(Book) Reproduction

Plant Reproduction

So what exactly is a flower? 

 This view is of a beautiful Prairie Rose flower. Notice the fine detail. There is a reason why this flower looks like it does. Do you know why flowers so colorful? What is the purpose of all the parts?

Lesson Objective 

Identify processes of sexual reproduction in flowering plants, including pollination, fertilization (seed production), seed dispersal, and germination. 

Lesson Vocabulary 

• reproduction 
• pollination 
• pollen 
• fertilization 
• seed dispersal

Flowering Plants

Flowering plants evolved millions of years ago. Unlike animals, there are no boy or girl plants. Plants have both male and female parts. That does not mean that plants do not need help to reproduce.

Some plants form seeds. These are called flowering seed plants. The seeds form in the plant's ovaries. A plant's fruit may fall to the ground and start to grow. A plant's fruit may get eaten by animals. If eaten, the seeds of the fruit are passed through the animal's digestive tract. As the animal eliminates its waste, the seeds are spread around. Because animals do not stay in one place, they help the plant populate a bigger area. The animal gets a tasty treat and helps the plant reproduce.

The ovaries are just one of the structures of a plant. The flower's job is to attract animals, such as bees and birds. The animals are able to spread the pollen to other plants. Animals are needed to get the pollen from the male to the female parts of the plant. It is important for diversity that the pollen is passed from one plant to another.

Parts of a Flower

Flowers have both male and female reproductive parts. The main parts of a flower are shown in Figure below. They include the stamen, pistil, petals, and sepals.

Flowers and Pollinators

Some flowers may have bright colors. Others may have strong scents. Some may have sweet nectar. All these traits help to attract animals. Why do you think it is important to attract animals? Animals are able to carry around pollen from one plant to another. The animals they attract may include insects, birds, mammals, and even reptiles. While visiting a flower, an animal picks up pollen from the anthers. The animal then travels to another flower. While there, some of the pollen brushes off on the stigma. This allows cross-pollination. By spreading pollen to different plants, it helps increase plant diversity.

Evolution of Flowering Plants

Flowering plants have been around for a really long time. Scientists think they evolved over 200 million years ago. Fossil flowers have male and female reproductive organs just like today. There have been changes over time as plants evolved new and better traits to help them reproduce. From fossil evidence, the ancient plants did not have petals or sepals as they do today.

Just like now, the earliest flowers most likely attracted insects and other animals. The insects and animals spread the pollen from flower to flower. This was a big help to flowers. Animals can be better than the wind at carrying pollen. Wind-spread pollen may or may not land on another flower. To take better advantage of this “animal labor,” plants evolved certain traits. These traits can include brightly colored petals, strong scents, and sweet nectar. These are all traits that help attract animals. The flowers are able to move around their pollen. The animals get a free meal.

Other plants developed additional traits that helped them reproduce.

Some plants rely on seeds getting stuck in animal fur. Once stuck, the seeds are carried off to another location. Eventually, the seeds fall off.

Other plants developed traits to better disperse their seeds using the wind. With time, plants develop better and better traits to help them reproduce.

Developing Specialized Traits

Relying on an animal to come by is risky. A flower may have to wait a long time for the right animals to come by. What if one never passes close enough to the flower?

Hoping the wind will blow is also risky for a plant. What if the wind does not blow? What if the blowing pollen does not land on another flower? The wind could even blow the pollen over the ocean where it is wasted.

Giving free nectar is costly. It is not a good use of the plant's energy. A plant uses a lot of energy to produce nectar. Some animals may just drink the nectar. They may not carry off any pollen in return. To improve their chances, plants evolved special traits. For example, they developed ways to “hide” their nectar. Only certain animals were able to get at the plant's hidden nectar. These specific animals might be more likely to visit only flowers of the same species. This was also a benefit for some animals. Animals also evolved special traits to get to the nectar. This is called co-evolution of traits.

Two examples of this type of co-evolution are shown in Figure below.

The hummingbird has a long narrow bill. The long narrow bill enables it to reach nectar at the bottom of the tube-shaped flowers. The bat is active at night. Bright white, night-blooming flowers attract it. In each case, the flowering plant and its animal co-evolved. They have become better suited for their roles.

Review

Lesson Summary 

• Most modern seed plants produce seeds in the ovaries. Ovaries may develop into fruits. 
• Flowers attract pollinators and fruits are eaten by animals. Both traits aid the dispersal of seeds. 

Lesson Review Questions
Recall

1. Describe the male and female reproductive structures of flowers and their functions. 
2. State how fruits help flowering plants reproduce. 
3. Explain how flowering plants and their animal pollinators co-evolved.