1. What are flowering plants?

Flowering plants, also known as angiosperms, are the most diverse land plants. They produce seeds, like the gymnosperms. Angiosperm seeds are contained in fruits, while the gymnosperm seeds develop on the leaf surface. Those leaves were often modified as cones like in conifers.

There are a lot of flowering plants producing flowers and fruits.

Not all fruits are what you may know as fruits, sweet and edible. What you may know as vegetables may also be fruit. Check chapter 2 for more details.

Per definition, fruit comes from the fertilization of an ovary, the female part of the flower. And the fruit contains the seeds. Let’s first see how a flower looks like.

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The flower

The flower is the reproductive structure of the flowering plant. Many flowers contain both the male and the female parts. We say there are bisexual or hermaphroditic.

Some other flowers only contain male or female parts. They are unisexual flowers. These flowers can be in the same plant. These plants are monoecious. It refers to the Greek word for “one household.” Pumpkin is a monoecious plant:

Alternatively, female and male flowers can be on different plants. Thus, there are male plants and female plants. We say the plants are dioecious. It refers to the Greek word for “two households.”

Kiwi is a dioecious plant:

The flowers are made of differents essential parts. A typical flower consists of four kinds of structures attached to a stalk, the pedicel. These structures are organised in a whorl.

The perianth, vegetative part, is composed of two whorls:

• The sepals, modified leaves protecting the flower buds before the flower opens. There are typical green.
• The corolla made of petals. There are typically colourful and help to attract pollinators.

The reproductive or sexual parts made the two next whorls:

• The androecium (from the Greek “ andros oikia”, the man’s house) is composed of the stamens. The stamens are made of a stalk, the filament, and the anther at its top. The anthers are containing the pollen.
• The gynoecium (from the Greek “gynaikos oikia”, the woman’s house) at the centre of the flower. It is also called pistil. It consists of three parts. At the top, there is the stigma where the pollen grains land to start the pollination of the female flower. Then there is the style, a stalk supporting the stigma through which the pollen tubes are growing towards the ovules. At the bottom, there is the ovary made of one or more (fused) carpels. The carpel forms a hollow structure containing the ovules. The ovules carry the female gametophyte, produced by meiosis. For more information about meiosis, see chapter 5.

Label the flower

The box gets green when the label is correct.

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Pollinators

To produce a fruit, the gynoecium needs to be pollinated by the pollen. Pollination is the movement of pollen from the anther to the stigma of the same or another flower to fertilize the ovule and produce seeds. After pollination, the ovary will become the fruit, the ovule will become the seed, and the egg cell produces an embryo that will form a new plant. But how the pollen reaches the stigma?

Flowers can be self-pollinated: the pollen from the flower is used to pollinate the stigma of the same flower. More often in nature, pollen moves from one plant to another. We call it cross-pollination. This requires some help from the environment (wind, water), animals and insects. These animals and insects are called pollinators.

To attract those insects and animals, flowers have developed strategies. The color of the petals, the shape, the smell of the flower and the production of attracting food like the nectar attract the insects. While pollinators feed on the nectar and the pollen, some pollen grains stick of their body and are carried to the next flower of the same plant species, where eventually they come in contact with the stigma. These pollinators are essential for the production of fruits and, therefore, of our food. They are bees, butterflies, hummingbirds, bats, moths. Watch the videos to learn more about how it works.

Worksheet

When pollination requires pollinators, it may be a random. And pollen grains from plants may land in the stigma of a flower from another species. In that case, the pollen will be rejected by the flower. It will not germinate. We say there is incompatibility. This process may also happen for self-pollination in a plant species that uses cross-pollination.