are related to each type, and the importance of pollination in the reproduction of flowering plants. So, keep reading to learn more about pollination!

Pollination definition

Plants can reproduce in two ways: sexually and asexually. Sexual reproduction happens when two gametes combine to form a zygote. But plants can't move to find a mate, so how do they reproduce? That's where pollen comes in. Pollen grains are the male reproductive structures that produce sperm in seed-producing plants like angiosperms. For pollination to happen, pollen has to move from the male part of the plant, called the stamen, to the female part, called the pistil or carpel. In angiosperms, all sexual reproductive structures are found in the flower. The anthers produce pollen, while the eggs are produced inside the ovules in the ovary. So, pollination in angiosperms happens when pollen is transferred from the anther to the stigma of a flower. In this article, we will focus on pollination in angiosperms.

Pollination process

Pollen grains need help to move and therefore rely on pollinating agents. These agents can be abiotic (non-living agents, like wind and water) or biotic (living agents, such as insects and birds). The specific traits a flower presents (color, size, shape, scent) are related to the type of pollination agents it relies upon.

Abiotic pollination

Did you know that around 20% of flowering plants don't rely on insects or animals for pollination? Instead, they use abiotic pollinating agents like wind or water. Wind-pollinated flowers don't need to attract pollinators, so they look very different from other flowers. They are usually small, inconspicuous, and green or brownish in color, and they might not even have petals (check out Figure 1). They also don't produce nectar or have a scent because they don't need to lure in insects. The stamens, which produce pollen, are usually exposed to the wind, and the stigma, which receives pollen, can be feathery to catch the pollen more easily. Keep reading to learn more about how plants reproduce!

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Wind-pollinated plants have evolved specific adaptations to make sure they can reproduce successfully. They produce a lot of pollen, creating "pollen clouds" that increase the probability of the pollen reaching another individual of the same species. In temperate species, wind-pollinated flowers usually appear early in the spring, before new leaves have developed and could block pollen movement.

Some examples of wind-pollinated plants are grasses, cottonwood, birch, oaks, and most gymnosperms. These plants produce so much pollen to ensure pollination that they can cause pollen allergies in some people. On the other hand, water is not a very common pollinating agent for angiosperms. Only a few aquatic grasses and weeds use water to transport pollen. In these plants, the pollen floats on the water until it reaches a flower. It's fascinating to learn about the different ways that plants have adapted to ensure successful pollination and reproduction!

Biotic pollination

Animal pollination is much more common in angiosperms, with about 80% relying on animal agents like insects, birds, and bats for pollination. Insects are the most common pollinators, pollinating around 65% of angiosperms. Flowers use color, shape, and scent to attract pollinators and often reward them with food in the form of nectar and pollen.

While most flowers can attract different pollinators, some have evolved specific traits to attract a particular group of pollinators or even a specific species. Specialized pollinators can be more efficient than generalists because they transfer pollen directly to another flower of the same species, rather than visiting multiple species and wasting some pollen. This process is called coevolution, where both the pollinator and the flower develop traits that make the pollination specific to them.

Bees are the most common pollinators, and many crop plants used for human consumption rely on them for pollination. Flowers pollinated by bees are usually brightly colored (yellow, blue, or purple), have strong fragrances, and may even have nectar guides to help guide bees to the center of the flower. These guides can be small structures in the petal or ultraviolet marks that form a path, which bees can see. The pollen attaches to the fuzzy body of the bees as they move from flower to flower, helping to pollinate the plants.

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In addition to bees, butterflies and moths are also common pollinators of certain types of flowers. Butterflies are attracted to brightly colored, sweet-scented day flowers with nectar guides, while moths pollinate pale or white flowers that bloom during the late afternoon or night.

Surprisingly, some species of flies are also pollinators. These flies are attracted to flowers that have an odor of rotten meat and are fleshy in color. The flies mistake these flowers for carrion and approach them looking for food or a place to lay their eggs, becoming covered in pollen in the process. Other flies feed on pollen or nectar and are active pollinators.

Some insect-pollinated flowers use deceptive pollination, where the flower attracts pollinators but offers no reward. Carrion flowers are one example, while some orchids use sexual deception by mimicking the female partner of a pollinator and producing a scent similar to corresponding pheromones.

Birds are also important pollinators for many angiosperms. Their flowers are brightly colored, usually yellow or red, and have fused petals that form a tubular shape to fit the bird's beak, such as hummingbirds. While birds do not have a well-developed sense of smell, these flowers produce high quantities of nectar that birds need to maintain their metabolic rates. The pollen covers the bird's head and neck as it feeds on the nectar.

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Bats are another important pollinator, particularly in desert and tropical regions. As they are nocturnal, the flowers they pollinate have some characteristics in common with moth-pollinated flowers. They tend to be open at night, with a pale or whitish coloration and a fruity scent. However, because bats have a large head, the flowers are usually large and have a wide opening or are arranged in a wide base. They also produce a lot of nectar, which the bats need to sustain their energy levels. The pollen attaches to the fur on the bat's head as it feeds on the nectar.

Some of the plants that rely on bat pollination include agave, guava, and cacti. Bats are particularly important for the pollination of agave, which is used to produce tequila. Without bat pollination, agave plants would not be able to reproduce, and the production of tequila would be affected. Overall, bats play a crucial role in pollinating many plant species and maintaining the balance of ecosystems.

Mechanisms of pollination

There are two mechanisms by which pollination can occur: self-pollination and cross-pollination. Self-pollination occurs when pollen is transferred to the stigma of the same flower or another flower on the same plant. While this mechanism may decrease genetic diversity, it can be beneficial when pollinators are scarce.

Cross-pollination, on the other hand, occurs when pollen is transferred to the stigma of a flower on another plant of the same species. This mechanism ensures a higher genetic diversity, as the sperm and eggs come from different parents. Many plants rely solely on cross-pollination and have evolved different ways to avoid self-pollination. For example, some plants have stamens and carpels that mature at different times, or they have different lengths or positions, which prevents the anthers from touching the stigma. Some plants produce unisexual flowers, and female and male flowers can be separated on different plants. In many plants, the pollen cannot germinate if the pistil recognizes that it comes from the same plant (self-incompatible).

Overall, both self-pollination and cross-pollination are important mechanisms for the reproductive success of plants, and different plant species have evolved varied strategies to ensure pollination occurs in the most effective way.

Flowers that attract pollinators

Here we give a few examples of native flowers of the United States and the animals they attract (Table 1), for each major group of pollinators:

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Examples of other pollinators

We have mentioned several examples of common pollinators and the flowers they pollinate. However, there are some lesser-known pollinators.

Mammals: lemurs in Madagascar, tropical mammals like bush babies and sugar gliders, rodents, Australian marsupials. Invertebrates: ants, true bugs, gnats, and slugs. Reptiles: some lizards, geckos, and skinks.

Examples of coevolution between a pollinator species and a corresponding flower

Yucca plants have developed a unique partnership with a specific group of moths known as yucca moths. Interestingly, each plant species has its own specific moth pollinator. Unlike other pollinators, the yucca moths do not feed on the nectar or pollen of the yucca flower.

The female yucca moth collects pollen from several yucca flowers, ensuring cross-pollination, and then selects one flower in which to lay her eggs, inside the ovary of the flower. The moth then pollinates the flower by depositing the collected pollen on the stigma. Once the moth larvae hatch, they feed on some of the yucca seeds, which allows the plant to disperse its seeds more effectively. This partnership is so specific that the yucca moths have evolved specialized mouthparts to collect pollen, and they do not eat as adults.

Moreover, natural selection has favored moths that do not lay too many eggs, as the flower would abort and the plant would not reproduce. This relationship illustrates how plants and pollinators can evolve together, forming a close partnership that benefits both parties. The yucca plant relies on the yucca moth for pollination, while the yucca moth relies on the yucca plant for food and a place to lay its eggs. This is a fascinating example of coevolution and the intricate relationships that exist in nature.

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There is an endemic orchid in Madagascar called Angraecum sesquipedale. The petals of the orchid form a very long tubular corolla. It is pollinated by the moth Xanthopan morganii praedicta with a proboscis the same length. When Darwin saw this orchid, he predicted the existence of a moth with a proboscis of equal length, though he never observed it. The moth was discovered 20 years after Darwin’s death.

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Importance of pollination and pollinators in plants

Angiosperms are the most diverse group of plants, comprising over 290,000 species, which represent approximately 90% of all plants. Pollination is a critical process in the sexual reproduction of flowering plants. After pollination, fertilization takes place, leading to the formation of seeds and fruits that contain the next generation of plants. Animal pollinators are essential for the survival of about 80% of flowering plants globally.

The importance of pollinators goes beyond their role in crop pollination. Nearly 80% of the crops that provide us with plant-based food, products, and materials depend on animal pollinators. In addition, other angiosperms not directly used for consumption provide essential ecosystem services, such as oxygen production, carbon dioxide absorption, prevention of soil erosion, cultural significance, maintenance of animal populations, and more. Pollinators maintain plant genetic diversity through cross-pollination, which enhances ecosystem resilience to environmental changes.

Most plant species have coevolved with a specific group of pollinators, making the conservation of native plants and pollinators crucial for ecosystem persistence. Therefore, it is essential to protect and conserve pollinators by preserving their natural habitats and addressing the factors that threaten their populations, such as habitat loss, pesticide use, and climate change. By doing so, we can ensure the survival of not only plant species but also the entire ecosystem and its many services.

Pollination - Key takeaways

Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. There are two main types of pollination: abiotic pollination and biotic pollination. Abiotic pollination uses nonliving methods such as wind and water to move pollen from one flower to another, while biotic pollination uses living agents such as animals, birds, and bats to move pollen from one flower to another.

Wind-pollinated flowers typically have small, inconspicuous flowers with no petals or nectar, and the anthers and stigmas are often held outside the flower. The flowers are usually arranged in a way that maximizes the amount of pollen that can be released into the air. The plants also tend to be taller than animal-pollinated plants, so that the pollen can be released higher into the air.

Animal-pollinated flowers usually have large, colorful petals and produce nectar to attract pollinators. The anthers and stigmas are often held inside the flower, and the flowers are usually arranged in a way that makes it easy for pollinators to access the nectar and pollen. The plants are usually shorter than wind-pollinated plants, so that the flowers are more accessible to pollinators.

Most plants use cross-pollination instead of self-pollination because it increases the genetic diversity of the offspring, which can lead to increased fitness and better adaptation to the environment.[4] Cross-pollination also increases the chances of successful pollination, as it increases the number of potential pollinators. Self-pollination, on the other hand, can lead to inbreeding depression, which can reduce the fitness of the offspring.

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Pollination

What attracts pollinators to flowers?

Pollinators are attracted to flowers by their color, shape, size, scent, and most of the time, by food rewards (nectar and pollen). These flower traits are directly related to the type of pollinator that pollinates a plant.

What is pollination?

Pollination is the transfer of pollen from the male reproductive structure to the female reproductive structure in seed-producing plants. In gymnosperms, the transfer is from a male cone to a female cone. In angiosperms, it is from an anther to the stigma of another flower (in self-pollination it is to the stigma of the same flower).

Why is pollination important?

Pollination is a crucial step in the sexual reproduction of flowering plants. After pollination occurs, fertilization takes place which results in the production of seeds and fruits, that contain the next generation of a plant species. 

Is pollination and reproduction the same?

No, pollination and reproduction are not the same. Pollination is a step in the sexual reproduction of seed-producing plants where the pollen is transferred from the male reproductive structure to the female reproductive structure. After pollination, fertilization must occur to produce a zygote inside the seed contained in a fruit. The zygote grows into a new plant that starts the reproduction cycle again when mature.

What happens during the pollination process?

During the pollination process in angiosperms, the pollen is transferred from the anther of a flower to the stigma of another flower (sometimes to the same flower). The pollen is moved between flowers through abiotic (non-living agents like wind and water) or biotic (living agents, in this case, animals) pollinating agents. Pollinators are attracted to flowers by their color, shape, size, scent, and most of the time, by food rewards (nectar and pollen).

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