Lesson Video: Pollination Science • Second Year of Preparatory School

Video Transcript

In this video, we will learn how to define pollination and take a look at the different types of pollination: self-pollination and cross-pollination. We will also investigate how flowering plants can be pollinated by different methods, such as by wind, insects, or even humans via artificial pollination. And we will describe some of the adaptations plants have for wind and insect pollination specifically.

You may have heard of pollen grains before in the context of allergies. Have you ever wondered why some people are strongly affected by pollen grain allergies? Plants like trees and grasses release pollen grains that can be inhaled into our bodies. In people with such allergies, the body mistakenly treats the pollen grains, which are actually harmless, as if they were dangerous trespassers. In response, the body produces some unpleasant symptoms like coughing, sneezing, and itchy red eyes.

But why do plants release pollen grains? To answer this question, we first have to discuss what pollen grains are. Pollen grains are plant structures that are involved in sexual reproduction in flowering plants. Let’s take a closer look at this process. You may remember that sexual reproduction requires two gametes. Gametes are an organism’s reproductive cells, which contain half the genetic material of a normal body cell. For sexual reproduction to occur, a male gamete and a female gamete fuse together in fertilization. This forms a cell called a zygote, which can then develop into a new organism.

What are the gametes of a flowering plant, and where can they be found? Gametes of flowering plants are actually called the same as in humans. The female gamete is called an egg cell, and the male gamete is called a sperm cell. Pollen grains are protective structures that contain the male gametes. Gametes are produced in flowers, which contain all the reproductive parts of a flowering plant.

Let’s take a look at some typical flower structures by looking at a closer view of a cross section through this flower. Flowers often contain petals, which are modified leaves that may be colored and sometimes scented. Within the petals, many flowers contain both male and female sex organs. If a flower like this one contains both male and female sex organs, it is called a bisexual flower and can produce both male and female gametes. Unisexual flowers are those that have only male or female sex organs but not both.

First, let’s discuss the composition of the male sex organs, where the male gametes are formed, by looking at a bisexual flower. Each male sex organ in a flower is called a stamen, which is made up of a filament and an anther. The filament is a stalk that supports the anther. The anther is the male structure in which pollen grains, which you’ll recall contain the male gametes, are produced. When pollen grains have finished developing, the anther splits lengthwise. This allows the mature pollen grains to burst out of the anther, as we can see here.

Now, let’s investigate the female sex organs of a flower in more detail. Each female sex organ in a flower is called a carpel, or pistil, and it includes structures called the stigma, style, and ovary. In order for a sperm cell to reach the egg cell, pollen grains containing the sperm cells need to be deposited onto the stigma. Then, the male gametes within the pollen grains can access the egg cells, one of which is located in each ovule within the ovary.

Earlier, we described that the splitting of an anther causes pollen grains to burst out of it. Then, pollen grains can be transferred from a male anther to a female stigma on the same flower, a different flower on the same plant, or a flower on a different plant of the same species. This process is called pollination. From the stigma, sperm travels down the style to reach the ovary, where the ovules are found, so the egg cells within them can be fertilized.

Let’s take a look at the different types of pollination in more detail. There are two main types of pollination: self-pollination and cross-pollination. Let’s look at self-pollination first.

Self-pollination is the process by which a pollen grain is transferred from an anther to a stigma on the same plant, which means that a sperm cell within a pollen grain will fertilize an egg cell of the same plant. Self-pollination can only occur in bisexual flowers or plants that have unisexual male and female flowers in flower at the same time. There are two main ways in which a plant can be self-pollinated. Pollen grains can be transferred from an anther to the stigma within the same flower. Alternatively, pollen grains can be transferred from an anther to the stigma of a different flower but on the same plant.

Self-pollination produces offspring that are almost identical to the parent plant. This means that there is little variation in the genetic material of offspring produced by self-pollination. This is not ideal, as variation in genes makes organisms more flexible to changes in their environment. For example, if a plant which has little resistance to cold weather self-pollinates, its offspring would also have little resistance as they are almost genetically identical. This means if an extremely cold winter struck, neither the parent plant nor its offspring would survive.

To increase genetic variation in their offspring, many flowering plants use cross-pollination. Cross-pollination is when a pollen grain is transferred from an anther of one plant’s flower to a stigma of a flower on a different plant of the same species. This means that a sperm cell will fertilize an egg cell of a different plant with different genetic material. Cross-pollination can occur with both bisexual and unisexual flowers. Cross-pollination is more likely to increase the genetic variation in the offspring it produces than self-pollination. Therefore, the offspring produced by two different plants may have a greater chance of survival.

There are several different mechanisms by which self-pollination or cross-pollination may occur. Let’s look into this next. Some plants rely on animals to transport their pollen grains from an anther to a stigma. The animal that does this is called a pollinator. The most common pollinators are insects, like bees, flies, butterflies, and beetles. However, other animals such as birds and even mammals, like bats and humans, are capable of being pollinators. Let’s focus on insect pollination specifically and how a plant’s flowers may be adapted for it.

An adaptation is a characteristic that a specific organism has, which makes it well suited to its environment, often increasing the chance of survival. If a flowering plant is adapted to a specific type of pollinator, it may increase the chance of that plant successfully reproducing. A pollinator would not usually visit a flower unless it had a good reason. Some flowers entice pollinators like insects by producing nectar. Nectar is a sugary fluid that encourages insects to visit a flower, especially bees, which use nectar to make honey. Some insects like bees also actively collect the pollen as it is highly nutritious.

When insects land on, or crawl into, a flower to access their nutritious nectar, some pollen grains are likely to rub off from the flower’s anthers onto the insect’s body. Then, if this pollinator visits another flower for some more nectar, the pollen grains on their body can be rubbed onto this different flower’s stigma. In insect-pollinated flowers, the anthers and stigma are often found inside the flower in front of the nectar. This enables the anthers to deposit as many pollen grains as possible onto the pollinators’ bodies when they contact the flower. It also makes it more likely for pollen grains to land on the stigma, which is often sticky.

How does an insect like a bee know that a flower might contain nectar? Well, flowers have other adaptations which advertise to specific pollinators that they contain a tasty reward. For instance, insect-pollinated flowers tend to have especially large and brightly colored petals. This allows pollinators with good eyesight to distinguish them from the mostly green vegetation surrounding them. This visual signal publicizes that this flower might have nectar for the insect to eat.

Incredibly, some flower petals even appear to have landing pads for insects or markers believed to function like runways at an airport, thought to direct insects like bees toward nectar. You may have noticed that some flowers, like lilies or roses, often smell pleasant. This is because petals can sometimes be scented to attract pollinators.

As insect-pollination is efficient, flowers that use this method tend to only produce a moderate amount of pollen grains. These pollen grains are often sticky or spiky so that they can attach easily to the pollinator. The same pollinator may pollinate the flowers of many different plant species. A particular species of flowering plant may also be pollinated by different pollinators. However, a flower will only be successfully pollinated when pollen grains from a plant of the same species land on its stigma.

Some plants can cross-pollinate without relying on pollinators. This saves the energy they would otherwise use to produce nectar and colorful, scented flowers. Species like grasses and some trees like oak and beech rely on wind or air for successful cross-pollination. This is called wind pollination. Let’s compare some of the typical adaptations of wind-pollinated flowers to those of insect-pollinated flowers in a table.

As wind does not specifically deposit pollen grains onto another flower like pollinators do, wind-pollinated plants produce a large amount of pollen grains. It’s usually pollen grains from those plants that are blown by the wind around our environment, enter our bodies, and lead to the allergic reactions we described earlier. Wind-pollinated flowers tend to have many anthers, located on long filaments that often hang outside the flower so pollen grains can easily be picked up by the wind. The pollen grains themselves are usually smooth, dry, and light in weight so that they can be efficiently transported via wind without clumping together.

Like their anthers, the stigmas of wind-pollinated flowers tend to be very exposed and they are often feathery, with a large surface area. This helps them to catch pollen grains drifting in wind currents. As they don’t require pollinators to transport pollen, wind-pollinated plants do not usually waste energy in producing nectar. Their petals are typically dull or green in color and are usually unscented as they do not need to attract pollinators. Sometimes, wind-pollinated plants do not have petals at all.

We mentioned earlier that humans can sometimes be pollinators. This process, which is called artificial pollination, involves a human transferring pollen grains from the anther of a flower to the stigma of the same flower or another flower. Often, artificial pollination is carried out by gardeners or farmers to ensure that all of their plants are pollinated and to increase fruit production.

As humans are responsible for transferring pollen grains, artificial pollination removes the reliance on other pollination methods, such as wind or insects. Artificial pollination can also be carried out by scientists to control which plants are self-pollinated, cross-pollinated, or not pollinated at all. You can even try this with a plant at home, by transporting pollen grains using a paintbrush.

Let’s apply what we’ve learned about pollination to a couple of practice questions.

Which of the following adaptations would be most useful for a wind-pollinated plant to have? (A) Large and brightly colored petals, (B) sticky pollen grains, (C) large quantities of nectar, or (D) light and easily dispersed pollen.

Pollination is the process by which pollen grains are transferred from a flower’s male anther to a female stigma on the same flower or a different flower. From the female stigma, the male gametes within pollen grains can access the female gametes to fertilize them. There are a few different mechanisms by which pollination can occur, such as via wind pollination and insect pollination.

Different species of flowering plants are often adapted to their specific mode of pollination. For instance, insect-pollinated flowers typically have large and brightly colored petals and often produce large quantities of sugary nectar to attract insect pollinators. Wind-pollinated flowers do not need to attract pollinators, so they do not expend energy on producing such costly features. Therefore, options (A) and (C) are incorrect.

Many wind-pollinated flowers produce large amounts of pollen grains in numerous anthers that hang outside the flower to easily allow the pollen grains to be picked up by the wind. The pollen grains themselves are often light in weight, dry, and smooth. This allows them to be easily dispersed by the wind currents and transported to a stigma on a different flower. In comparison, the pollen grains in insect-pollinated flowers are usually sticky or spiky so that they can attach more easily to the body of a pollinator and are not produced in such large numbers. This means that option (B) is not a feature of wind-pollinated flowers.

This means that we know the correct answer to this question. The adaptation that would likely be found in the flower of a wind-pollinated plant is given by answer (D), light and easily dispersed pollen.

Let’s try another question together.

Plants can also be pollinated artificially. What does this mean? (A) Plants are pollinated by pollen from a different species. (B) Humans transport pollen from one plant to another. Or (C) plants are pollinated outside of their natural habitat.

Pollination is the process by which pollen grains are transferred from a flower’s male anther to a female stigma on the same flower or a different flower. From the stigma, the male gametes within pollen grains can access the female gametes within the flower’s ovules to fertilize them. Sometimes, humans transfer pollen grains from the anther of a flower to the stigma of the same flower or a different flower. This is called artificial pollination. Artificial pollination removes the reliance on other pollination methods, such as wind or insects.

For successful fertilization to occur, a plant must be pollinated by pollen from the same species, not a different species, regardless of whether artificial or natural pollination is occurring. This means option (A) is incorrect.

Artificial pollination can either occur within or outside of a plant’s natural habitat, which means that option (C) is incorrect.

This means that we know what it means for a plant to be artificially pollinated and thus the correct answer to this question. The correct answer is given by (B). Humans transport pollen from one plant to another.

Let’s recap some of the key points we have covered in this video about pollination. Pollination is the transfer of pollen grains from an anther to a stigma, either on the same plant or a different plant. Plants can either be self-pollinated or cross-pollinated. Pollen grains will be released as an anther matures. Plants are often specifically adapted for their pollination method, such as those that are wind-pollinated or insect-pollinated. Humans can artificially pollinate plants.