# Growing Pollen Tubes

In this worksheet, students will be taken through an investigation to generate pollen tubes from pollen grains and then look at the variables involved in performing the experiment.

Key stage:  KS 3

Curriculum topic:   Biology: Structure and Function of Living Organisms

Curriculum subtopic:   Reproduction

Difficulty level:

### QUESTION 1 of 10

Mrs. Bates's science class had been studying how plants reproduce and also how to use a microscope, so in today's lesson they are going to try growing pollen tubes.

In flowers, when the pollen arrives on the female stigma, it starts to grow a tube down towards the ovary in order to fertilize the ovule and form the beginnings of the seed.

Let's join the class and see how they get on.

"Today we're going on a magical journey," Mrs. Bates's first words thrilled the class. "We're going to watch pollen grains growing their little tubes that reach down deep into the flower to join with the ovule."

"Who can tell me - what part of the flower is the pollen made in?"

petals

stigma

stamens

carpel

"To make the pollen grow we're going to place some pollen grains into different concentrations of sugar solution," Mrs. Bates explained to the class. "How many different concentrations do you think we should try?" she asked.

What do you think?

2

5

10

100

"About how many pollen grains do you think we should try at each concentration?" she asked her class.

What do you think is best?

5

20

50

100

"First of all we're going to find out just how small a pollen grain is," Mrs. Bates told the class. "Use your microscopes and the microscopic rulers and I'll give you some pollen grains."

Poppy was looking down her microscope - this is what she saw:

The microscopic ruler is marked in 0.1mm divisions. Use that to estimate the width of Poppy's pollen grain.

0.1mm

0.15mm

0.5mm

"Now this is quite a tricky experiment," Mrs. Bates warned the class. "I have prepared several concentrations of sugar solution for you as well as some special microscope slides."

She divided the class into several groups and handed out the instructions. The scientists followed their instructions, as well as they could, and observed the pollen grains over the next hour.

Tick any TWO observations that you think the students are likely to have made during that hour.

some pollen grains grew tubes out of them

some pollen grains shrivelled up

some pollen grains floated away on the breeze

some pollen grains didn't do anything much

some pollen grains swelled up much bigger than usual

Mrs. Bates did the same experiment herself, but used pure water rather than sugar solution to place the pollen grains in.

What is her experiment called?

a blank test

a control

a base line

After the hour was up Mrs. Bates asked her students to count the number of pollen grains that had grown tubes out of them. The results were plotted on a bar graph. Here it is:

Look carefully at the graph. Which sugar solution concentration do you think was the most successful?

15%

20%

100%

Predict how many pollen grains produced a pollen tube in Mrs. Bates's experiment using plain water with no sugar in.

0

5

12

Here's the graph from Q7 again:

The sugar solution mimics (that is, it sort of copies) the kind of conditions that the pollen grain would find when it arrives on the stigma of a flower.

What sort of sugar concentration does this suggest is found on the stigma?

0-10%

10-20%

20-30%

50-100%

Apart from the sugar concentration, what other sorts of factors might affect whether a pollen grain grows a tube out of it? Tick THREE factors you agree with.

temperature

wind speed

light

air pressure

gravity

time

• Question 1

"Today we're going on a magical journey," Mrs. Bates's first words thrilled the class. "We're going to watch pollen grains growing their little tubes that reach down deep into the flower to join with the ovule."

"Who can tell me - what part of the flower is the pollen made in?"

stamens
EDDIE SAYS
The pollen is the male GAMETE (sex cell) of the flower and is made in the top part of the stamens (called the anther). It's easy to remember as staMENS are male!
• Question 2

"To make the pollen grow we're going to place some pollen grains into different concentrations of sugar solution," Mrs. Bates explained to the class. "How many different concentrations do you think we should try?" she asked.

What do you think?

10
EDDIE SAYS
A greater number of different concentrations gives a more accurate answer, but it's a school science lesson, and they don't have forever. So, 10 gives a good spread of results, involving everyone, without giving way too much data and taking ages.
• Question 3

"About how many pollen grains do you think we should try at each concentration?" she asked her class.

What do you think is best?

20
EDDIE SAYS
Again, it's better to have lots and lots, but it'll take ages to count as many as 50 and as for watching all of them to see if they're growing tubes....! 20 should give sufficient results to use in a science lesson.
• Question 4

"First of all we're going to find out just how small a pollen grain is," Mrs. Bates told the class. "Use your microscopes and the microscopic rulers and I'll give you some pollen grains."

Poppy was looking down her microscope - this is what she saw:

The microscopic ruler is marked in 0.1mm divisions. Use that to estimate the width of Poppy's pollen grain.

0.1mm
EDDIE SAYS
Look carefully at the pollen grain - it's bang next to one mark and the other side is just past another. That makes it closest to 0.1mm - it's not as much as halfway between marks, so it's not as big as 0.15mm, but you're pretty close if you chose that.
• Question 5

"Now this is quite a tricky experiment," Mrs. Bates warned the class. "I have prepared several concentrations of sugar solution for you as well as some special microscope slides."

She divided the class into several groups and handed out the instructions. The scientists followed their instructions, as well as they could, and observed the pollen grains over the next hour.

Tick any TWO observations that you think the students are likely to have made during that hour.

some pollen grains grew tubes out of them
some pollen grains didn't do anything much
EDDIE SAYS
The point of the experiment is to watch pollen grains producing tubes, so that should be one of your choices. Of course, lots of them won't do anything, so that's your second choice.
• Question 6

Mrs. Bates did the same experiment herself, but used pure water rather than sugar solution to place the pollen grains in.

What is her experiment called?

a control
EDDIE SAYS
In science we have 'fair tests' and 'controls'. In a fair test you change ONE variable at a time, to see what it's effect is. A control is different because it is showing you what happens to pollen grains if the thing you're testing (sugar solution) is not given to them - it's really important because it allows you to compare the other results to the control to see if there's been a change. Mrs. Bates did it herself because most of the students would feel it was 'boring'!
• Question 7

After the hour was up Mrs. Bates asked her students to count the number of pollen grains that had grown tubes out of them. The results were plotted on a bar graph. Here it is:

Look carefully at the graph. Which sugar solution concentration do you think was the most successful?

20%
EDDIE SAYS
The one with the highest bar is the one with the most pollen tubes produced - that was 14 tubes in the 20% sugar solution.
• Question 8

Predict how many pollen grains produced a pollen tube in Mrs. Bates's experiment using plain water with no sugar in.

0
EDDIE SAYS
It's likely that no pollen grains grew a pollen tube in water alone; looking at the results, it's clear that more tubes resulted from grains in 10-20% sugar solutions, so it appears that sugar is important in inducing the grain to grow a tube, so the probability is that those in water would not grow tubes..
• Question 9

Here's the graph from Q7 again:

The sugar solution mimics (that is, it sort of copies) the kind of conditions that the pollen grain would find when it arrives on the stigma of a flower.

What sort of sugar concentration does this suggest is found on the stigma?

10-20%
EDDIE SAYS
The best growth of pollen tubes was in the 10%, 15% and 20% sugar solutions, so that suggests that these solutions most closely match the conditions found on the actual stigma of the flower.
• Question 10

Apart from the sugar concentration, what other sorts of factors might affect whether a pollen grain grows a tube out of it? Tick THREE factors you agree with.

temperature
gravity
time
EDDIE SAYS
Temperature is likely to have an affect, with warmer conditions allowing the grains to produce their tubes more quickly. Gravity is important in ensuring that the tube grows DOWN towards the ovary. Time will have an effect in terms of how long it takes for the grain to respond to the conditions on the stigma. The others won't affect whether a pollen tube grows, although they may affect whether the pollen grain reaches the stigma in the first place.
---- OR ----

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