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Pollination

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Pollination

Why would a flower look and smell like a piece of rotten meat? Traits like color, shape, odor, and size, are used by flowers to attract pollinators. Depending on the pollinator, flowers have evolved the corresponding traits to attract them. Thus, reddish fleshy flowers that smell like rotten meat are pollinated by some types of flies, like blowflies. We describe the pollination types and mechanisms for angiosperms, the flower traits related to each type, and the importance of pollination in the reproduction of flowering plants.

Pollination definition

Plants can reproduce sexually and many can also reproduce asexually. Sexual reproduction occurs through the fusion of two gametes to form a zygote. But adult plants are sessile, meaning they cannot move. How do plants find a “mate” to reproduce sexually? This is the function of pollen. In seed-producing plants (gymnosperms and angiosperms), pollen grains are the reproductive structures that produce the male gametes (sperm). Therefore, pollen must move and reach the female reproductive part of the plant that contains the female gametes (eggs). This is known as pollination.

We will be focusing on angiosperm (flowering plants) pollination. In angiosperms, all sexual reproductive structures are found in the flowers.

Stamens are the male parts, and the pollen is produced in the anthers. Pistils or carpels are the female parts, and the eggs are produced inside the ovules that are contained in the ovary. Thus, pollination in angiosperms consists of the transfer of pollen from the anther to the stigma of a flower.

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

About 20% of flowering plants rely on abiotic pollinating agents (98% by wind, and only 2% by water). Flowers that rely on wind for pollination do not need to attract pollinators; thus, their flowers are usually inconspicuous and small, with green or brownish coloration, sometimes lacking petals (Figure 1). For the same reason, they typically do not produce nectar (a solution of water and sugars) and have no scent. The stamens are usually well-exposed to facilitate pollen dispersion and the stigma can be feathery to catch pollen more easily.

Pollination Wind-pollinated flowers StudySmarter

Figure 1: A wind-pollinated grass with a cloud of pollen. Source: Alex Jones, CC0, via Wikimedia Commons

Wind-pollinated plants have developed other traits to facilitate this type of pollination. They produce pollen in abundance (causing pollen clouds), to increase the probability of landing on another individual of the same species. In temperate species, the flowers usually appear early in the spring. This way, new leaves have not yet developed and cannot block pollen movement.

Grasses, cottonwood, birch, and oaks are wind-pollinated. Most gymnosperms are also wind-pollinated. Because of the copious amount of pollen these plants produce to ensure pollination, they are related to pollen allergies.

Few angiosperms use water as a pollinating agent. In some aquatic grasses and weeds, the pollen floats on the water until it reaches a flower.

Biotic pollination

Most angiosperms (80%) rely on animal agents for pollination, including a wide variety of insect, bird, and bat species. The most common pollinators are insects, which pollinate about 65% of angiosperms. Flowers attract pollinators with their color, shape, and scent, and reward them most of the time (some attract pollinators but give them no reward!) by offering food in the form of nectar and pollen.

Although most flowers can attract different pollinators, and many pollinators are therefore generalists, some flowers have evolved traits to attract a specific group of pollinators and, sometimes, even a specific pollinator species. Having a specific pollinator can be more efficient than a generalist one because the pollinator transfers the pollen directly to another flower of the same species (instead of visiting several species, wasting some of the pollen). Thus, specialized pollinators can be favored by natural selection. This provokes both the pollinator and the pollinated flower to develop corresponding traits that make the pollination specific to them (like tubular flowers with specific shapes to accommodate birds’ beaks or bats’ tongues). This is called coevolution.

Coevolution is the evolution between a pair of species that interact in a long-term relationship, where one species exerts a selective pressure over the other.

Insect pollinators

Bees are the most common pollinators and most crop plants used for human consumption rely on them for pollination. Flowers pollinated by bees are typically brightly colored (yellow, blue, or purple; bees cannot see red) and have strong fragrances (Figure 2). Besides the color, flowers can have nectar guides to help guide bees directly to the center of the flower. These guides can be small structures in the petal or ultraviolet marks that form a path. Bees can see ultraviolet colors contrasting with the rest of the flower. The pollen attaches to the fuzzy body of the bees.

Pollination Insect-pollinated flowers StudySmarter

Figure 2: Insect-pollinated flowers. A bee pollinating an aster flower (left) and a fly on a carrion flower (right). Source: left, Shari Grenier; right, Larry Deack, both images Public domain, via Wikimedia Commons.

Butterflies are also common pollinators of brightly colored, sweet-scented day flowers with nectar guides. Moths, on the other hand, pollinate pale or white flowers that are open during the late afternoon or night (see figures 4 and 5).

Although we do not think of flies as pollinators, some species are. These flowers are characterized by an odor of rotten meat and are fleshy colored. Flies mistake them for carrion and approach looking for food or to lay eggs, becoming dusted with the pollen. Some flies also feed on pollen or nectar (flower flies) and are active pollinators.

Carrion flowers are an example of “deceptive pollination” in insect-pollinated flowers, where a flower attracts pollinators, but gives no reward (e.g., no meat to feed the fly or its larvae). Other flowers (mainly orchids) use sexual deception, where the flower looks like the female partner of a pollinator and even produces a scent similar to the corresponding pheromones. The male pollinator approaches and tries to mate, getting dusted with pollen in the process.

Bird pollinators

Numerous angiosperms are pollinated by birds. Their flowers are brightly colored (usually yellow or red) and the petals are often fused into a tubular shape to fit the bird’s beak (e.g., hummingbirds, see Figure 3). They are not heavily scented, as birds’ sense of smell is not well developed, but they produce high quantities of nectar that birds need to maintain their metabolic rates. The pollen covers the bird’s head and neck.

Pollination Hummingbird pollinating a flowers StudySmarter

Figure 3: A Ruby-throated hummingbird pollinating a flower. Source: freko1947, Public domain, flickr.com

Bat pollinators

As bats are nocturnal, the flowers they pollinate share some characteristics with moth-pollinated flowers. They are open at night, have a pale or whitish coloration, and are aromatic, commonly with a fruity scent. But because of the large bat head, they are usually large flowers with a wide opening or group of flowers arranged in a wide base. They also produce a lot of nectar. The pollen attaches to the fur in the bat’s head. Bats are especially important pollinators in desserts and the tropics, pollinating plants like agave, guava, and cacti.

Mechanisms of pollination

Pollination can occur by two mechanisms:

  • Self-pollination: when the pollen is transferred to the stigma of the same flower or another flower of the same plant. Although this mechanism may decrease genetic diversity, it can be beneficial if pollinators’ abundance decreases.
  • Cross-pollination: when the pollen is transferred to the stigma of a flower in another plant of the same species. This mechanism ensures a higher genetic diversity, as the sperm and eggs come from different parents. Many plants only cross-pollinate and avoid self-pollination in different ways. The stamens and carpels can mature at different times (thus pollen release and stigma receptivity are not synchronized), or they can have different lengths or positions (thus the anthers do not touch the stigma), many plants produce unisexual flowers, and female and male flowers can be separated in different plants. In many plants, the pollen cannot germinate if the pistil recognizes that it comes from the same plant (self-incompatible).

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:

Table 1: Examples of pollinators and the native flowers they pollinate.

Pollinator group

Examples

Flowers

Insects

Bumblebees

Tomato, peppers, and chokecherry flowers

Bees

With ultraviolet nectar guides: buttercups, black-eyed Susans, dandelions

Butterflies

Violets, serviceberry, chokecherry

Sphinx moths

Columbine, honeysuckle, Yellow Evening Primrose

Beetles

Yarrow, sunflowers

Hoverfly

Golden currant, rabbitbrush, sunflowers

Blowflies

Carrion flowers (Stapelia)

Pollen wasps

Penstemons, phacelias

Birds

Hummingbirds

Honeysuckle, columbines, trumpet creepers

Bats

Bats

Guava, banana, mango, saguaro and other cacti flowers

Lesser

long-nosed bat (Leptonycteris

curasoae yerbabuenae)

Agave and cacti flowers

Source: USDA, https://www.fs.fed.us/wildflowers/pollinators/

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 evolved a close partnership with a group of moths commonly called yucca moths (figure 4). Each plant species has its own moth pollinator species. This is an interesting case because the yucca moths do not feed on the yucca flower’s nectar or pollen.

A female moth collects pollen from a few yucca flowers (ensuring cross-pollination) and then chooses one on which to lay her eggs (inside the ovary of the flower). Then she pollinates the flower depositing the pollen on the stigma. When the moth larvae are born, they feed on some of the yucca seeds. This partnership is so specific that these moths have evolved specialized mouthparts to collect the pollen (and not for eating, adult yucca moths do not eat). Furthermore, natural selection has selected against individuals that lay too many eggs, as the flower would abort and the plant wouldn’t reproduce.

Pollination Yucca flower and its moth pollinator StudySmarter

Figure 4: A yucca moth pollinating a yucca flower. Source: Joshua Tree National Park, Public domain, Flickr.com

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.

Pollination Angraecum orchid and its moth pollinator StudySmarter

Figure 5: The orchid Angraecum sesquipedale (left) and its moth pollinator (right). Source: left, Biodiversity Heritage Library, Public domain, Flickr.com; right, Nesnad, CC BY 4.0 , via Wikimedia Commons.

Importance of pollination and pollinators in plants

Angiosperms, with more than 290,000 species, represent about 90% of all plants. Pollination is a crucial step in the sexual reproduction of flowering plants. After pollination occurs, fertilization takes place, resulting in the production of seeds and fruits that contain the next generation of plants. With 80% of flowering plants relying upon animal pollinators, the maintenance of these symbiotic relationships is critical for plant survival.

Globally, almost 80% of the crops that provide us with all our plant-based food, products, and materials, are pollinated by animals. And this does not consider the ecosystem services provided by other angiosperms not directly used for consumption, but that are part of the ecosystems. These services include oxygen production, carbon dioxide absorption, prevention of soil erosion, cultural significance, maintenance of animal populations, and more. Pollinators ensure the maintenance of plant genetic diversity through cross-pollination, which enhances ecosystem resilience to environmental changes.

Since most groups of plants have coevolved with a specific major group of pollinators (besides the specific pairs of plant species-pollinator) conservation of native plants and pollinators is crucial for ecosystem persistence.

Pollination - Key takeaways

  • Pollination is a step in the sexual reproduction of seed-producing plants (gymnosperms and angiosperms).
  • Pollen grains rely on abiotic (non-living agents like wind and water) or biotic (living agents, in this case, animals) agents to reach another flower.
  • The specific traits a flower presents (color, size, shape, scent) are related to the corresponding type of pollination agent.
  • Pollen grains of wind-pollinated flowers are small, non-sticky, light-weighted, and produced in copious amounts, while the pollen of animal pollinated flowers is usually heavier, sticky, and produced in less quantity.
  • Wind-pollinated flowers are typically inconspicuous (small with green or brownish colors), sometimes lacking petals, unscented, do not produce nectar, and usually with well-exposed stamens and a feathery stigma.
  • Animal pollinated flowers usually have bright colors (except for nocturnal pollinators), strong scents (except for birds), and produce nectar as a reward (in larger quantities for birds and bats).
  • Most plants use cross-pollination instead of self-pollination because it increases genetic diversity.

References

  1. Lisa Urry et al., Biology, 12th edition, 2021.
  2. Mary Ann Clark et al., Biology 2e, Openstax web version 2022
  3. United States Department of Agriculture. Pollinators. https://www.fs.fed.us/wildflowers/pollinators/ Image link
  4. Figure 5, right: https://commons.wikimedia.org/wiki/File:Xanthopan_morganii_praedicta_Sep_16_2021_03-58PM.jpeg

Frequently Asked Questions about Pollination

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.

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).

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. 

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.

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).

Final Pollination Quiz

Question

Pollination is related to: 

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Answer

plant sexual reproduction

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Question

Which of the following are abiotic agents of pollination? 

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Answer

non-living agents

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Question

Which of the following are flower traits related to wind-pollination? 


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Answer

bright coloration

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Question

Which of the following are flower traits related to biotic pollination?


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Answer

feathery stigma

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Question

.Which of the following options shows the correct order from the most common pollination type to the less common one, in angiosperms?

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Answer

wind, water, biotic

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Question

Because of the coevolution of traits between a flower and its pollinator, what do you think would happen if the pollinator goes extinct?


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Answer

The plant would probably go extinct too if it is a highly specialized relationship. The plant can have other pollinators, but their visits are probably less frequent or they do not successfully pollinate the plant, thus the reproduction success of the plant population could decrease enough to endanger the population. 

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Question

Why do many plants avoid self-pollination? 


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Answer

because it decreases genetic diversity 

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Question

In what ways can a plant avoid self-pollination? 


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Answer

By not synchronizing pollen release and stigma receptivity, positioning anther and stigma at different lengths, self-incompatibility, production of unisexual flowers, or separation of female and male flowers in different plants. 

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Question

Bee pollinated flowers usually present these traits: 


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Answer

pale or white color

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Question

If a flower has a pale or white coloration, is aromatic, and is open at night, it is most likely pollinated by: 


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Answer

bees

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Question

Which of the following are traits of flowers pollinated mostly by birds?


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Answer

yellow and red colors

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Question

This is a trait shared by flowers pollinated by birds and bats: 


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Answer

bright colors

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Question

Gymnosperms are mostly pollinated by____ while angiosperms are mostly pollinated by____. 


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Answer

wind, animals

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Question

Pollination is critical for plant reproduction because it enables: 


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Answer

fertilization

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Question

Describe one example of “deceptive pollination”, when a flower attracts a pollinator but gives no rewards. 


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Answer

Carrion flowers that attract flies through an odor of rotten meat and with a reddish or brownish coloration. 

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