CP 4

Question 1

IV DV

Answer 1

IV = Concentration
DV = Length of Pollen Tube/Number of Germinating Pollen Grains

Question 2

What is the pollen tube?

Answer 2

It is a path through the stigma, made by digestive enzymes, down which the pollen grain travels to reach the embryo sac, and hence fertilise the ovum

Question 3

What structure controls the production of these enzymes?

Answer 3

Pollen tube nucleus.

Question 4

How is an eyepiece graticule calibrated to make measurements?

Answer 4

By using a stage micrometer.

Question 5

What are the controlled variables of this practical?

Answer 5

Environment - humid chamber

Volume and content of nutrient salt solution

Volume of sucrose solution

Time allowed for growth

Question 6

Outline the procedure to this practical.

Answer 6

1. Dilute the stock sucrose solution to several set concentrations.
2. Place a moist piece of filter paper into a petri dish to form a humid chamber.
3. Put a few drops of sucrose solution and an equal volume of mineral salt medium onto a clean microscope slide.
4. Use a mounted needle to rub the anther of the flowers so they shed some pollen onto the microscope slide.
5. Place the slides into the petri dish until it is time to observe them.
6. Start the stop clock. Place the slides under the microscope and use a calibrated eyepiece graticule to measure pollen tube growth.

Question 7

Why should a cover slip not be used?

Answer 7

To allow oxygen to reach the pollen, preventing conditions from being anoxic.

Question 8

You were instructed to use pollen from only one flower for all the concentrations.

Comment on any advantages and disadvantages of this approach in terms of the validity and reliability of results.

Answer 8

Advantage:

• the use of pollen from the same flower improves the validity of the results because the maturity of the flower is a controlled variable.
• This means that any differences in the results between treatments are more likely to be a result of the differences in sucrose concentration.

Disadvantage: the results may not be reliable.

• Different results may arise if the experiment is repeated because of differences between flowers.
• Flowers may differ slightly in maturity or there may be genetic differences in pollen tube growth.
• Ideally, the investigation would be repeated with a large number of flowers and a mean would be taken.

Question 9

What is the role of the pollen tube?

Answer 9

The pollen tube carries the male gametes to the ovule for double fertilisation

Question 10

Suggest what might cause pollen tubes to grow in the correct direction in the style

Answer 10

Pollen tube growth may be attracted towards the micropyle by chemicals secreted by the embryo sac.

The pollen tube may be positively chemotropic

Question 11

A student needs to measure the length of a pollen tube

Explain the steps needed to set up the microscope apparatus to make it ready for measuring length (4)

Answer 11

• The graticule needs to be calibrated
• This uses an eyepice graticule with a stage micrometer
• Select magnification. focus and align graticule and micrometer
• Count how many micrometers on stage micrometer correspond to one eyepiece graticule
• Use this ratio to calculate distances