Lesson Explainer: Reproduction in Flowering Plants Biology

In this explainer, we will learn how to identify the structures in a flower and describe the processes of gamete formation, pollination, fertilization, and fruit and seed formation.

More than of Earth’s green plants belong to a group called angiosperms. Angiosperms are flowering plants, and they first evolved around 140 to years ago! Angiosperms are the largest group of plants that are highly adapted to terrestrial life, as opposed to life in water, thanks to their seeds. Seeds enable flowering plants to pass on their genetic information easily on land or via air, water, or animals to other locations to help them spread their genetic information.

In order to understand more about how they reproduce, let’s first look at the structure of a typical angiosperm.

Angiosperms are flowering plants. Flowers are short stems surrounded by modified leaves that usually arise from leaf-like structures, sometimes called bracts. Some flowers are solitary such as the tulip flower. Other plants have their flowers grouped into clusters called inflorescences. Some flowers are supported by small stalks called pedicels, while other flowers have no stalk and are therefore called sessile.

Flowers contain the specialized male and female reproductive organs of the plant. Angiosperms have their ovules, which contain the female reproductive cells enclosed in an ovary. The ovary is the female reproductive organ. The ovule will usually develop into a seed when it is fertilized, and the ovary will often develop into a fruit.

The diagram below in Figure 1 shows the basic structure of an angiosperm as a cross section, including the location of the ovules within the plant’s ovaries.

Angiosperms can be male, female, or often both. This is because they are capable of having both male and female reproductive organs.

You can see the female reproductive organs, or the carpel, in Figure 1 labeled in red. The carpel includes the stigma, an often sticky organ onto which pollen grains, which contain the male gamete, are deposited in a process called pollination. The carpel includes the style, which connects the stigma to the flower’s ovary. The ovary contains ovules, which contain the plant’s female reproductive cells, or gametes. The sum of all the female reproductive organs in an angiosperm, the style, stigma, and ovary, is called the gynoecium.

You can see the male reproductive organs, or the stamen, in Figure 1 labeled in blue. The stamen includes the anther from which pollen grains, which contain the plant’s male gametes, are produced. The stamen consists of two parts: an anther and a supporting filament. The plant in Figure 1 has 6 stamens that surround a central carpel, but the number of stamens will vary between different flowering plant species. The sum of the male reproductive organs in a plant is called the androecium.

Flowers are arranged into whorls. Whorls are circular arrangements of the leaves, sepals, petals, stamens, or carpels in a flower surrounding the stem. Angiosperms are based on four types of whorls, which you can see in Figure 2.

The calyx is the outermost whorl, consisting of sepals. Sepals are modified green leaves that function to protect the young developing flower when it is in its bud stage. When the flower blooms, the petals and other reproductive structures emerge out of the calyx of sepals. The calyx still functions to protect the inner parts of the flower after it has bloomed. You can see these comparatively small green sepals in Figure 1.

Moving inward from the calyx is the corolla, the second outermost whorl that consists of petals. Petals are also modified leaves that are often brightly colored and sometimes scented. This is helpful to attract pollinators that function to spread pollen ideally from one plant to another. This whorl is found more centrally within the flower than the calyx, as the sepals protected the petals in the corolla and the reproductive organs while they were developing.

In some plants, the leaves of the calyx and corolla are very similar, so both whorls are grouped into a single outer whorl called the perianth, or sometimes the perigonium. Each individual modified leaf part in a perianth is called a tepal.

The other two whorls of an angiosperm are the gynoecium and the androecium, both of which are found even more centrally in the flower than the corolla. The androecium tends to surround the whorl of the gynoecium, which is usually found right in the center of a flower.

Example 1: Identifying the Different Structures of a Flower

Identify the labels on the figure shown.

1. 1: corolla, 2: stigma, 3: anther
2. 1: petal, 2: corolla, 3: stigma
3. 1: sepal, 2: anther, 3: stigma
4. 1: sepal, 2: style, 3: stigma
5. 1: corolla, 2: anther, 3: stigma

Answer

Flowers contain the reproductive organs of angiosperms, one of which is displayed in the image above. Angiosperm flowers are arranged into four main whorls that radiate around the stem of the plant. Let’s look at these different whorls, starting from the exterior of the flower structure.

The outermost whorl is the calyx, which describes the sepals, shown in the diagram labeled with a 1. Sepals are small modified leaves that protect the flower when it is in bud.

The second outermost whorl is the corolla, which describes the petals. The corolla is not labeled in the diagram but is composed of several brightly colored and often scented petals that attract pollinators.

The third outermost whorl is the androecium, which is the male part of the flower. It consists of stamens, which are long filaments with anthers on the top. The anther is responsible for producing pollen, the male gamete, and is labeled on the diagram with a 2.

The innermost whorl is the gynoecium, which is the female parts of the flower. The gynoecium consists of the ovary, which contains the ovules with the female gamete, egg cells. It also includes the style, which leads up from the ovary, and the sticky stigma on the top, labeled in the diagram with a 3. The stigma is the part of the female organ that receives pollen grains, the male gamete.

Let’s fill in those different labels in the diagram.

Therefore, the labels are as follows: 1: sepal, 2: anther, 3: stigma.

Let’s look at how the female and male reproductive parts of a flower produce their gametes.

Gametes are formed by meiosis. The female gamete in an angiosperm is the egg cell. The egg cell is produced in an ovule within the plant’s ovary.

This begins as spore mother cells, sometimes referred to as megasporocytes or megaspore mother cells, develop within the ovule. Spore mother cells are diploid as they have a full set of chromosomes, represented as “.” You can see the process by which a spore mother cell produces an egg cell in Figure 3.

A structure called a funicle, sometimes called a funiculus, develops in the ovule. The funicle is similar to the umbilical cord in humans, as it is a stalk-like structure that connects the developing ovule to the ovary wall. It is believed that the funicle has a role in providing the ovule with nutrients during seed maturation, as this is the only channel of communication between the seed and the parent plant. This would help the seed to grow, but the details of how the funicle provides these nutrients is an ongoing area of research.

The ovule is surrounded by integuments (in this case, two integuments) that form an outer layer that you can see in Figure 3. You may notice that there is a small gap in these layers, however. This gap is called the micropyle and will be the space through which a male nucleus can access and fertilize the egg cell once it has formed.

The spore mother cell divides by meiosis, forming four haploid cells called megaspores . These cells are haploid, as they have half the number of chromosomes of a normal cell, represented as “.” Three of these haploid cells degenerate, while the fourth grows and develops into an embryo sac, sometimes called the functional megaspore or the megagametophyte. The embryo sac is contained within a tissue called the nucellus. Usually, the nucellus breaks down after fertilization has occurred to provide the developing embryo with nutrients.

The haploid functional megaspore within the embryo sac then divides by mitosis three times. This means that eight nuclei are produced. Figure 4 shows a closer look at the final stage shown in Figure 3. You can see where all the nuclei have moved to and what they are now called.

Two of the eight nuclei move to the center of the embryo sac and are called polar nuclei.

The other six nuclei move to opposite ends, or poles, of the ovary. One pole will be next to the micropyle, which you can see at the top of the ovary in Figure 4. The other pole is at the base of the ovary in Figure 4. The nuclei at the poles of the ovary become enveloped by cytoplasm and a thin membrane forming six distinct cells, three at the top of the ovary and three at the bottom. The three cells at the bottom of the ovary, far from the micropyle, are called antipodal cells.

The cells at the other pole, near to the micropyle, differ in their development. The cell in the center closest to the micropyle will grow and develop into an egg cell. The two cells on either side of the egg cell develop into cells called synergids. When the egg has grown sufficiently, it is ready for fertilization.

We will look more closely into the role of the polar nuclei and synergids in fertilization soon, but first, let’s see how the male gametes are produced.

The male gamete in an angiosperm is pollen. Pollen grains are produced in the plant’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 have formed during flower development, these sacs are filled with large spore mother cells, one of which you can see in Figure 5 below.

Figure 5 shows how each of these spore mother cells can produce sperm nuclei capable of fertilizing an egg cell. Each spore mother cell is diploid and divides by meiosis to form four haploid cells called microspores .

The microspore nucleus then divides unequally into two very differently sized cells by mitosis. One of these new cells is called the generative cell, which contains the generative nucleus, and the other is called the tube cell, which contains the tube nucleus. The generative nucleus will typically divide by mitosis to give rise to two sperm nuclei that might fertilize an egg cell. The tube nucleus will control the development of a pollen tube once successful pollination has occurred, which is helpful in the process of fertilization, as we will see shortly.

The microspores are now called pollen grains, and their walls thickens to provide protection to the reproductive cells from environmental factors such as desiccation. As the anther matures, the walls between pollen sacs disintegrate, causing the sacs to open and pollen grains to be released.

Example 2: Identifying the Location of Meiosis in a Flower

Where in the figure of the half flower shown does meiosis happen?

1. 2 and 4
2. 1 and 2
3. 2 and 3
4. 4 and 5
5. 1 and 4

Answer

Gametes are formed by meiosis. The female gamete in an angiosperm is the egg cell. The egg cell is produced in an ovule within the plant’s ovary. The male gamete in an angiosperm is the pollen grain. Pollen grains are produced in the plant’s anthers, which are where meiosis will occur in a similar way to the development of the egg cell in the ovary.

Let’s label the diagram to show where these different structures are.

Therefore, the locations of meiosis in the flower are 2 and 4.

In order for fertilization to be successful in angiosperms, pollination must first occur. Pollination is the process by which pollen grains are transported from the anther to the stigma of a flower.

There are two main types of pollination: self-pollination and cross-pollination.

Let’s look at self-pollination first.

As mentioned, plants are often hermaphroditic, so they contain both male and female reproductive organs. This means that they can technically reproduce with themselves to produce offspring that are almost identical to the parent plant. Self-pollination is the process by which pollen from one plant’s flower travels to the stigma of the same flower or a different flower on the same plant to eventually fertilize that plant’s egg cell.

Self-pollination does not require a parent plant to find another plant to reproduce with. It also removes reliance on pollinators to transfer pollen to other flowers.

The “aim” of the plants is to pass on their genes to offspring that will survive and pass on their genes for many more generations. However, there is no variation in the genetic material of the offspring produced by self-pollination, so they are technically clones of the parent. This can be a disadvantage to a plant, as genetic variation makes any species more resilient to changes in their environment.

For example, if a parent plant that has little resistance to cold weather self-pollinates, they would produce offspring with similarly little resistance, as they are genetically identical to their parent. This means that, if a sudden extremely cold winter struck, the parent plant and its offspring would not survive and the whole family would be wiped out.

Plants have a solution to reduce the chances of self-pollination and increase the genetic diversity and survivability of their offspring. This solution is called cross-pollination—let’s look at this process next.

Cross-pollination is when the pollen from the anther of one flower is transported to the stigma of a different plant of the same species. This is where pollinators, such as insects and other modes of long-distance pollination such as wind, come in useful. Cross-pollination has a much greater chance of increasing the genetic diversity, and so, the survivability, of offspring by avoiding self-pollination.

Interestingly, different plant species adapt their structures to suit their mode of pollination. For example, wind pollination can cause a large, random scattering of pollen over a wide area. Insect pollination, however, is a lot more precise as pollen is carried to specific flowers by small organisms. As a result, wind pollination can cause the loss of many pollen grains, so plants that use this mode of pollination are adapted to produce large numbers of pollen grains.

You may be wondering how the plants can increase the chances of cross-pollination, as opposed to self-pollination. Flowers can adapt to become more suited to cross-pollination in numerous ways.

Plants being hermaphroditic, or unisex, increases the chances that pollen from one flower will be able to pollinate any other flower it comes into contact with. If the flowers of a plant species are discrete sexes, just male or female, there is a risk that pollen might land on a flower with only male sex organs. If this happened, there would be no ovules to be fertilized, so fertilization cannot occur. Hermaphroditism in all flowers of a certain species therefore increases the chance that successful fertilization will occur.

In some plants, one set of sex organs develops before the other to avoid inbreeding in a process known as dichogamy. For example, the female organs might mature and be receptive to pollen before the male organs mature. Some plants experience the opposite effect. As different plants of the same species will mature at different times, it can be helpful to prevent self-pollination and enhance cross-pollination.

Another adaptation to avoid self-pollination can be in the different relative positioning of the anthers and stigma, known as herkogamy. Anthers are sometimes located deeper into the flower than the female stigma, making self-pollination difficult. This means that pollinators need to move deeper into the flower to locate the nutritious reward of pollen or nectar. Nectar is the nutritious sugary liquid produced by some flowering plants that many pollinators use as their food source. When the pollinators visit another flower for another meal of nectar, they are very likely to spread the pollen caught on their bodies from many different flowers as they move past the stigma. Sometimes, the stigma is buried deeper into the flower than the anther, which may be beneficial for pollination by different species of pollinators.

Now that we know more about how pollen can be transported from one flower to another by pollination, let’s find out what happens when the pollen meets the stigma. This process leads to fertilization and is summarized in Figure 6 below.

When a pollen grain falls onto the stigma, the pollen grain is said to germinate. The tube nucleus begins to form a pollen tube, growing through the stigma and down the style as shown in stage A of Figure 6. It releases enzymes that facilitate its passage through the style until it reaches the micropyle of the ovule as you can see in stage B of Figure 6.

The tube nucleus enters the ovule through the micropyle by lengthening the pollen tube a little more and then the tube nucleus stops its growth, indicated in stage C of Figure 6.

The generative nuclei, which have been following the tube nucleus down the pollen tube, now enter the ovule through the micropyle. The generative nucleus divides by mitosis to form two sperm nuclei, and one of the sperm nuclei fuses with the egg cell nucleus in a process called fertilization. This produces a zygote , which starts to divide by mitosis forming an embryo . The two synergid cell nuclei on either side of the egg cell are theorized to aid the generative nucleus in reaching the egg for fertilization.

The other sperm nucleus fuses with the central cell with two polar nuclei in the embryo sac . This forms the endosperm nucleus, which is now described as triploid ! This process is called triple fusion, as three nuclei are fusing together. The endosperm nucleus then repeatedly divides and forms endosperm tissue. The role of the endosperm is to provide the developing embryo with a food supply, and it surrounds the embryo to eventually develop into part of the seed. Figure 6 shows the final stages of seed development in stages D and E as the endosperm and embryo become visible.

As the process of fertilization in plants involves two major fertilization events, one sperm nucleus and the egg (to form the seed embryo) and the other sperm nucleus and the polar nuclei (to form the endosperm), it is called double fertilization. This process is summarized in the box below.

Example 3: Identifying the Number of Nuclei which are not Involved in Double Fertilization

How many of the eight haploid nuclei inside the embryo sac (female gametophyte) are not involved in double fertilization?

Answer

Gametes are formed by meiosis. The female gamete, containing a haploid nucleus, in an angiosperm is the egg cell. The egg cell is produced in an ovule within the plant’s ovary from a single diploid cell called a spore mother cell. Seven other haploid nuclei are formed from successive divisions of the spore mother cell in the ovary during egg cell formation.

You can see all eight nuclei, some of which have formed cells, in the image of an ovary below.

Two of the eight nuclei move to the center of the embryo sac and are called polar nuclei.

The other six nuclei move to opposite ends, or poles, of the ovary and become independent cells. One pole will be next to the micropyle, which you can see at the top of the ovary in the image above. The cell in the center, closest to the micropyle will grow and develop into an egg cell. The two cells on either side of the egg cell develop into cells called synergids.

The other pole is at the base of the ovary. The three cells at the bottom of the ovary, far from the micropyle, are called antipodal cells.

When the egg has grown sufficiently, it is ready for fertilization.

One of the male sperm cells fuses with the egg cell . This produces a zygote , which starts to divide by mitosis, forming an embryo . The two synergid nuclei on either side of the egg cell are theorized to aid the pollen nucleus reach the egg for fertilization but are not directly involved in fertilization itself.

The other male sperm cell nucleus fuses with the two polar nuclei in the embryo sac . This forms the endosperm nucleus which is now described as triploid !

As the process of fertilization in plants involves two major fertilization events, one of the male sperm cells and the egg cell and the other of the other sperm cell nucleus and the polar nuclei, it is called double fertilization. This process is summarized below:

Of the eight haploid female nuclei, three, the two polar nuclei and the egg nucleus, are involved in double fertilization. The other five, two synergids and three antipodal cells, are not directly involved.

Therefore, the number of nuclei in the embryo sac that are not involved in double fertilization is 5.

Example 4: Describing the Sequence of Pollen Tube Growth

What is the correct sequence of structures through which a pollen tube grows?

1. Stigma style integuments ovule
2. Style filament micropyle ovule
3. Stigma style micropyle ovule
4. Stigma filament micropyle ovule
5. Style stigma micropyle ovule

Answer

Pollen contains the male gamete of angiosperms. Its “aim” is to reach and fertilize an egg cell, the female gamete. The egg cell is contained within the ovule of an angiosperm. When a pollen grain first lands on the female part of a flower in pollination, it makes contact with a sticky structure called the stigma, as you can see on the image below.

A pollen tube grows down from the stigma, through the style and toward the ovule, which contains the egg. The male nuclei travel along the pollen tube. When the pollen tube reaches the ovule, it enters through a small gap called a micropyle. The pollen tube now provides an entrance point for the male gamete (sperm) nucleus to enter the female ovule and fertilize the egg cell.

The sequence of structures through which a pollen tube grows is therefore the following: Stigma style micropyle ovule.

Once the embryo has formed, how is it going to be moved to soil to germinate into a new plant? Let’s look at how seeds and fruits can form to develop into new angiosperm organisms.

There are two main types of seed that an angiosperm can form: a monocotyledonous seed, sometimes called a monocot, or a dicotyledonous seed, sometimes called a dicot. A cotyledon is a structure that will eventually develop into the embryo’s first leaves. You can see the main differences between monocot and dicot seeds in Figure 7 below.

Monocot seeds have one cotyledon, as mono means one. Monocots also usually have a prominent endosperm that supplies the developing embryo and the germinating seedling with food before they can photosynthesize. Therefore, they are sometimes called endospermic seeds. Common examples of monocots are barley, maize, and wheat, which are species providing a large proportion of the human diet. The nutrition humans gain from eating them is obtained from the monocot seeds’ endosperm!

Dicot seeds have two cotyledons as di means two. Common dicots are plants like beans and peas. The two cotyledons in dicot embryos sometimes absorb the nutrition in the endosperm, leaving little behind. Therefore, dicots are sometimes called non-endospermic or exendospermic seeds.

In both monocots and dicots, the integuments of the ovule develop into a hard seed coat, sometimes called a testa, which helps to protect the embryo against desiccation or mechanical damage among other stresses.

Now, we know how seeds are formed. Let’s see how fruits develop around them.

Fruits are really helpful, as they provide an incentive to hungry herbivores to eat a plant’s seeds and then deposit them in another location in their feces so the seeds can germinate.

If fertilization is successful, in many angiosperms, the calyx (sepals), corolla (petals), androecium (stamens), style, and stigma wilt and drop. This leaves just the ovary behind, with the developing embryo and its food stores within. The diagram in Figure 8 shows a comparison between the ovary containing a developing embryo and the structures within it that change when it forms a fruit.

Hormones, such as auxins, may accumulate in the ovary causing it to ripen and develop into a fruit. The wall of the ovary is transformed into a structure called the pericarp, which surrounds the fruit. The ovule develops into a seed within the fruit and contains the developing embryo within it. In some cases, the two synergid cells and antipodal cells within the ovary disappear. In other cases, the synergid cells fuse with the endosperm after fertilization. The micropyle remains to allow water to enter the seed for germination to occur.

There are some fruits that develop slightly differently. The pomegranate, for example, keeps the calyx and some stamens. Eggplants and dates keep their calyxes too, while the fruits produced by marrow plants, which belong to the species Cucurbita pepo, retain the petals of the corolla as they develop.

True fruits, such as mangos, develop from the ovary. In false fruits, also known as accessory fruits, a part of the plant other than the ovary develops into a fruit. For example, in apples and strawberries, the receptacle beneath the ovary develops into a fruit instead of the ovary itself. Other examples of false fruits include figs and pears.

Though pollen is needed for the development of fruit because it has been shown that it contains auxins that accumulate in the ovaries, in some cases, the fertilization that is also caused by pollen grains is not essential for a fruit to develop. The process by which fruits are produced without the fertilization of ovules is called parthenocarpy, and it can be induced either naturally or artificially.

For example, an artificial solution of pollen extracts, indole or naphthol acetic acid, sometimes known as IAA, can stimulate fruit growth even though no fertilization has occurred. These fruits will not contain seeds, as the seed is the developing embryo of a plant that has been fertilized. Fruits without seeds, such as banana and pineapple, can be produced commercially through this process, which is called artificial parthenocarpy.

If a flower is neither pollinated nor fertilized, it will drop without fruit formation. Perennial plants, such as strawberry plants, live for years, sometimes known as growing seasons, or more. Annual plants, such as watermelon plants, complete their life cycles in one growing season. This means that, after it has produced flowers, seeds, and fruit, it dies. Therefore, it is very important that seeds are dispersed to suitable locations to pass on the genetic material of the plant to the next generation successfully, as this individual plant would not get a second opportunity!

Let’s recap some of the key points we have covered in this explainer.