Biology Lessons Part 2: Population Biology

 

 Lesson 2.6: How Do Flowering Plants Reproduce?

Time
To Ponder
Supplies
Objectives
Introduction and Background
Resources
AAAS Benchmarks

Exercise 1: Flower Dissection

Exercise 2: Fruit Dissection

Grade Level

Prospective and Practicing K-8 Teachers; may be adapted for use in K-12 classes

Time

Exercises 1 and 2 will take approximately 1/2 hour.

To Ponder

1.

Why do some plants produce flowers?

 

2.

Some people are allergic to the pollen in flowers. What is pollen and why do flowers make it?

 

3.

Why do plants grow fruits?

 

Supplies

These are some materials you may want for the sample experiments supplied with this lesson.

For students working in pairs, per pair:

  • large flowers (ex. hibiscus)
  • apples or pears
  • cutting board
  • scalpel or razor blades
  • paring knife
  • magnifying glass

  Objectives

    Once you have completed this lesson you should be able to:

1.

Identify the sexual organs of angiosperms, including: carpel, stigma, style, egg, stamen, anther, pollen, sepal.
  2.  Describe the functions of the major sexual organs of a flower.
  3. Explain how an egg combines with pollen inside the flower to produce a fertilized seed.
  4.  Describe basic differences between monocot and dicot plants.
  5.  Identify the parts of a fruit and explain how and why some angiosperms produce fruits.
 

Background

Information

Exercise 1: Flower Dissection

1. In this lab, you will examine the intricate structures that compose a flower.
2.  Many angiosperms have, in a single flower, both the male and the female sex organs surrounded by petals. The egg (female haploid cell) and the pollen (containing one or more haploid sperm nuclei) are contained in the same flower.
   3. Angiosperms may self-fertilize if pollen from a flower is transferred to egg cells in the same flower, or they may cross-fertilize. Carried on the wind or by other means, pollen grains from other flowers may land on the sexual organs of a flower and fertilize it.
Question 4.

Can you think of other ways that cross-fertilization of flowers might happen (besides by the wind)?

 

 

 Background 5.

The various parts of the flower help with the transfer of the pollen to the egg. There are typically four rings of structures in flowers, from outside to inside they are:

  • sepals
  • petals
  • stamens
  • carpels

Figure 1: Cross-section of an angiosperm
 

 To Do 6. Obtain a large flower and examine it, using the diagram in Figure 1 as a reference. Look for the sepals of your flower. The sepals are typically on the outside of the flower, often green, sometimes small and withered, sometimes as large as the petals. The sepals protect the bud before it opens.

  

7. The petals compose the next "ring" of flower structures. You can think of petals as modified leaves. Examine the texture and color of the petals using a magnifying glass. If your flower is colored, pinch a small piece of a petal between your fingers and examine the colorful pigment released.
 Question  8.

 Why do you suppose the petals of flowers are so colorful, fragrant, uniquely shaped?

 

  Background  9. The structures inside of a flower produce the gametes, or eggs and pollen. The male reproductive structures of the flower, called stamens, may be T-shaped, colored, straight or gently curved. They consist of an anther supported by filament.
To Do  10.

Carefully pull back the petals of the flower to expose the stamens. If necessary, use a razor blade or scalpel to help expose the internal structures. How many stamens do you see in your flower?

 

Background   11. The stamens each have an anther at the top of the filament shaft. Pollen grains are released from the anther. Each stamen will produce hundreds of pollen grains. Contained inside of each pollen grain there are two sperm nuclei.
To Do   12. Examine the anther using a magnifying glass and touch the tip of your finger to the anther. Did any pollen rub off on your finger?
 Background  13. Making up the innermost ring of structures is one or more carpels. A carpel (see Figure 2) is a floral structure enclosing an egg in angiosperms, typically divided into ovary, style, and stigma. A flower may have one or more carpels, either single or fused. (A single carpel or a group of fused carpels is also known as a pistil.)
 To Do   14.

To see the carpel clearly, gently separate the flower from the green sepals and base. The stamens will usually stay with the flower petals and the carpel remains attached to the base. This separation occurs naturally when a tree or plant sheds its flowers.

Figure 2: A carpel
 

  15.  Notice that the carpel has three parts: a sticky stigma at the top, a long shaft called a style, and an ovary at the bottom. Cut open the carpel to see the ovary. The ovary contains the haploid eggs.
 Question  16. Why do you think the stigma is sticky?
Background 17. 

Once a pollen grain has become stuck onto a stigma, it begins to grow a tube through which the sperm nuclei travel down to the ovary. There, the haploid sperm nuclei from the pollen unites with the haploid egg cells to produce diploid zygotes.

  18. There are two classes of angiosperms: the monocots and the dicots. The seeds in monocots have only one cotyledon, while in dicots, the seeds contain two cotyledons. You can typically characterize a flower as monocot or dicot by looking for identifying physical characteristics. The floral parts in a monocot generally occur in multiples of three and the leaves have parallel veins. In dicots, floral parts are usually in multiples of four or five and leaves are net-veined.
 To Do  19.

Characterize, to the best of your ability, your flower as a monocot or a dicot. Write a brief explanation below about the way you made this judgment.

 

 

 To Notice 20.  In the flower you examined, you were probably able to see both male and female organs. If a flower contains both male and female parts, botanists call them perfect flowers. Most flowers (roughly 85%) are perfect flowers and are able to self-fertilize. Imperfect flowers contain either male or female parts, but not both, and therefore, cannot self-fertilize.

Background

Information

Exercise 2: Fruit Dissection

   1. When the plant sheds its flower, the fertilized egg develops into a seed. The ovary wall surrounding the seed often develops into a fruit.
 To Do 2. Look at a fruit (an apple or a pear) to see the remnants of a flower. At the base of the fruit, notice the tiny withered sepal. If you pull back the edges of the sepal, you may be able to make out the remains of the stigmas and the styles. (You may need to use your magnifying glass to see this.)
3. Using a paring knife, cut open the fruit lengthwise. Look at the "core." Inside, you will see the seeds. The ovary walls are the tough structures which separate the core from the receptacle, the part of the fruit that we like the best!
 Question 4.

What is the primary function of fruits? Why do you think some angiosperms put so much energy into synthesizing sugars, carbohydrates, etc. to put into fruits?

 

 Question  5.

Is your fruit a product of a monocot or a dicot? How do you know?

 

 

Supplementary
Resources

Postlethwait, J. H. & Hopson, J. L. (1995). The Nature of Life, Third Edition. San Francisco: McGraw-Hill, Inc.

Klare, Roger. Gregor Mendel: Father of Genetics. Enslow Publishers. 1997.

http://www.urbanext.uiuc.edu/gpe/ "The Great Plant Escape." University of Illinois and Champaigne-Urbana. Website to introduce 4th and 5th graders to the mysteries of plants.

http://versicolores.ca/seedsoflife/ehome.html "Seeds of Life." A magnificent site by Francois Brenkmann full of colorful pictures and descriptions of unusual seeds and fruits.

Related
AAAS
Benchmarks

Chapter 5: THE LIVING ENVIRONMENT

Section B: Heredity

Grade K-2 Benchmark 2 of 2

Offspring are very much, but not exactly, like their parents and like one another.

Grade 6-8 Benchmark 1 of 3

In some kinds of organisms, all the genes come from a single parent, whereas in organisms that have sexes, typically half of the genes come from each parent.

Grade 6-8 Benchmark 2 of 3

In sexual reproduction, a single specialized cell from a female merges with a specialized cell from a male. As the fertilized egg, carrying genetic information from each parent, multiplies to form the complete organism with about a trillion cells, the same genetic information is copied in each cell.

Section D: Interdependence of Life

Grades K-2 Benchmark 1 of 2

Animals eat plants or other animals for food and may also use plants (or even other animals) for shelter and nesting.

Grades 3-5 Benchmark 3 of 5

Organisms interact with one another in various ways besides providing food. Many plants depend on animals for carrying their pollen to other plants or for dispersing their seeds.