Term 1 - Experimental Assessment

Experiment: Flower Dissection

Due date:  to be announced separately by teachers in various classes, since experiments will be performed at different times.

Marks: 5 marks for practical procedure in class + 10 marks for formal report. (See your booklet for marking criteria)

Background Information:

Flowering plants (angiosperms) reproduce sexually.  Angiosperms first appeared in the Cretaceous period (approximately 146 to 65 million years ago) and have flourished since.  The main reproductive structure is the flower.  Most flowers have both male and female reproductive organs (hermaphrodites), whilst others have separate male and female flowers.  Some plants are either male or female, and so only the female plant bears fruit, (e.g. common varieties of pawpaw).

Generally, most flowering plants avoid self-fertilisation.  The male organs ripen earlier of later than the female organs.  Thus, only the male cells from one plant can fertilise the female cells of another plant.  This ensures a greater variety in the next generation.  Pea plants studied by Mendel in his heredity research are an exception - they only self-fertilise.  In this case both male and female organs mature before the flower opens.

A simple flower consists of four different types of structures attached above one another on the expanded end of the stalk.  The outermost circle is composed of green leaf-like structures called sepals, which together form the calyx.  Attached just inside the sepals are the petals (white or coloured).  The petals may secrete a scent or nectar from special cells.  Collectively these petals are known as the corolla.  Inside the petals is a third group, the stamens, which are the male reproductive structures of the flower.  Each stamen consists of a filament with a yellow anther at the end.  When mature, the anthers contain many small pollen grains (male gametes).  The innermost structure of the flower is the pistil.  This is the female reproductive organ of the flower.  Each pistil consists of an expanded tip, the stigma, an elongated stalk, the style; and an enlarged base, the ovary, which contains one or more rounded structures, the ovules (female gametes).

Pollination is the process by which pollen is transferred from the anther to the stigma.  Pollen can be transferred between flowers by insects, birds, the wind and even fruit bats.  When the pollen from the stamen of a flower passes to the stigma of the same flower, self-pollination occurs.  Cross-pollination results when the pollen fertilises the stigma of another flower on another plant of the same species.  When the pollen grain lands on the hairy or sticky stigma, it develops a very fine tube, which grows down the style and into the ovary.  The male gamete passes down the tube and into ovule through a hole called the micropyle.  There it unites with the ovule.  The fertilised ovule grows into a plant embryo inside the seed.  The seed is a mature fertilised ovule.  As the ovules inside an ovary mature into seeds, the ovary develops into a fruit.  A fruit is a ripened ovary containing seeds.  In the bean plant, the fruit is the bean and the seeds are the beans inside the pod.

Part A - Dissection of a flower

Materials:

Procedure:

1. Copy the table below into your Activity book and identify and describe the flower parts.
2. If your flower has protective leaves (coiled leaves), peel them off to where the flower joins the stem leaving the stalk and the petals.   
3. Bend back the sepals until they break off, and then remove the petals form one side of the flower in the same way.  Observe the veins in the petals and the sepals.  
4. Use a scalpel to cut vertically through the central part of the flower and stem.  This should leave you with the flower cut in half.   
5. Use a hand lens or magnifying glass to observe the central sections.    
6. Draw in pencil a sketch of your flower and label the following parts: sepals, petals, stamen, filament, anther, pistil, stigma, style & ovary.  (Note: Biological drawings should be clear and accurate; they are not artistic representations; use outlines without shading.)

Questions:

1. complete the table outlining the function of each part of the flower.
2. Which parts of the flower could be removed without preventing reproduction?
3. What is the function of the sepals, which form the calyx?
4. What advantage would a strong scent or nectar have for the flower?
5. Outline some ways in which plants can avoid self-fertilisation?
6. Nectar producing flowers often make the nectar difficult to access.  How would this assist the pollination process?
7. Explain why the stigma is sticky or hairy?
8. Some plants can reproduce both sexually and asexually.

   (a)     What would be the advantage to the plant of reproducing sexually?
   (b)   Under what circumstances would it be advantageous to reproduce asexually?

9. Use the information below to compare and contrast the features of wind and insect pollinated flowers.

   - Insects are attracted to flowers by their colour and fragrance. Their prize is a meal of nectar.

   - Pollen released into the wind is much less likely to land on a stigma than that carried by an insect.

   - Wind is much more likely to catch pollen from dangling exposed anthers than from anthers enclosed in petals.

   - Flat or lobed, sticky stigmas can trap pollen from the body of an insect.

   - Feathery stigmas can trap pollen from a relatively large area of air.

   	Wind pollinated flowers	Insect pollinated flowers
   Colour and fragrance
   Position of anthers
   Shape of stigmas
   Presence of nectar

10. Fertilisation in flowering plants is a complex process. The mechanisms involved must provide advantages since flowering plants dominate the terrestrial environment. Outline the main advantages.

PART B - Pollen Grains

Materials:

  Procedure:

1. Remove a stamen and place it on a slide.  Crush the tip with your tweezers.
2. Put a drop of water on the tip and gently place a cover slip on top. View the slide under a microscope.  Remember to start with the lowest power lens.  
3. You should be able to see the pollen grains.  Describe their shape and estimate their size.  Record your observations in the appropriate section of your table. Draw a diagram of your pollen grain.

Questions:

1. How do pollen grains from two different flowers compare? 
2. The number of pollen grains produced by one anther is much greater than the number of ovules in one ovary.  How is the high production of pollen an advantage to the plant? 

PART C - Seed Structure

Procedure:

1. Examine the outside of the bean seed.  Note the scar called the hilum, where the ovule was attached to the ovary.  Near the hilum is a small hole called the micropyle, where the pollen tube entered the ovule at fertilization. draw a diagram of the outside of the seed.
2. Use the razor blade to carefully remove the outer covering of the bean.  Open the seed into tow halves longitudinally. These halves are called the cotyledons.
3. Observe the embryonic plant. you should be able to identify the plumules (leafy structures that will become the plant's first true leaves and the sstem to which they are attached) and the radicle (that will become the root structure.)
4. Draw a diagram of the closed bean seed and label the hilum, the micropyle, testa, cotyledon, tirst leaves, stem and root.

Questions:

1. Flowering plants can be divided into two groups according to the number of cotyledons in the seed. plants that have seeds with two cotyledons are known as dicotyledons or dicots, whilst plants that have one cotyledon are monocots. Is the bean seed a monocot or a dicot?
2. If one ovary contains nine ovules, how many fruits and seeds will develop after fertilization?
3. The bean seeds were soaked in water before class so that it is easier to remove the seed coat and open the halves. T hrough what structure could the seeds have absorbed the water?
4. Examine the diagram of the corn seed. Is the corn seed pictured a monocot or a dicot? Explain.

Practical report format for the method to write up your formal report.

Internet Reference Sites:

• Flower dissectionImages of Pollen 1
• Images of Pollen 2