Lesson Video: Sexual Reproduction in Flowering Plants Science • Second Year of Preparatory School

Video Transcript

In this video, we will learn how to identify and describe the parts of a flowering plant that are involved in sexual reproduction. We will also take a look at a few examples of the great diversity of structures and shapes that the flowers of these flowering plants can take.

You’ve probably noticed that a bunch of flowers is a widely used gift. Someone might offer a rose to a loved person or bring a bunch of flowers when visiting a friend. Did you know that more than 88% of the Earth’s green plants can produce flowers? This includes flowers that you might not even spot at first glance, like those of the wheat plant. This raises the question: what exactly is a flower?

A flower is the reproductive structure of a flowering plant. Reproductive structures are those which are necessary for reproduction, the process by which new offspring are produced from one or two parents. Flowers arise from modified leaves, called bracts, at an axil on a stem. The term “axil” refers to the angle between the stem and the structure branching from it. When one axil contains only one flower, like in the tulip we can see here, this is called a solitary flower. Sometimes, one axil may have several flowers, like in the geranium plant here. This is called an inflorescence.

Let’s have a look at the different structures of a typical flower. Starting at the base of this flower, we can see the pedicel. This is the stalk that branches from the stem in an inflorescence and holds an individual flower. Above the pedicel, we can see the receptacle, which attaches the stalk to the flower itself. Before a flower opens, it is enclosed in a bud. Sepals are modified green leaves, which surround the delicate structures of the flower when it is in this unopened bud form. This helps to protect the innermost parts of the developing flower from possibly harmful environmental influences.

Some flowers have separate sepals, while other flowers have fused sepals, which means that these modified leaves are joined together. As a flower blooms, its petals usually emerge from within the bud. A petal is a different form of modified leaf, enclosed within the sepals when in bud. Petals are often brightly colored. Some flowers, like those of the onion plant, may have petals that are difficult to distinguish from the sepals. In these cases, the sepals and petals together are called the perianth. Other flowers may not have petals at all. In certain species of flowering plant, like roses or lilies, the petals may also be scented. This can attract certain animal pollinators, such as insects like bees and butterflies. Like sepals, some flowers have separate petals while others have fused petals.

Moving inward from the petals of a flower, we can find its reproductive organs. Depending on the species of flowering plant, their flowers may contain male reproductive structures, female reproductive structures, or both. When a flower contains both male and female reproductive organs, it is called bisexual, or sometimes hermaphroditic. The prefix bi- means two, referring to the fact that this particular flower has two sexes. Most flowering plant species produce bisexual flowers, including tulips, petunias, and wallflowers.

If a flower only contains male or female reproductive organs, it is called unisexual. We can remember this, as the prefix uni- means one. Unisexual flowers are produced by fewer flowering plant species than bisexual flowers. Some examples of plants that produce unisexual flowers include palms, maize, and pumpkins. Let’s take a closer look at the reproductive organs that can be found in flowers by exploring the structures in a bisexual flower.

In a bisexual flower like this one, the male reproductive structures are usually found just within the petals. All the male reproductive organs in a flower are collectively called the androecium. We can remember this as the prefix andro- means male. Each male reproductive organ is called a stamen. This flower has six stamens in total, but this number varies between flowering plant species. Each stamen consists of two parts: the filament and the anther. The filament is a thin, stalklike structure, which supports the anther. Let’s take a closer look at one of these anthers.

An anther is divided into two main parts, and within each part there are two chambers full of pollen grains. The anther is the part of the flower that produces these pollen grains. Pollen grains contain the male gametes of flowering plants. Just like in animals such as humans, the male gametes of flowering plants that are found within these pollen grains are called sperm cells, and their job is to fertilize the female gametes: the egg cells, or ova.

Moving inward from the androecium in a bisexual flower, we can find the female reproductive organs of the flower. These are sometimes collectively called the gynoecium, as the prefix gyno- means female. The gynoecium consists of one or more carpels. Each carpel itself, which is sometimes called a pistil, is a female reproductive organ. The carpel includes a stigma, style, and ovary. The stigma forms the top part of the carpel. It is responsible for receiving, or catching, pollen grains. It is often sticky, which helps it carry out this function. The style is a long stalk located just below the stigma, connecting it to the ovary.

The ovary contains one or more ovules. In turn, each ovule contains a female gamete: an egg cell, or ovum. As this flower contains eight ovules, it will likely contain eight egg cells, or ova. For the sperm cell in the pollen grain to reach the egg cell in an ovule to fertilize it, first, pollen needs to be deposited onto the stigma. This process is called pollination, and it is what animal pollinators and other pollination methods, like wind, are useful for. Pollination allows flowering plants to reproduce sexually.

The parts of the flower that we previously mentioned are arranged into whorls. Whorls are circular arrangements of the sepals, petals, stamens, or carpels in a flower surrounding the stem. Bisexual flowers are made up of four distinct whorls. The sepals form the outermost whorl, which is called the calyx, outlined here in blue. Moving inward from the calyx is a whorl of petals called the corolla, outlined here in pink. Moving inward from the corolla, we would find the androecium, which contains all the male stamens, outlined here in green. The innermost whorl is usually the gynoecium, which contains all the female carpels, outlined here in orange.

As we mentioned earlier, some flowers are not bisexual but unisexual. In unisexual flowers, like those of the papaya plant, there will only be three whorls, not four. The innermost whorl will either be the gynoecium or the androecium, depending on whether the flower is female or male, respectively. Let’s apply what we’ve learned about sexual reproduction in flowering plants to a couple of practice questions.

Which of the following are the parts of a female organ in a flower? (A) Calyx, corolla, and style; (B) corolla, style, and ovary; (C) stigma, style, and ovary; or (D) stamen, stigma, and ovary.

The female organ of a flower is called a carpel or sometimes a pistil. Each carpel is made up of several different parts, as we can see in the image. Let’s work our way from the top down to see what these different parts are.

The stigma is the top-most part of the carpel. It is an often sticky structure that is responsible for receiving or catching pollen grains. Pollen grains contain the male sex cells of flowering plants, which are capable of fertilizing a female sex cell. Pollen grains are produced in the male organs of a flower, the anthers. Each anther, supported by a stalk called a filament, makes up one stamen. As the stamen is a male organ, not a female organ, we can eliminate answer option (D).

In order to access the female sex cells once pollen grains have landed on the stigma, the male sex cells must travel through the style to reach the ovary. The ovary contains one or more ovules. Each ovule contains a female sex cell. From what we’ve learned, it seems likely that the correct answer is option (C), but let’s check the remaining options to make sure.

The calyx is a group of sepals in a flower, which are modified green leaves that protect the inner parts of the flower when it is in its bud form. As the calyx is not part of the female organ in a flower, option (A) is incorrect.

The corolla is a group of petals in a flower, which are modified leaves that are often brightly colored and sometimes scented to attract pollinators. As the corolla is also not part of the female organ in a flower, option (B) is also incorrect. So, we can confirm that the answer option listing the parts of a female organ in a flower is (C) stigma, style, and ovary.

Let’s have a go at another question together.

A student was examining a flower that they found in the wild. They found that it had four distinct whorls. What is the sex of this flower? (A) Bisexual flower, (B) female flower, or (C) male flower.

Flowers are the reproductive structures of a flowering plant. Depending on the species of flowering plant, its flowers may contain male reproductive organs, female reproductive organs, or both. When a flower only contains male or female reproductive organs, it is referred to as unisexual. The prefix uni- means one, indicating that this flower only has one sex: male or female. On the other hand, if the flower has both male and female reproductive organs, it is referred to as a bisexual flower. The prefix bi- means two, so this flower has two sexes.

A whorl is each of the sets of flower organs arranged in successive circular patterns surrounding the stem. Most flowers are based on four types of whorl, which we can see in a typical bisexual flower here. The outermost whorl of both unisexual and bisexual flowers is called the calyx. The calyx is made up of modified green leaves called sepals, which function to protect a flower while it is in its bud stage before it is bloomed.

Moving inward from the calyx is a whorl of petals called the corolla. The corolla is present in both unisexual and bisexual flowers. Petals are also modified leaves, which in some plants are colored or even scented. If a flower is bisexual, we first come across the male reproductive organs, which are called stamens, as we move inward from the corolla. The group of stamens found within a flower is called the androecium whorl. The innermost whorl of a bisexual flower consists of the female reproductive organs, the carpels. These carpels make up a whorl called the gynoecium.

So, if a flower is bisexual, it contains four different whorls: the calyx, corolla, androecium, and gynoecium. Alternatively, a unisexual flower will only contain three whorls: the calyx, corolla, and either the androecium or the gynoecium. As the flower found by the student in this question has four distinct whorls, we know the correct answer. The sex of a flower with four distinct whorls is described in answer option (A), bisexual flower.

Let’s recap some of the key points we have covered in this video about sexual reproduction in flowering plants. Flowers are the reproductive structures of flowering plants. The male reproductive organs are called stamens, and each consists of an anther supported by a filament. The anther produces pollen, which contains the plant’s male sex cells. The female reproductive organs are called carpels. Each carpel consists of a stigma connected to an ovary by a style. The ovary contains ovules, which contain the plant’s female sex cells.

Pollination allows the male sex cells to be transported to the female reproductive parts of a flower so that the female sex cells can be fertilized. Bisexual flowers are made up of four whorls: a calyx of sepals, a corolla of petals, an androecium of stamens, and a gynoecium of carpels. Unisexual flowers are made up of three whorls: a calyx, corolla, and either an androecium or gynoecium.