Sexual Reproduction in Flowering Plants

Flower Anatomy detail  http://www.enchantedlearning.com/subjects/plants/printouts/floweranatomy.shtml

Pollination occurs when pollen grains land on the sticky surface of the stigma and are trapped there.  The pollen grain germinates and a pollen tube emerges from the grain.  It releases special enzymes that digest a cell the wall on the surface of the stigma.  The pollen tube grows down through the style to the ovary and enters the ovule, making a continuous passageway for the two sperm nuclei to enter the ovum.  Fertilization occurs when the sperm nuclei join the egg nuclei. 

The fertilized egg becomes an embryo.  The wall of the ovule thickens and forms a seed, thus enclosing and protecting the embryo.  The ovary wall also thickens and develops into a fruit.  In some plants such as apples, the ovary walls become fleshy and contain stored sugars and starches.  In other plants such as walnuts, the ovary walls become dry and hard. 

Purpose

1. To study the structure of a typical flower.
2. To study the male and female reproductive organs needed for sexual reproduction in flowering plants.

Materials

Procedure

1.  Separate the last page of this lab to make the Observations Chart accessible.

2.  Obtain a single flower and observe its parts carefully.  Flower parts are

     arranged in a circular pattern.  Each circle is called a whorl.  The whorls are

     attached at the enlarged receptacle located at the base of the flower. 

     Please read this overview before you begin your flower dissection:   

     As you examine your flower, you will be carefully removing parts beginning

     with the outer whorl and working your way in towards the pistil.  You will

     arrange each whorl in a circle on the plain paper, beginning with the sepals as

     the largest outermost circle.  As you proceed with your dissection, you will  

     carefully tape each whorl of flower parts into position and label them (please

     use pencil!).  As each whorl is observed and removed, you will complete the

     appropriate information in the Observations column of the chart.  Use the

     information in the handout to complete the Function column of the chart.

3.  The sepals form the outermost whorl of the flower.  The sepals are leaf-like

     structures that are usually green in color.  Sometimes, the sepals are the

     same color as the petals, or appear to be  another set of petals of a different

     color.  The function of the sepals is to protect the inner part of the flower

     before it blossoms.  Gently remove the sepals, tape them into position onto

     the paper, and label them.  On the chart, record the following observations:

a)     How many sepals does your flower have?

b)     Describe the appearance of the sepals (color, markings, etc.).

4.  The petals are found directly under the sepals.  The color and odor of the

     petals help to attract birds and insects to the flower for pollination.  Gently

     remove the petals, tape them into position onto the paper, and label them. 

     On the chart, record the following observations:

a)     How many petals does your flower have?

b)     Describe the appearance of the petals (color, markings, etc.).

5.  The stalk-like structures inside the petals are the stamens, the male

     reproductive organs.  Depending on the species, the stamens may be

     attached  to the receptacle, to the petals, or to the pistil.  The enlarged portion

     at the top of the stamen is the anther.  Inside the anther are pollen sacs,

     which produce pollen grains.  When the pollen grains mature, the pollen

     sacs split open, releasing the dust like pollen grains. The filament is the thin

     structure that supports the anther.  Gently remove the stamens, tape them

     into position onto the paper, and label them.  On the chart, record the

     following observations:

a)     How many stamens does your flower have?

b)     To which structure(s) were the filaments attached?

c)      Have the pollen sacs opened?  How can you tell?

d)     If pollen grains are visible, describe their appearance.

6.  The central structure of the flower is the female reproductive organ, the pistil.   The top of the pistil is the stigma.  When mature the stigma is enlarged, and its surface is moist and sticky.  The style is the middle portion of the pistil.  It supports the sigma.  Some flowers lack a style.  The ovary is the enlarged structure at the bottom of the pistil.  The ovary contains one or more hollow compartments known as locules.  The locules contain ovules, which in turn,     contain the egg nuclei.  Carefully remove the pistil by cutting it from the stem just under the ovary.  Using pencil, make a life-sized sketch of the entire pistil (just the outline) in the center of the plain paper and label it.  Cut the style just at the top of the ovary, tape it next to your sketch, and label the stigma and style.  Using the ruler, measure the length of the style in millimeters.  Cut a thin cross-wise section of the ovary and tape it under the stigma and style.  Label the ovary wall, locules and ovules.   Using the dissecting needle, carefully pick the ovules out of one of the locules.  Try to estimate how many ovules are contained in one locule.   On the chart, record the following observations:

a)     What color is the pistil?

b)     Describe the appearance of the stigma.  Is the stigma mature?  How can you tell?

c)      How long is the style (in mm)?

d)     Describe the appearance of the ovary.

e)     How many locules does the ovary contain?

f)        Approximately how many ovules are contained in one locule?

7.  Check your flower parts sheet and your chart for the following:

·        All flower parts are correctly taped in place.

·        All flower parts are labeled correctly (in pencil).

·        The pistil is drawn on the paper (in pencil).

·        Your name(s) and class period are written on the paper.

·        The Observations column of your chart is completed.

Discussion  - Please write the answers to the following questions in your lab databook using complete sentences.

1. Which does your flower produce in greater numbers:  ovules or pollen grains?  Explain why this would be important in terms of reproductive success.

2. What are some adaptations of flower petals to help attract pollinators?

3. How is the stigma of your flower adapted to capture and hold pollen?

   4.   Describe where pollination and fertilization occur.

   5.   Explain the differences between pollination and fertilization.

6. a) In which part of the male reproductive organ are the pollen grains

          made?

b) In which part of the female reproductive organ are the egg cells made?  c) By which nuclear process are these gametes formed?

7. How do the sperm nuclei in a pollen grain reach the egg nucleus in an

      ovule?

8. a) Which part of the flower becomes the seed? 

b) Which part becomes the fruit?

      c) Which part of the fruit contains the embryo?

Conclusion

Check your chart to be sure that the function column is complete.  Then summarize the information presented in this lab by creating a brief outline and writing it in your lab databook.  Your outline should include information about types of flower parts, male and female structures, pollination, fertilization, and development of seed and fruit.  Use the Introduction as your guide!

Name _____________________________                                            Tape Your Flower Parts here.