This justifies my prediction. As leaves are taken off there will be fewer stomata for water to escape through so I would expect there to be less water loss and therefore less water take-up.
So leaf area is a good choice because of the stomata. Through preliminary work (below) I found there to be 75 stomata per mm2. This is a large area for water to escape through so cutting leaves off should have a large effect on the readings. I measured this by painting nail varnish on the leaves taking it off and counting the stomata under a microscope. This is described in greater detail in the preliminary work below.
Accuracy
- I will take a plant with enough leaves on so I can get 6 -10 different readings i.e. six leaves or sets of leaves removed.
- I will take the readings for each number of leaves 5 -10 times and take a mean from the readings which will increase reliability and accuracy.
- When taking readings I will wait for three consecutive readings that are similar/the same i.e. +/- 1mm from each other.
- The readings will be taken at eye-level with the meniscus to ensure accuracy by reducing parallax error.
- I will do one minute trials i.e. measure how far the meniscus travels in one minute.
- The equipment is reliable to +/- 1mm.
PRELIMINARY WORK - Stomatal Densities
A thin layer of clear nail varnish (about 1 cm2 area) is painted on the upper and lower surfaces of a leaf. This leaf is left to dry.
Using a pair of fine forceps the film of varnish is peeled off. This is placed onto a drop of water on a clean slide, covered with a cover slip and examined under a microscope on high power. The number of stomata that can be seen is counted.
Results
Upper surface= 0 stomata
Lower surface= 15 stomata
You can then work out the number of stomata in 1mm2.
The actual diameter of the field of view can be read from the foot of the microscope-0.5mm, so the radius is 0.25mm.
From this you can calculate the area you are looking at (πr2).
(1/the area of your field of view x number of stomata)
Radius of microscope
πx0.252=0.196 area of microscope
0.196 goes into 1mm 5 times
15x5=75 stomata per 1mm2
This is a large area for water to be lost through, so I would expect that cutting leaves off will have a dramatic effect on water uptake as the leaf area will be greatly affected.
GCSE BIOLOGY COURSEWORK: Obtaining
Safety
I took into consideration many safety points when obtaining the evidence:
- The glassware can be broken easily - glass should be pulled from the holder not levered.
- Laurel is poisonous - hands should be washed after handling it.
- Secateurs are sharp - you should be careful not to cut yourself, especially when cutting the plant underwater.
I also made sure I took accurate readings by:
- Labelling plastic bags before leaves were put in so they did not get mixed up.
- Squeezed the air out of the bags to stop the leaves changing shape.
- Looking from the level of the meniscus to reduce parallax error – readings were taken from the top of the meniscus.
- I also waited for three consecutive near-identical readings (+/- 1mm) before recording them to increase accuracy.
GCSE BIOLOGY COURSEWORK: Analysis
The evidence shows that the fewer leaves the plant has, the less water it takes up.
The graph shows strong negative correlation.
The less leaf area left, the less water taken up.
Conclusion
Water is lost through the stomata on the underside of the leaves. There are about 75 stomata per mm2.
Quote from Green, Stout and Taylor biology book:
“Stomata: by evaporation of water from cells and diffusion of the water vapour through stomata, the pores found in the epidermis of leaves and green stems (about 90 %*).”
*90% of water is lost through the leaves. The other 10% is through lenticels. The rest of the plant (not the leaves) is not 100% waterproof.
60072(total leaf area)/7(number of leaves=8581.714283.
So the average leaf area is about 8581.7mm2.
The average number of stomata per leaf was 643628.5714.
This is a large area for water to be lost through.
The results show us that when we remove leaves, less water is lost. This is because there is less stomata for water to be lost through. The top-side of the leaf has no stomata.
Diagram of how water is lost through stomata
Contours of equal concentration of water molecules; steeper potential gradient=closer contours and faster rate of diffusion. The fastest rate is at the edge of the pores. Water loss and gas exchange is more rapid through a large number of small holes than a small number of large holes with the same total area (edge effect). Water is lost very rapidly through these holes. By cutting leaves off, so there are fewer of these holes, it is bound to have a dramatic effect on the water uptake because such a large amount of water is lost through them. My results confirmed this.
The graph shows strong negative correlation. The rate of water loss when leaves are removed changes evenly.
I predicted:
1. Plant will take up most H2O with all leaves on.
2. When the plant has half the leaves it started with it will take up half the H2O.
3. Even with all leaves off, 10% of water will be lost /taken up (GST).
The graph shows us that my first prediction was correct. The graph slopes downwards showing that as leaves were removed the water uptake was less.
The graph also shows that my second prediction was correct. It did roughly have half the water loss/take-up when there were half the leaves.
6 leaves on plant = 38mm travelled by meniscus in one minute
3 leaves on plant = 18mm travelled by meniscus in one minute
38-18=2.1 This is close to 2 which would have been the result if everything was perfect.
However, my third prediction did not turn out to be true. There was less than 1mm3 of water taken up with no leaves. However, this could not have worked properly (explained in more detail in evaluation).
GCSE BIOLOGY COURSEWORK: Evaluation
The procedure was reliable once set up. The setting up was the most difficult part.
The evidence was accurate. The error bars on the graph were quite close to each other. Sometimes each reading was identical each time for the number of leaves. As long as the calculations were done correctly the final data is accurate. One problem though was air bubbles which would often become trapped by the cut part of the plants stalk. This left less area for water to be taken up so the accuracy would be affected if the bubbles were not removed.
The procedure was quite good. It produced consistent results when set up properly. However, it could be improved. One problem was that there were just too many variables, e.g. air pressure, light intensity, wind speed etc. A better experiment would be one that did not have so many variables. Also, the plant is living and there would be a certain amount of shock to it when it was cut apart. A better experiment would be one where it is not cut. The experiment also needed time to get going and the plant needed time to settle down and produce consistent readings. Things that I would change are monitoring air pressure and choosing a time to do the experiment when this is constant. I would do it in a darkened room using an artificial light source which could be monitored. Temperature would also be kept constant and doors and windows would be shut to prevent air turbulence. I would cover the leaves with cling film rather than cut them off. This would reduce stress to the plant and improve the accuracy of the readings.
The evidence was reliable. The error bars were close to each other meaning the readings were (sometimes identical). The reliability though could be affected by the airlocks, other uncontrolled variables and water on the leaves. Care had to be taken to wipe off any water on the leaves. The Vaseline also had to be secure on the tips so no water could escape. You also had to make sure that your eyes were at the level of the meniscus to make accurate readings.
Overall, the conclusion is sufficiently supported by the evidence.
Mini-plan for second experiment
If I did an experiment like this again I would do it a different way. I would measure the water loss by measuring the mass of the plant and water being taken up. Because 1g of water=1cm3 it would be easy to measure the water uptake, and therefore loss. I would take six consistent readings from the balance. This would have greater accuracy than the three readings I took in the previous experiment.
Diagram of Apparatus
The variables will be the same as in the last experiment but I would try to keep more of them controlled. Air pressure would be monitored. I would do the experiment in a dark room using artificial light as a light source. This would enable me to monitor the light that gets to the plant. This would improve the accuracy. The humidity could also be kept constant in the room as could temperature. Air turbulence would be kept to a minimum by closing all windows and doors and making sure that no one enters while I am doing the experiment. Another problem which I will address from the first experiment is that cutting off the leaves gave the plant shock affecting the readings. To combat this I would cover the leaves with cling film instead of cutting them off. This would cause no distress to the plant so the readings will be more accurate.
The plant and apparatus would be weighed first, then I would see how the mass changes as water is sucked from the beaker. The balance would be accurate enough to detect the small weight change. I would wait for the plant to settle down and for the water uptake to become constant. I would perhaps measure the weight change in one minute and repeat six times to increase accuracy. Before I cover the leaves with cling film I will weigh the cling film first. This will stop the weight of the cling film from affecting the readings.
This method would be more accurate than the last because the balance is more accurate than judging from a scale as in the last experiment. There would be no errors such as parallax with this experiment and no stress to the plant because of leaves being cut off, making this experiment much more reliable than the original.
