Issues highlighted by my preliminary experiments
- Considering the results of my preliminary tests, I will be growing my cress inside by a window so that it has plenty of access to light and warmth. I will be growing my cress in petri dishes on two circular pieces of kitchen paper.
- I will be covering each dish with a piece of transparent film slightly larger than the dish itself. This is because the kitchen paper tends to dry out quickly, which would obviously prevent germination and negatively effect plant growth. The reason for using a piece of transparent film is because this will let light through. I will not be sealing these, as this will prevent air getting to the cress, which it requires for growth. This method prevents the seeds from drying out too quickly, although I will still be watering them at intervals specified before.
- In my preliminary tests, I measured the seed growth by their length above the seed and root length. However, this may not be accurate as some cress seedlings may have longer roots, and longer roots also indicate growth. For this reason I have decided to measure the shoot and root length combined of my seedlings.
Ensuring consistancy
- I need to ensure that the amount of liquid present stays constant, and thus need to ensure that the dilutions of detergent liquid have appropriate amounts of distilled water. I will be using 20ml of water for the control, and various concentrations made up to 20ml for the other experiments.
- I need to ensure that the environment of the plants stays as consistent as possible throughout the experiment, and thus need to choose a window with enough space beneath it to give equal light to all the plants.
- I need to ensure that the cress is spread in the same way in each experiment. To give maximum space to each seed, I have chosen the following layout:
This gives me enough room for 25 cress seeds in each experiment.
- I will need to ensure that the temperature around the petri dishes is kept constant between all petri dishes, as an inconsistency will have a small effect on growth. To do this, I shall measure the temperatures near each dish twice daily and record and adjust any anomalies.
- The petri dishes will need to be clearly labeled with concentration and experiment number (either 1 or 2) so as to avoid confusion when collecting results.
Preparation
As the Fairy Active Burst Red detergent comes in capsules, I need to extract the liquid for dilution. To do this, I must prick the tablets with a pin and squeeze the liquid into a clean beaker.
Equipment
- 6 x petri dish
- 150 x cress seed
- A thermometer
- Distilled water
- Tweezers
- Fairy Active Burst Red liquid
- Various pipettes
- Pipette filler
- Sturdy card
- Conical flask
- Kitchen paper cut into circles the size of the petri dish
- Transparent film cut into circles slightly larger than the petri dish
Method
- Ensure that all equipment is clean and ready.
- Using the card, measure out 8 pieces of card, 5cmx1cm rectangular. Make 4 slits of around 0.75cm, one every 1cm along the long sides, and slot these together in the shape of the seed layout given above.
- In a conical flask, measure out the correct volumes of distilled water and detergent as per the table below using the pipette and pipette filler to ensure accuracy of measurement. Keep the pipettes for distilled water and the pipette for detergent completely separated.
- When this is complete, place the 2 layers of kitchen paper into the base of the petri dish.
- Place the card template on top of this, and place one seed in every compartment made using the tweezers.
- Remove template.
- Pipette the solution (using a clean pipette) made in step 3 as evenly as possible across the petri dish.
- Cover each petri dish with one piece of transparent film, and leave them by the window, ensuring that the temperature is steady and the cress receives enough light.
- Twice every day, measure the temperature of the environment, note and fix any anomalies that occur.
- Every other day, add an extra 5ml of distilled water to each dish, at the same time each day.
- After 7 days, count the number of seeds that have germinated and measure heights of the seeds from base to highest point, including root. Record.
I plan to repeat the experiment at each concentration twice, as mentioned in my justifications.
Risk Assessment
- Some detergents can cause irritation to skin. To prevent this, rubber gloves and goggles will be worn throughout the experiment to prevent painful skin irritation and contact with the eyes.
- Spillages must be cleaned up as soon as they occur as they may damage surfaces and skin.
- All breakages of glass pipettes must be cleared away carefully as soon as they occur to prevent injury.
Results:
To calculate an average, I must first calculate the sum of all the numbers desired as part of the average, and then divide by the number of figures used. For example:
48.3+52.1 = 100.4
100.4/2=50.2
Test 1 - Control
Test 2 – 10%
Test 3 – 20%
Test 4 – 30%
Test 5 – 40%
Test 6 – 50%
Analysis:
Trends and patterns:
- As I predicted, the percentage germinated and average height are both negatively affected by high concentrations of detergent, whereas smaller concentrations (10%-20%) have a slight positive effect on average height and 10% has a slight positive effect on percentage germinated.
- There is a high standard deviation in the smaller concentrations (10%-20%) of detergent, whereas 0%,30%,40% and 50% all produce roughly the same standard deviation (between 8 and 12)
- The percentage germinated seems to be only very slightly affected by lower concentrations, as 0%, 10% and 20% all have germinations of between 80-90%.
- The percentage germinated is, however, very strongly affected by stronger concentrations, with 50% producing only 50% germinated seeds.
- The average height is very similarly affected – smaller concentrations of detergent all produce between 55mm and 65mm plants, including the root, whereas this drops drastically with 50% concentration, to an average of only 32mm.
Anomalies:
There are no distinct anomalies in the data after averages have been taken.
Variability:
Obviously, cress is a naturally occurring plant and thus there is not much that can be done to change the varying results due to natural variation. Some minor mistakes may have been made during measuring, as the microscope and micrometer eyepiece I was using was difficult to use and I may have caused small movements.
Spearman’s Rank Correlation Coefficient:
Rs = 1- (6 x 0/210)
Rs = 1-0
Rs = 1
And thus there is perfect correlation between the percentage of germinated seeds and average height of the cress seeds.
Evaluation:
Explanations:
I predicted that the presence of phosphates in the detergent would have a small positive effect at low concentrations and that this effect would be cancelled out by the presence of other pollutants present in the detergent. This hypothesis has obviously been proved right by my results, as explained in the Analysis section.
The reason I believed that this would happen is because phosphates are present in ATP. Pentasodium Triphosphate, the phosphate present in the detergent used, is only present in detergents that display a warning, as in certain conditions it breaks down to an orthophosphate which may cause algal blooms and other abnormal growth patterns. This is obviously what has happened in the case of my cress experiment.
The presence of citral and limonene, perfumes, and the colorants may have, however, had a negative effect on the germination and growth. It may have been the presence of these that, at higher concentrations of 30% and above, damaged the cell walls of the seeds and obviously meant that they could not germinate. The ones that did germinate would have been affected in a similar way, in that their roots will have been damaged and the cell walls of the root cells may have been broken down by these “additional” ingredients and thus disturbed and stunted the growth of the cress plant.
There is perfect correlation between the average height in millimeters and the percentage of seeds germinated. This shows that phosphates affect both growth and germination in a similar way.
Limitations:
- Although the mass is a more accurate way of measuring growth, as it allows for root growth as well as visible growth, more accurate still would be using the dry mass, although this was not possible.
- I believe that using a microscope and the small measuring eyepiece was a disadvantage as it wasn’t terribly accurate and results could have been skewed by any small movements.
- The packet of seeds I was using recommended a 2 week gestation period for the seeds before they are edible. Perhaps after 2 weeks, rather than 1, the growth may have evened out somewhat.
- This experiment only showed me the effect of a detergent as a whole rather than the actual effect of phosphates. While this is important in reality, as it is often detergent as a whole that gets washed into water sources, I cannot be sure that it is the pentasodium triphosphate giving me these results and not an alternative chemical.
- Only using 50 seeds per concentration in the overall, single experiment has obvious percentage error and data collection limitations.
- The cress variety used in this experiment doesn’t grow in the wild and thus wouldn’t come into contact with phosphates in the same way wild plants would. This means that the varieties of plants and algae that grow in the wild and come into contact with phosphates as part of irrigation systems, lakes, ponds and rivers may not respond in the same way as the cress I used to experiment with.
Further Work:
- If I was to do this again, I would have measured the dry mass to get more accurate results for growth. Obviously drying them removes the water and gives a more valid and reliable measurement, and using mass instead of length is both simpler and more useful than measuring length.
- I would also try and do as many repeats as possible, as using only 50 seeds for each concentration could still have given quite significant percentage error. For example, 1 anomaly from this would have given a 2% error, whereas 1 anomaly from 500 would only be 0.2%.
- If repeating this experiment I would carry it out over a longer period of time, for example 2 weeks as opposed to one.
Bibliography:
23 February 2009
NEC AS Biology course materials
Roberts, Reiss and Monger Advance Biology Textbook
------------------------------------------------------------------------------------------------------------------------------------------
Appendix:
Preliminary Tests:
First of all, I want to find the best environment for cress seeds to germinate in. For this, I will find:
- The medium in which the cress seeds grow most effectively in
- The environment at which the cress seeds grow most effectively in
Test 1
The aim of this test is to discover which medium the cress seeds best grow in. For this, I had to consider many possibilities, such as cotton wool, soil, compost, tissue paper, kitchen paper and flannel. I felt that soil and compost would not be ideal to use in this experiment as they have uncertain amounts of minerals and ions present in the composition. Tissue paper, whilst being easy and cheap to obtain, was too thin to work in a single layer, and thus I decided it would not be a fair test to use 5+ layers of tissue paper for every one layer of the other mediums. I decided to carry out my preliminary tests on cotton wool, kitchen paper and flannel.
Equipment:
- 3 petri dishes
- 1 circular piece of flannel
- 2 circular pieces of kitchen paper
- Cotton wool
- Water
- 30 Cress seeds
- Marker pen
- Pipette
- Tweezers
- 3 circular pieces of transparent film, slightly larger than the petri dishes
Method:
- Place the appropriate medium (2 pieces of kitchen paper) into the bottom of each separate petri dish, and label each one correctly.
- Carefully measure 15ml of distilled water into the bottom of each petri dish, using a pipette to ensure that the water soaks into as much of the medium as possible.
- Place each of the 10 cress seeds required into the petri dish containing cotton wool using the tweezers, ensuring that they are as widely spaced as possible.
- Repeat step 3 for each medium.
- Cover each petri dish with one piece of transparent film, and leave in a warm place which receives plenty of sunlight.
- Every day, add an extra 5ml of distilled water to each dish, at the same time each day.
- After 5 days, count the number of seeds that have germinated and measure their height from base to highest point, not including root.
Although the cress seeds grown in cotton wool grew higher, I decided to use kitchen paper as more seeds germinated in this medium. I understand that this may have been due to an error margin, however, kitchen paper is easier to keep consistent as it can be easily cut and measured.
Test 2
The aim of this test is to discover which environment the cress seeds best grow in. As there were a limited amount of different places I could put my cress seedlings, I immediately decided upon placing one dish by the window, one on a patio outside, and one in the airing cupboard. This gave me a variety of different temperatures and effects, all of which could affect the cress.
Equipment:
Equipment:
- 3 petri dishes
- 6 circular pieces of kitchen paper
- Water
- 30 Cress seeds
- Marker pen
- Pipette
- Tweezers
- 3 circular pieces of transparent film, slightly larger than the petri dishes
Method:
- Place the circular pieces of kitchen paper (2 in each) into the bottom of each separate petri dish.
- Carefully measure 15ml of distilled water into the bottom of each petri dish, using a pipette to ensure that the water soaks into as much of the kitchen paper as possible.
- Place each of the 10 cress seeds required into the petri dishes containing kitchen paper using the tweezers, ensuring that they are as widely spaced as possible.
- Cover each petri dish with one piece of transparent film, and leave one in each of the places specified above.
- Every day, add an extra 5ml of distilled water to each dish, at the same time each day.
- After 5 days, count the number of seeds that have germinated and measure their height from base to highest point, not including root.
Considering these results, it seems best to place the cress seeds by the window, as this gave the most seeds germinated, although the height was generally shorter. This may be due to differences in the light available, rather than the temperature.