IB Biology HL Y1
15 January 2009
The Effect of Humidity on Transpiration in Plants
How does placing a plant in a more humid location for 50 minutes affect its rate of transpiration?
Transpiration is the loss of water from a plant, mainly through the stomata of leaves. Darkness, internal water deficit, and extremes of temperature tend to close stomata and decrease transpiration; illumination, ample water supply, and optimum temperature cause stomata to open and increase transpiration. Its exact significance is disputed; its roles in providing the energy to transport water in the plant and in aiding dissipation of the sun's heat (by cooling through evaporation of water) have been challenged. Since stomatal openings are necessary for the exchange of gases, transpiration has been considered by some to be merely an unavoidable phenomenon that accompanies the real functions of the stomata. ("Transpiration")
Hypothesis and Variables
If a plant is placed in a more humid location, there will be more water available to absorb, thus encouraging more transpiration within the plant. Independent variable: The amount of moisture residing in the air in the immediate vicinity of the plant. Dependent variable: The plant’s rate of transpiration.
Control: A plant placed in a standard indoor environment, with standard room temperature, relatively consistent CO2 and light, as well as level air pressure. Equipment/Materials
Experimental Design or Methods
Time (min + 1 min)
| First run (mL+ 0.005 mL)
| Second run(mL+ 0.005 mL)
Table 1. Water loss in plants; the first run took place in a standard indoor environment, while the second run took place in a sealed plastic bag, lightly misted with water. The same plant was used for each experiment. Data Processing
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