I have decided that the most suitable method to measure rate of transpiration would be using a potometer to take the volume of water taken up a branch of each of the two plants which would indicate the rate at which the plant is transpiring. And to measure the wind speed an anemometer which will measure the wind speed.(3)
I predict that the graph of transpiration rates of the 2 different species of plant at different wind speeds would look like this:
The reason I think the graph would look like this is for several reasons:
- holly with no wind would transpire less due to its thick waxy cuticle and lower stomata density.
- Holly would plateau earlier as less water vapour is being transpired out therefore a lower velocity of wind will be required to remove the water vapour from near the plants surface.
- The acceleration of the rate of transpiration will be higher for holly as the humidity percentage outside the leaf will be lost at a higher percentage than that of the privet therefore a steeper gradient in relation to water potential.
Apparatus
1 branch of holly
1 branch of privet
1 fan
1 potometer with mm scale (only simple one required i.e. capillary tube (300mm long) with rubber connector and a mm scaled ruler taped to it)
1 clamp stand with clamp
1 meter ruler
1 large bowl filled with water
1 stop clock
1 anemometer
1 pair secatures
Method
Whilst preparing the potometer no air must come into contact with the end of the shoot (of either plant) therefore the first stage (preparation) must be carried out under water.
- Set up fan and clamp stand firstly at 10cm away from each other (make sure blinds are down and lights are on to keep light intensity as a constant and take temperature using thermometer throughout to make sure temperature is also kept constant)
- Submerge the capillary tube under the water in the large bowl ensuring no bubbles are in it
- Cut the shoot of the plants end off under the water slanted as this will reduce the chance of air being taken up if the shoot happens to come out of the water momentarily.
- Insert the end of the shoot into the water-tight rubber seal (ensuring water-tight by applying petroleum jelly around joins).
- Turn fan on then take shoot and place on clamp in line with flowing air then start timer (first time don’t turn fan on as it is the control with no wind speed)
- Leave for 5 minutes so that the plant can settle then after a minute take the reading of how many units (mm) the inverted meniscus has moved from the point at which it was after 5 mins (indicating how much transpiration took place over the min)
- Repeat for 10cm, 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100cm (refilling capillary tube each time by squeezing the rubber tubing whilst the capillary tube is under the water and releasing making the water fill the tube due to the reduced pressure)
- Repeat for other plant ensuring that the blinds are still down and temperature remains the same.
- Repeat whole experiment for both plants (but different shoots) to ensure that results are not anomalous and an average can be calculated.
Safety
There are little hazards in this experiment as there are no chemicals being used however there will be an electrical fan and water being used at the same time therefore to avoid electrocution or water making contact with the electrics I will cover the socket and avoid spilling the water. Another hazard is the secatures as you may cut yourself with them due to this you must be careful whilst cutting the plant. You must also take care when putting the capillary tube onto the plant as if you slip the capillary tube will could pierce your hand.
Set up apparatus as shown below
To calculate the average you add the results for that distance for the certain plant species and divide by 2.
These results however do not give the volume of water transpired which is easily enough worked out.
To work this out cm2 paper and a small measuring device (mm ruler) are required.
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Draw around every leaf on squared paper and count how many cm2 surface area they take up.
- work out volume of capillary tube
After working out the radius which is the width of the hole/2 you must put it into this equation for each of the average lengths of the two plants giving you the volume of water transpired over 1 min (mm-2min-1)
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Then you must work out how much is transpired per cm2 of surface area which is a simple equation:
Volume of water transpired over 1min / surface area of leaves (cm2)
This will produce you with another table which looks like this:
It would now be sensible to convert from distance to an actual wind speed using the digital anemometer as now you have more time and don’t have to worry about the potometer and stop clock.
E.g. 10cm = 35mph
20cm = 32mph (not accurate just estimates)
Etc…
From the table of results on the previous page and the wind speeds produced from the anemometer you can then produce a graph similar to the one I drew earlier for my predictions. This graph will show the relationship between wind speed and transpiration rates and how they differ between the 2 species of plant.
Bibliography
(1) Biology 1 :Jones, Fosbery, Taylor,. Cambridge university press
(2)
(3) Letts Revise AS & A2 Biology : John Parker