Constant : Intensity of light, humidity of air, type of plant, numbers of leaf.
Data collection
Quantitative data
SET A
Table 1; the mass of entire apparatus in SET A experiment for rate of transpiration of plant in different light intensity with respect to time.
SET B
Table 2; the mass of entire apparatus in SET B experiment for rate of transpiration of plant in different humidity of air with respect to time.
SET C
Table 1; the mass of entire apparatus in SET C experiment for rate of transpiration of plant in different speed of wind with respect to time.
Qualitative data:
It was observed that mass of entire apparatus of every set of experiment shows decrease in mass.
Data processing
Rate of transpiration of plant (hibiscus can be calculated by finding the average changes in mass of entire apparatus with respect to time by using formula:
Since the mass of entire apparatus is in Newton, it must be converted into grams. To convert it, we use the ratio:
1 Newton=100 grams.
Example; 0.82N x 100
=82 grams
The changes of mass for entire mass of apparatus can be calculated by;
SET A
Table 4; the rate of transpiration of plant at different intensity of light with respect to time.
Graph 1; the rate of transpiration versus different intensity of light.
SET B
Table 5; the rate of transpiration of plant at different humidity of air with respect to time.
Graph 2; the rate of transpiration versus different humidity of air.
SET C
Table 6; the rate of transpiration of plant at different speed of wind with respect to time.
Graph 3; the rate of transpiration versus speed of wind.
Discussion and evaluation
- Transpiration is a process where plants losses its water in form of water vapour, to surrounding via stomata. This process can be affected by atmospheric condition namely light intensity, humidity of air, and speed of air.
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According to theory, as the light intensity is higher, the rate of transpiration will also increase. This is because; guard cells will close the stomata during darkness. According to graph 1, the rate of transpiration is higher during low intensity of light as compared to rate of transpiration during high intensity of light. This observation does not agree with theory stated before. There are other possibilities to cause this rate of transpiration to occur. When the plant is under water stress, the need to preserve its water source is higher than its need of carbon dioxide for photosynthesis. Under these conditions, the plant will secretes abscisic acid, (ABA). This chemical is a messenger which causes stomata to close. ABA triggers a metabolic pump which actively secretes potassium ions out of guard cells, causing the cells to lose water and become flaccid. The stomata will close and the rate of transpiration is slower.
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High humidity of air will results in low rate of transpiration. According to graph 2, low humidity of air shows high rate of transpiration. This is because; water diffuses out to the leaf when there is a concentration gradient between the air spaces inside the leaf and the air outside. The air spaces are always nearly saturated. The lower the humidity outside the leaf, the steeper the gradient and therefore the faster rate of transpiration.
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Rate of transpiration is directly proportional to speed of the wind. Referring to graph 3, the higher the wind speeds, the faster the rate of transpiration. Pockets of air saturated with water vapour tend to form near stomata in still air, which reduce the rate of transpiration. Wind blows the saturated air away and so increases the rate of transpiration.
Conclusion
From the observation throughout the experiment, the rate of transpiration is higher when the humidity of air is low and speed of wind is high.
However, the rate of transpiration is lower when the intensity of light is high. This is not suit the hypothesis that stated as before. This reaction occurs was explained in the discussion.
Limitation and suggestion
Questions
- How would an air column drawn into the xylem affect your result?
An air column would disturb the continuous flow of water in the xylem vessel. This will cause huge impact on the normal function of the cohesive forces as it is vital for transpiration to occur. As cohesive forces can only be formed between molecules which are alike, the presence of air column will not allow any cohesion of water to take place. This will cause the water column below not to be pulled together and hence cannot be transpired.
- State the other precautions you take?
- A layer of oil is added in the boiling tube so that to ensure that the weight loss is caused solely by transpiration, not via evaporation.
- The stem was cut under water to prevent air column to be drawn into the xylem.
- Is there a trend in the result?
No. The rate of transpiration showed different rate according to different atmospheric condition.
- Under what condition does transpiration occur at greatest rate?
Transpiration will occur at greatest rate under conditions whereby light intensity and wind speed are relatively high but at low level of humidity.