Investigating seed growth under different conditions

In a nutshell

The effect of light on the growth of newly germinated seedlings can be measured by allowing mustard seeds to germinate in petri dishes in different light intensities. The height of each seedling will be measured and compared. The effect of gravity on the growth of newly germinated seedlings can be measured by allowing mustard seeds to germinate in petri dishes and then transferring one to a clinostat and another to a vertical support. The height of each seedling will be measured and compared.

Equipment list

The following equipment can be used to investigate the effect of light and gravity on the growth of newly germinated seedlings.

Equipment	Use
White mustard seeds	These will be newly germinated seeds that you are measuring the height of.
Petri dishes	The seeds will be growing in petri dishes.
Cotton wool	Cotton wool will line the petri dish and the seeds will be planted on the cotton wool.
A $30\ cm$ ruler	Used to measure the height of the newly germinated seedling.
Water	Used to water the seedlings.
Access to a light windowsill and a dark cupboard	To create different light intensities for the seedlings to grow in.
A wooden support	To hold the petri dish upright.
A clinostat	To rotate the petri dish to negate the effect of gravity.

Safety precautions

When carrying out experiments, it is very important to consider the safety precautions so nobody gets hurt.

Hazard	Risk	Safety measure
Seeds	The seeds may present a biohazard as they contain a toxic compound called isothiocyanate. Inhaling powder from the seeds can irritate your airways.	Wash your hands after touching the seeds.

Experiment 1: Investigate the effect of light on the growth of newly germinated seedlings.

Experimental variables

Every experiment will have an independent, dependent and constant variable. The independent variable is the variable that you change. The dependent variable is the variable that depends on the independent variable; therefore, it is the one you measure. The control variable is what is kept the same during the experiments.

Independent variable	$The\space light\space intensity.$
Dependent variable	$The\space height\space of\space the\space plant.$
Control variables	$The\space length\space of\space time\space that\space the\space plant\space grows\space for,\space the\space temperature,\space\\ the\space water\space availability,\space the\space carbon\space dioxide\space concentration,\space the\space size\space\\ of\space the\space petri\space dish,\space the\space number\space of\space seeds\space per\space petri\space dish\space and\space the\space type\space of\space seeds.$

Method

The following method is used to investigate the effect of light on the growth of newly germinated seedlings.

1.	Soak cotton wool in equal amounts of water.
2.	Line three petri dishes with equal amounts of cotton wool.
3.	Put ten mustard seeds in each petri dish.
4.	Put all three petri dishes in a warm location and do not move them until they germinate. Note: Water the seeds daily with the same amount of water for each dish.
5.	Once the seeds have germinated, count the number of seedlings in each dish. Remove any seeds that did not germinate and ensure each dish has the same number of seedlings.
6.	Measure the height of each seedling.
7.	Move one petri dish to a location with full sunlight, like a windowsill that is in the sun all day. Move a second petri dish to a location with partial light, like a windowsill that is in the sun for half of the day and shaded for the other half of the day. Move the third petri dish to a dark place, like a cupboard.
8.	For five consecutive days, measure the height of each seedling.
9.	Record the heights in a table.

The experimental set-up should resemble the diagram below.

1.	A ruler used to measure the height of the seedlings.
2.	Measuring the height of the mustard seedlings.
3.	Petri dish used to grow the seeds in.
4.	Cotton wool to act as a growth medium for the seedlings.

Analysis

This is how you would use your data to form conclusions. At this stage you should have three tables containing your data like the one shown below. You should have one table for the seedlings grown in full sunlight, one table for seedlings grown in partial light and one table for seedlings grown in the dark.

Table of results

	Height of seedling in full sunlight in $mm$
Day	$1$	$2$	$3$	$4$	$5$	$6$	$7$	$8$	Mean
$1$
$2$
$3$
$4$
$5$

Using the data from your table, you can perform your data analysis.

1.	Calculate the mean height of each seedling for each day. $mean=\frac{height\space on\space day\space 1 + height\space on\space day\space 2 + height\space on\space day\space 3 + height\space on\space day\space 4 + height\space on\space day\space 5}{5}$
2.	Using this data, plot a graph. The $y$ -axis should have the mean height in millimetres and the $x$ -axis should have the day number. *Note:* Plot all data points for full sunlight, partial sunlight and darkness on the same graph.

Conclusion

The plants in the sunlight will grow towards the sun. Auxin is the plant hormone that controls the growth of the roots and shoots of the plant. Auxin gathers on the shaded side of the plant and stimulates growth of the cells on the shaded side of the plant. Therefore, the seedling will bend towards the light. The control of the growth towards the sun is called phototropism.

Evaluation

After completing the experiment, you must comment on the quality of your data. You should also think about how your method could be improved.

A common source of error is that the seeds are old and they do not germinate or grow well. It is important that all the seeds come from the same batch and that they are fairly new seeds.

It is also important that each petri dish receives the same amount of water. This is an easy mistake to make and will impact the growth of the seedlings. It is also best to measure the growth of the seedlings at the same time each day to ensure the growth you record is over a $24$ hour period.

Experiment 2: Investigate the effect of gravity on the growth of newly germinated seedlings.

The effect of gravity on the growth of newly germinated seedlings can be measured by allowing mustard seeds to germinate in petri dishes and then transferring one to a clinostat and another to a vertical support. The height of each seedling will be measured and compared.

Experimental variables

Every experiment will have an independent, dependent and constant variable. The independent variable is the variable that you change. The dependent variable is the variable that depends on the independent variable; therefore, it is the one you measure. The control variable is what is kept the same during the experiments.

Independent variables	$The\space presence\space or\space absence\space of\space gravity.$
Dependent variable	$The\space growth\space pattern\space of\space the\space seedlings.$
Control variables	$The\space length\space of\space time\space that\space the\space plant\space grows\space for,\space the\space temperature,\space\\ the\space water\space availability,\space the\space carbon\space dioxide\space concentration,\space the\space size\space\\ of\space the\space petri\space dish,\space the\space number\space of\space seeds\space per\space petri\space dish,\space the\space type\space of\space\\ seeds\space and\space the\space light\space intensity.$

Method

The following method is used to investigate the effect of gravity on the growth of newly germinated seedlings.

1.	Soak cotton wool in equal amounts of water.
2.	Line two petri dishes with equal amounts of cotton wool.
3.	Put three mustard seedlings in each petri dish. *Note:* Place one seedling with the radicle facing downwards, one seedling horizontal and one seedling with the radicle facing upwards (see reference diagram).
4.	Put a lid on each petri dish.
5.	Place on petri dish on a support so it is on its side.
6.	Place the other petri dish on a clinostat.
7.	Place both petri dishes in a dark box.
8.	Leave for two days.
9.	Observe the growth of the seedlings.

The experimental set-up should resemble the diagram below.

Biology; Biology practicals; KS4 Year 10; Investigating seed growth under different conditions

A.	This is the seedling with the radicle facing downwards.
B.	This is the horizontal seedling.
C.	This is the seedling with the radicle facing upwards.
1.	This is the radicle which will develop into the root.
2.	This is the plumule which will develop into the shoot.

Analysis

Record your observations. Determine whether there has been a positive or negative gravitropic effect and what the effect of clinostat is.

Conclusion

Auxin is the plant hormone that controls the growth of the roots and shoots of the plant. Auxin promotes the growth of the shoots (A) but inhibits the growth of the roots (B). Therefore, when auxin gathers in the plant root it stimulates growth downwards. The control of the growth towards gravity (1.) is called gravitropism (or sometimes geotropism).

Biology; Biology practicals; KS4 Year 10; Investigating seed growth under different conditions

Petri dish	Observation
Set-up A	This is the petri dish that was placed in the upright support. All of the radicles (1.) have grown downwards regardless of the way they were initially facing. This demonstrates a positive gravitropic response and all roots have grown in the direction of gravity. All of the plumules (2.) have grown upwards. This demonstrates a negative gravitropic response as all shoots have grown against the direction of gravity.
Set-up B	This is the petri dish that was placed on the clinostat. All radicles (1.) and plumules (2.) have grown straight outwards from whichever direction they were placed in. There is no gravitropic response as the rotation of the clinostat negates the effect of gravity.

Biology; Biology practicals; KS4 Year 10; Investigating seed growth under different conditions

Evaluation

After completing the experiment, you must comment on the quality of your data. You should also think about how your method could be improved. A source of human error in this practical would be forgetting to turn on the clinostat. This would mean that gravity could still impact the growth of the seedlings.