Video Transcript
In this video, we will learn how to identify the structures in a flower and relate these to the function of flowers in reproduction. We will also describe how gametes are formed in flowering plants, which are otherwise known as angiosperms. Finally, we will look at how pollination, fertilization, and fruit and seed formation can lead to the production and dispersal of new organisms.
Flowering plants are also called angiosperms. They represent 80 percent of the variety of the green plants on Earth. Angiosperms are characterized by their flowers producing seeds that are enclosed in a fruit. These seeds are the units of reproduction and are capable of developing into a new organism. As we’ll learn in this video, fruits are a great adaptation of flowering plants to spread their genetic information contained in the seeds on land via air, water, or animals to other locations. In order to understand more about how they reproduce, let’s first look at the basic structure of a typical angiosperms flower, one of which we can see as a cross section in the diagram here. Angiosperms can be male, female, or often both. This is because they’re capable of having both male and female reproductive organs contained within their flowers.
Reproductive organs are mainly responsible for producing gametes, which are otherwise known as reproductive or sex cells. The male reproductive organs produce the male gamete, pollen. These male reproductive organs are called the stamens. Each stamen consists of two parts, the anther that produces pollen and the filament that supports the anther. This flower has six stamens, but this number will vary depending on the species of angiosperm. All of the male reproductive organs together in a flower are called the androecium. This can be remembered as the prefix andro- means man. The female reproductive organs of a flower are both responsible for receiving the male gamete and for producing the female gamete, the egg cell. The stigma is an often sticky organ onto which pollen grains, which contain the male gamete pollen, are deposited in a process called pollination.
The style connects the stigma to the ovary. The ovary usually contains several ovules, which contain the egg cell. A distinguishing feature of angiosperms is that they are flowering plants whose ovules are contained within an ovary. After fertilization, the ovule tends to develop into a seed, while depending on the species of flowering plants, the ovary might develop into a surrounding fruit. All the female reproductive organs together in a flower are called the carpel or sometimes the gynoecium. We can remember this as the prefix gyno- refers to a woman or a female.
Flowers are arranged into whorls. Whorls are the circular arrangements of the different sets of flower organs surrounding the stem. Angiosperm flowers are based on four types of whorls, two of which we’ve looked at already. The gynoecium is a whorl that’s usually located right in the center of the flower, with another whorl, the androecium, surrounding it. Moving outwards from the androecium is a whorl of petals, known as the corolla. Petals are modified leaves that are often brightly colored and sometimes scented. This is helpful to attract pollinators that function to spread pollen to the female stigma, preferably on a different plant.
The calyx is the outermost whorl, and it consists of sepals. Sepals are modified green leaves that protect the young flower when it’s in its bud stage. This bud can protect the developing flower from a range of environmental pressures and when the flower blooms. The petals and other reproductive structures emerge out of the calyx of sepals.
Let’s have a look at how the male and female reproductive organs in a flower form their gametes. Gametes are formed through a type of cell division called meiosis. Meiosis is a process that halves the number of chromosomes in a normal diploid cell to produce haploid gametes. Haploid cells are often represented as 𝑛.
It is vital that gametes have half the genetic material of other body cells, as when a haploid male gamete fuses with a haploid female gamete in fertilization, this produces a diploid zygote, with a full set of chromosomes that’s often represented as two 𝑛. This process occurs slightly differently in the male and female reproductive organs of an angiosperm. So let’s get started by looking more closely at the production of female gametes. Remember that the female gamete in an angiosperm is the egg cell, which is produced in the ovule within a plant’s ovary. Let’s take a closer look at an ovule to see how this process occurs.
Egg cell production begins as spore mother cells develop within the ovule. Spore mother cells are diploid cells as they have a full set of chromosomes. A structure called a funicle or sometimes a funiculus connects the ovule to the ovary wall. The ovule is surrounded by an outer layer of integuments, in this case two integuments, but this number will vary between species. There is a small gap in the integuments called the micropyle that might eventually allow a male nucleus to access and fertilize the egg cell once it’s formed. But let’s get back to this formation process. The spore mother cell divides by meiosis. This forms four haploid cells called megaspores. Three of these megaspores degenerate, while the fourth, which is sometimes called the functional megaspore, grows and develops into the embryo sac.
The embryo sac, which is sometimes called the megagametophyte, is called the embryo sac as it’s destined to host the plant embryo after fertilization has occurred. The haploid functional megaspore within the embryo sac then divides by mitosis three times. This means that eventually eight nuclei are produced. Two of these haploid nuclei move to the center of the embryo sac. These are called the polar nuclei. The other six nuclei move to opposite poles or ends of the ovule. The nuclei at the poles of the ovule become enveloped by cytoplasm and a thin membrane, forming six distinct cells. The three cells that are located furthest from the micropyle are called the antipodal cells, while the three cells at the opposite pole of the ovule closer to the micropyle differ in their development. The cell in the center closest to the micropyle will develop into the egg cell, which when it’s grown sufficiently is ready for fertilization. The two cells either side of the egg cell develop into synergids.
Let’s see how the male gamete, pollen, is produced next. Pollen grains are produced in a flower’s anthers, which are where meiosis will occur in a similar manner to the development of the egg cell in the ovule. Each anther typically contains four sacs of pollen grains. Before the pollen grains are formed during flower development, these sacs are filled with large spore mother cells, one of which we can see here. In the anther, each diploid spore mother cell will divide by meiosis. This forms four haploid cells called microspores. Each microspore then divides by mitosis. But this division is unequal and produces two very differently sized cells.
One of these cells is called the generative cell and contains the generative nucleus. The other cell is called the tube cell and contains the tube nucleus. Eventually, after pollination has occurred, the generative nucleus will divide by mitosis. This gives rise to two sperm nuclei, which might be capable of fertilizing an egg cell. In order to allow the sperm nuclei to access the egg cell, the tube nucleus controls the development of a pollen tube. This will only occur after successful pollination. It’s helpful in the process of fertilization, as we’ll see shortly.
Once the tube cell and generative cell have formed, the microspores can be referred to as pollen grains, and their walls thicken. This protects the reproductive cells from environmental pressures like desiccation. As the anther matures, the walls between the pollen sacs disintegrate, causing the sacs to open and pollen grains to be released. As we now know, in order for fertilization to be successful in angiosperms, pollination must first occur.
Pollination is the process by which pollen grains are transported from an anther to a stigma. There are two main types of pollination: self-pollination and cross-pollination. Let’s take a look at self-pollination first.
Self-pollination is the process by which pollen from one plant’s flower travels to the stigma of the same flower or to the stigma of a different flower on the same plant to eventually fertilize that plant’s egg cell. Self-pollination can be advantageous as it does not require finding another plant to reproduce with. As in self-pollination the pollen is usually traveling such short distances, it also removes the reliance on pollinators to transfer the pollen to other flowers on different plants. However, there is no variation in the genetic material of the offspring that are produced by self-pollination, as they will be clones of the single parent that produced them. This means that plants produced by self-pollination are not particularly resilient to changes in environmental conditions, like the introduction of new diseases, which makes them vulnerable to extinction. Cross-pollination avoids this issue. Let’s look at this next.
Cross-pollination is when the pollen from the anther of one plant is transported to the stigma of a flower on a different plant of the same species. Cross-pollination has a much greater chance of increasing the genetic diversity and so the survivability of the offspring produced by avoiding self-pollination. However, this process can be much slower, and it relies on both finding a mate and on long-distance pollination methods, such as by animals, which are called pollinators, wind, water, or various other long-distance pollination mechanisms that angiosperms can adopt to transfer their pollen to a different plant. Once pollination of either kind has successfully occurred, the fertilization process can begin.
For fertilization to occur, when a pollen grain like this one lands on a stigma, it needs to reach the egg cell, which as we learned earlier is located nearest to the micropyle within the ovule in the ovary. Let’s find out how the pollen grain can cover this distance. Well, once it’s landed on the stigma, the pollen grain is first said to germinate. Remember, in the pollen grain at this point, there are two nuclei: the tube nucleus and the generative nucleus. Next, the tube nucleus begins to form a structure called the pollen tube, which grows down the style in the female part of the flower. The tube nucleus moves down this pollen tube, and the generative nucleus follows closely behind it. The tube nucleus releases enzymes that facilitate its passage through the style, until it reaches the micropyle of the ovule. The tube nucleus then enters the ovule through the micropyle before stopping the growth of the pollen tube.
During its travel down the pollen tube, the generative nucleus has divided by mitosis to form two sperm nuclei. These sperm nuclei can then enter the ovule through the micropyle. As the tube nucleus has now served its purpose, it will then disintegrate. Both of the sperm nuclei that have entered the ovule will be capable of fertilizing the egg cell. The process of fertilization in angiosperms involves two major fertilization events. For this reason, it’s often called double fertilization. One of these fertilizations involves the haploid sperm nucleus fusing with the haploid egg cell nucleus. This produces a diploid zygote.
The two synergid cell nuclei on either side of the egg cell are theorized to aid the sperm nucleus in reaching the egg cell for fertilization. The other fertilization event occurs in the embryo sac, as the other sperm nucleus fuses with the two polar nuclei in the embryo sac. This forms the endosperm nucleus, which is triploid and so often represented as three 𝑛. This process is often called triple fusion, as three nuclei are fusing together. While the endosperm nucleus continually divides and forms endosperm tissue, the zygote also divides several times and eventually forms an embryo full of diploid cells. The role of the endosperm is to provide the developing embryo with a food supply. It surrounds the embryo to eventually form part of the seed.
Let’s take a look at seed development. There are two different types of seed that an angiosperm can form depending on its species: a monocotyledonous seed, which is sometimes known as a monocot, or a dicotyledonous seed, which is sometimes known as a dicot. A cotyledon is a structure that will eventually develop into the embryo’s first leaves. The prefix mono- means one, and monocots typically have just one cotyledon, while the prefix di- means two, describing how dicotyledonous seeds have two cotyledons.
Some common examples of monocotyledons are maize, which is sometimes known as corn, and barley. Monocot seeds usually have a very prominent endosperm that supplies the developing embryo and the germinating seedling with food before they’re able to photosynthesize. In fact, when we eat grains like these, we gain the nutritional value from their endosperm. Common dicots involve plants like beans and peas. The two cotyledons in dicot seeds sometimes absorb the nutrition in the endosperm, leaving little behind.
In both monocots and dicots, the integuments of the ovule sometimes develop into a hard seed coat, which is called a testa. The testa helps to protect the developing embryo against desiccation or mechanical damage, among other environmental stresses. Now that we know how seeds are formed, let’s find out how fruit can develop around them. Fruits are really helpful to plants as they provide an incentive to hungry herbivores to eat a plant’s seeds and then deposit them in their feces in another location so that the seeds can germinate. In many angiosperms, if fertilization is successful, the calyx of sepals and the petals that make up the corolla will wilt and drop. This will also usually happen to the androecium or stamens and even to the stigma and the style, leaving just the ovary behind and of course the developing embryo within it.
Hormones like auxins may accumulate in the ovary after pollination has occurred. This can cause it to ripen and develop into a fruit. The ovary wall is transformed into a structure called the pericarp, which surrounds the fruit. And the ovule develops into a seed within the fruit containing the developing embryo. The seed is surrounded by its tough testa coat. The red part of the diagram on the left represents the fleshy fruit itself, which usually develops from the ovary and acts as an incentive to herbivores. Fruits that develop from the ovary are often called true fruits, an example of which is the mango. However, different species of angiosperms can develop their fruit very differently. Some fruits retain different organs. For example, the pomegranate, which also develops from the ovary, retains a cluster of stamens and its calyx of sepals.
False fruits, which are sometimes known as accessory fruits, develop from a plant part other than the ovary. For example, in apples, it’s the receptacle that’s located just below the ovary that develops into the fleshy fruit instead of the ovary itself. The ovary and the real fruit whorl are located in the middle of the apple and contains the seeds within it. Interestingly, some fruit can be produced without prior fertilization. This occurs through a process called parthenocarpy. Parthenocarpy can be natural, but it’s usually induced artificially to commercially produce lots of fruit without seeds. For example, if you sliced a fertilized banana in half, you’d see that it’s full of seeds. But in artificial parthenocarpy, an artificial solution of pollen extracts, which is sometimes known as IAA, can mimic the effects of auxins that usually stimulate fruit growth, even though no fertilization has occurred. These fruits will not contain seeds as the seed is a developing embryo of a plant that has been fertilized.
Let’s recap the key points that we’ve learned in this video. Angiosperms usually produce seeds. Their flowers consist of four main walls. While pollen forms within the anthers, egg cells form within the ovary. Angiosperms can either carry out self-pollination or cross-pollination. And double fertilization occurs when the sperm nuclei fuse with the egg cell and the two polar nuclei. This eventually forms a seed and possibly a fruit to surround the seed.
