Hi everyone! In this video I'm going to get into a little more detail on the process of pollination and take a closer look at how flowers are specialized for particular types of pollinators. So we know that the flower is going to be used for pollen transfer and pollination refers to the point when a pollen grain containing the microgametophyte actually lands on the stigma of the carpal on a flower.
That could potentially be self-pollination, which is when the pollen grain is transferred from an anther to a stigma on the same plant. It's actually usually on the same exact flower. It's fairly easy for that to happen since the anthers and the stigma are right next to each other, so it's not that hard for pollen to just land on the stigma directly from the anthers right next to it. So that's one possibility.
Another possibility is to do cross-pollination, where pollen from one plant will have to travel and land on the stigma on flowers on a different plant. This is essentially reproduction between different individuals. Cross-pollination will require some kind of method to transport the pollen from one plant to another plant.
That could be wind, as it usually is in the gymnosperms and sometimes in the angiosperms, or it could be an animal who would actually pick up the pollen and move it from one plant to another. Both of these strategies have some advantages. As we know, there are always trade-offs in biology, so see if you can come up with some ideas about the benefits and costs of self-pollination versus cross-pollination.
Most plants will have various strategies that they use to try to promote cross-pollination between different plants. So first of all, if you think about flower structure, do you think cross-pollination is more likely with perfect flowers or imperfect flowers? And if you have imperfect flowers, do you think cross-pollination is more likely if the plant is monoecious or dioecious?
Think about each scenario and whether you would be more likely to get self-pollination or more likely to get cross-pollination. Even if you do have perfect flowers on a plant, you can still try to prevent self-pollination in a few different ways. For example, even if a flower will develop both stamens and carpels, those don't necessarily have to develop at exactly the same time. They could mature at slightly different rates so that they are not both active at the same time. For example, here we are looking at a flower where it develops stamens first and carpels later.
So young flowers, which would be located towards the top of the plant where it's actively growing upwards, are going to have stamens with pollen on them, but you can see the stigma of the carpal is still closed up. As the flowers age, the pollen will fall off of the stamens and then the stigma will open up. So the stamen matures first and the carpal matures later.
The reason this prevents self-pollination is because when an insect pollinator approaches the plant, they often have a stereotyped flight path that they will use. They always approach it in the same way. For example, a bee going to collect pollen from this plant would always start at the bottom of the plant. When it flies to the bottom of the plant, it's going to encounter older flowers which have open carpels. As it travels up the plant, it will start to encounter younger and younger flowers which will have stamens on them.
So it will start to pick up pollen as it reaches the top of the plant. Once it hits the top of the plant, it's then going to leave and go to a second plant, and when it approaches the second plant, it will again start at the bottom, where we'll encounter flowers with mature. So it will carry pollen from the top of one plant with the young flowers to the bottom of the next plant with the older flowers.
The pollen will rub off on those older flowers and then by the time it gets to the top plant it can pick up new pollen. So by having the stamens mature first and the carpels mature later, we can ensure that the insect is cross-pollinating the flowers from younger flowers to older flowers as it approaches a new plant. Even if the stamens and carpels do mature at the same time, you can still prevent self-pollination by having flowers be self-incompatible.
In order for pollination to occur, it's similar to what we saw with the acrosome in animal fertilization. In order for the pollen to successfully germinate down the stigma into the style, there are proteins that have to match so that the pollen can bind to the stigma. And it is possible for a plant to be genetically incapable of mating with itself if the proteins on the stigma and the pollen don't match up properly. So there are a lot of ways to promote cross-pollination through the flower structure, whether the plant is monoecious or dioecious, even in a perfect flower having the parts mature at different times, or by preventing the stigma from successfully binding the pollen.
If you are going to do cross-pollination, that will require pollen to be transferred from one plant to another. And there's a couple of different ways that that could happen. You could either use wind to transfer pollen, or you could have an animal pick up pollen and actually move it from one plant to another.
Again, there are always trade-offs, so there are costs and benefits of each of these. What do you think is a possible advantage of wind pollination? But why do you think also that animal pollination might be much more common? We do see animal pollination in the majority of species, which suggests that it has a major advantage. So what do you think the advantage of wind pollination is?
What do you think the advantage of animal pollination is? You are going to see flowers specialized for these different forms of pollen transfer. So by looking at the flower in terms of its shape, its appearance, its color, its smell, you can figure out how it is likely pollinated. We call these pollination syndromes, and they're not 100% reliable, but are pretty easily recognizable as a flower that uses a particular form of pollination. We'll start with wind pollination.
Wind pollination is done in about 20% of angiosperms, in particular a few trees, and most notably the grasses. So grasses, as you can see here, are going to be pollinated largely by wind and will not use animal pollinators. Wind pollination is relatively inefficient in comparison to animal pollination.
As a result, wind pollinated plants have to produce really large amounts of pollen. because they're not exactly sure where the pollen is going to end up. It's a little bit unreliable, so you have to produce a lot as extra insurance that some of the pollen will actually end up getting transferred.
They also have to live relatively close together as a population because wind doesn't carry pollen that far. The pollen will have to be very lightweight and picked up by the wind to be traveled around. But it can't go very far distances, so plants that are wind pollinated have to live in fairly close populations.
Since wind pollination requires you to produce a lot of pollen and have it get picked up and blown around, it is usually these plants that are responsible for our allergies, because it is wind pollen that is being carried around in large amounts and is very lightweight and can get into our respiratory tracts and cause those allergies. When you look at the flower of wind pollinated plants, usually the flowers are going to be fairly small, often with reduced petals and no scent to them. This is because the plant is not trying to attract a particular pollinator and as a result is not going to spend energy on those attractive structures like colorful petals or a scent.
They're gonna save some energy on the flower where they can. Animal pollination, in contrast to wind pollination, is actually a lot more common, so the majority of angiosperms are going to use animal pollination. In an animal pollinated plant, the flower will be specifically designed to attract a specific type of pollinator.
So again, you can identify the pollinator by looking at the flower. In exchange for the pollination service provided by the animal, where it is transferring pollen to help with fertilization, the animal is going to get a benefit which is food of some kind. So this is usually a mutualistic relationship. The plant benefits through sperm transfer and the animal benefits by getting food. That food could be in the form of some pollen itself, so it will transfer some pollen but also eat some pollen, or it could be in the form of nectar which is a sugar solution that the plant produces specifically to feed the pollinator.
The reason the animal does this is not to help the plant reproduce, it's to get this food reward out of the relationship. In animal pollinated flowers, we usually do see very close co-evolution between the traits of the flower and the traits of the pollinator. The flower will have higher fitness if it can successfully attract a pollinator and transfer pollen very efficiently to that animal. So for example, in this flower you can see the stamens are arranged in such a way that when a bee crawls into the flower to get its meal, the stamens will rub pollen all over its fuzzy little butt and accomplish a huge amount of pollen transfer, which is going to enhance the fitness of the flower.
Conversely, the pollinator will also have higher fitness if it can successfully get a food reward from the flower. So you can see on this flower, it has a very, very long tube, which is going to have nectar at the base. In order to get into that long tube to access the nectar, you need a very, very long tongue, which is what this moth has evolved. So we will see close coevolution between the traits of the flower and the traits of the pollinator because this relationship increases the fitness of both partners. We'll start with the most common animal pollinator, which is bees.
Bees are a very widespread animal pollinator, and this is not only going to be honeybees like you see around, but also wild bees and things like wasps as well will fall into this category. You can recognize bee pollinated flowers by the fact that they are usually fragrant with a kind of strong sweet smell. Bees like sweet things, so that sweet smell will help attract them.
In terms of color, the flower will usually be quite bright, very often yellow, because yellow is a color that bees see really well. Besides the yellow though, there are usually also some patterns visible in the UV wavelengths on the flower. These are sometimes called nectar guides because they help guide the bee to the nectar within the flower. So a flower that looks to us like it is solid yellow may actually have a bull's eye on it or a runway on it to a bee's eyes.
This is due to differences in the cones that are in the eye of an insect versus the cones that are in our eyes. But you can see that the flower is guiding the insect exactly to where it needs to go. When the insect pollinates the flower, it will get covered in pollen. Some of that pollen the bee may collect to feed its own offspring or the offspring of the colony.
The pollen is usually not consumed by the bee right away. Instead, it will be taken back to the nest and given to offspring. The bee itself will feed on nectar as a food reward.
Bees have pretty short tongues though, so the flower usually has to be fairly open in shape and pretty shallow for the bee to be able to get into the nectar. Bee pollinated flowers are bright, yellow, shallow, and open, and then will also show up under UV. Next we have butterfly pollinated flowers, and these are going to be pretty different in appearance to bee pollinated flowers.
They will be fragrant, but it's a milder scent because butterflies are more cued in to visual signals than to odor signals. The color of the flower will be very fancy and showy and bright, often pink or purple, and this is again because butterflies are really visually oriented. They have lots of bright colors themselves that they use to signal for themselves, and they are attracted to bright colors in plants as well, especially things like pink and purple flashy sorts of appearance.
In terms of shape, the flower will often have a place where the butterfly could land, but on the inside will offer mostly nectar, since butterflies do have that tongue kind of mouth part to slurp up the nectar. On the inside of the flower, the shape will usually be tubular so that the tongue goes down into the nectar. So a butterfly pollinated flower would have more of a tubular interior than, say, a bee pollinated flower.
We can contrast that to moth pollinated flowers. Moth pollinated flowers are going to have an extremely strong smell. It's a very strong, sweet, vegetable kind of smell, the kind that would hit you in the face as you were walking by. really strong smell. In terms of color, they will be quite different from butterfly pollinated flowers.
Not purple, but something white or pale yellow. So very, very pale in color. I'd like you to think about why these would be good for a moth pollinated flower. Why do you want a strong smell?
Why do you want a pale white or yellow color if you are trying to attract a moth? But like butterflies, moths also are attracted mainly to nectar since they have that same kind of tongue mouth part, so they will acquire nectar as their food reward from the flower. Moving on to flies, flies have different kinds of lifestyles and as a result are attracted to different kinds of flowers. For daytime nectar feeding flies like this bee fly that you are seeing here The flowers they are attracted to will usually have a pale color and kind of an open shape and often will be quite small.
So small white, lilac, pale blue kinds of flowers are often fly pollinated and they will feed on nectar from the flower during the day. The other kind of fly pollinator is one that is attracted to a flower that mimics rotting meat. So it will have this vivid red pinkish kind of color. with what looks like mold coming off of it and a strong sickly kind of smell like rotting meat would have. The reason a fly would be attracted to this is because flies do, in some species, lay eggs in raw meat.
So they are looking for a place to lay their eggs and have their larvae hatch into maggots that would then digest the meat. This is not going to work out very well for the fly because of course if it lays its eggs in this flower, the eggs will have nothing to feed on because it is not in fact rotting meat. So this is an example of how the relationship between plants and animals is not always a mutualism and it can kind of shift depending on the pressures and the ways in which the two species are interacting with each other.
In this case the flower is essentially taking advantage of the fly and is basically using the fly in a parasitic relationship. It is not only insects that can do pollination, some vertebrates also can, including birds. Bird pollinated flowers are not going to have much scent to them at all, and this is because birds don't smell very well.
I told you previously, if you remember, that birds are one of the few groups that actually don't have a very good sense of smell, and so bird pollinated flowers often will not bother producing a scent. The color of bird pollinated flowers will be bright red or orange. This is a color that birds see very well, but it is not a color that insects see very well.
So the color is attractive to birds, but hides the flower a little bit from insects. The reason you want to hide these flowers from insects is because they contain really large amounts of nectar. Birds, of course, are much larger than insects, and as we know they are endotherms, that means they need a lot of food because their metabolism is a lot higher.
So in order for pollination to be worth it for them, they need to get a lot of food from the flower. So bird pollinated flowers have to offer a really substantial food reward. If an insect were to visit one of these flowers, it would get full really quickly and not have to visit a second flower. So it would just gorge itself and not actually do any pollen transfer. As a result, the plant wants to avoid insects visiting the flower, which is why it uses that.
red color. The shape of the flower will very often be tubular because birds that are pollinators are going to have a tubular kind of beak and you can see that in these flowers here where the beak is going to match the shape of the flower. One more pollinator is bats. Bat pollinated flowers will have some things in common with moth pollinated flowers and you can think about why that might be the case. So they will be very very strongly scented.
often with a fruity kind of smell because bats like fruit. The color will be white or pale yellow. But the big difference between moth pollinated flowers and bat pollinated flowers is that bat pollinated flowers are much bigger. This of course is because bats are much bigger and they don't have a really long tongue like moths do.
So in order for them to get the food reward from the flower, they have to actually just stick their whole little face in the flower. which means the flower needs to be pretty big. Now, flowers are made of leaves, and there is a problem with big leaves, which is evaporation.
Big flowers will also experience lots of evaporation. As a result, bat-pollinated flowers tend to close up during the day and then only open at night in order to prevent the greater amounts of evaporation that they would experience during the daytime. The food reward for the bat is usually going to be nectar. Again, because they are bigger and endotherms, they need a good food reward, and nectar has more nutritional content than pollen does. But they will also consume some pollen.
Usually they just kind of get it all over their little faces and then have to lick it off of their face. So you can see that there is a distinct difference between the flowers that are attractive to these different kinds of pollinators, and that is the result of coevolution. between the angiosperms and their animal pollinators. Thank you.