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Background Information

Transpiration is the process in which water moves through plants and evaporates into the environment. The water from the soil enters through the root hairs and the water passes into the xylem and travels up the xylem vessels and into the leaves. The water properties of cohesion and adhesion help the water molecules “stick” to each other and get pulled up through the plant’s stem and roots. The water is released from stomata, which are tiny openings on the underside of leaves. The stomata also allow the plant to take in CO2 for use in photosynthesis. Guard cells surround the stomata and facilitate transpiration by opening and closing the stomata. The rate of transpiration can be affected by various factors, including environmental conditions, which is the topic of this experiment.


Gather 5 impatiens plants and remove all buds, flowers, and loose leafs.

Remove plant from plastic container and wrap the soil with plastic wrap. Tie a string tightly around the excess wrap and plant stems.

Record mass of each plant (in grams).

Place one plant outside in natural sunlight (control). Place another directly under an artificial light source, one in a dark cabinet, one in front of a fan, and one under a plastic box that is filled with mist. Periodically spray water into box to maintain humid conditions.

For the next four days, take and record the mass of each plant.

On the final day, find the change in mass and calculate the percent change in mass.

Analyze results.

Experiment is finished.


Claim: The percent change in mass of the impatiens plants due to transpiration will decrease if the plant is subjected to a dark condition

Evidence: Our control group of impatiens plants were placed in room temperature conditions while our experimental group of impatiens plants placed in a dark environment to grow for one week. Over the course of the week, we were able to observe that the mass of the experimental plant decreased from the initial 111.82 grams to 93.52 grams, leading to an overall 16.37% decrease in the mass of the impatiens. The mass of the control plant decreased from its initial mass of 113.24 grams to 87.12 grams, leading to a 23.06% change in mass.

Reasoning: Impatiens plants are very sensitive to a lack of water so since we did not water the plants throughout the experiment, they lost mass due to transpiration. Plants need light to grow as it is a much needed component to lead to the plant carrying through with the process called photosynthesis. Since the experimental group was denied exposure to light, their stomata do not want to open up and will not go through transpiration as much as the control group plants did, which explains why the change in mass rate for the experimental was slower than the control group.

The independent variable in this experiment was the environmental condition the plant was placed in. The four independent variable levels were: light, dark, fan, and humid. The dependent variable was the transpiration rate, which was measured by % change in mass. Constants in the experiment include the type of plant and the time between each measurement.

Each condition affected the way transpiration worked on the plant in different ways. For the light condition, the plant had a large change in mass as it was exposed to light which is needed for a plant to go through photosynthesis, which uses up the water stored by the plant. The dark condition had a low change in mass as by limiting the light exposure to the plant, the growth of the plant itself will be stinted as it is missing a component needed for photosynthesis, so water will stay within the plant itself. The fan increases the change in mass as the wind created by the fan will make it easier for water vapor that is stuck on the leaves of the plant to go through the boundary layer that otherwise keeps the water vapor on the leaves. Humidity leads to very little change in mass as there is lots of water in the atmosphere as there is in the plant, so the plant has less of a driving force for water movement out to the atmosphere.

Temperature had an impact on the experiment as warmer surroundings will lead to water movement being increased in the plant while a colder environment will decrease the ability for water to move. This is due to the fact that if the temperature is warmer, the air around the environment will be able to hold more water, which in turn will promote transpiration as the stromata within the plants will want to open up their stomata and release water.

Water potential affects the movement of water because water tends to move to regions of low water potential. This is seen in transpiration because the water potential decreases as you progress down the transpiration pathway. Beginning in the soil and root hairs and then traveling through the xylem vessels, mesophyll of leaves, and finally the outside air of leaves, the water potential in each structure gets lower and lower, which causes water to move towards it, thus facilitating the movement of water through the plant.

The plants placed in front of the light and fan decreased the water potential in the leaves. The extra light caused more evaporation to take place on the leaves so water potential decreased. The fan also caused the water in the leaves to blow away after the water evaporated; so the water potential decreased. The plants placed in room temperature had no effect on the water potential gradient from the leaves to the stem. This was because there was no external factors affecting the plants and their water potentials. The humidity increased the plant’s water potential because there were many water molecules in the air in which the leaves were able to take in. Water is moving from the stem to the leaves so water potential in the stem is always decreasing.

When the plant closes the stroma due to lack of water supply the plant also halts the intake of carbon dioxide. This prevents the plant from going through photosynthesis, a processes that creates food for the plant to use.

One way a plant has adapted to be able to conserve water is by closing its stomata during hot days. A hot, dry environment would increase the rate of transpiration and cause excessive water loss in the plant, so instead they open their stomata at night to minimize water loss and to take in CO2 for photosynthesis. Guard cells are also an adaptation since they control the opening/closing of stomata (the part of the plant that allows the water to evaporate through the leaves) and therefore regulate water loss. Plants also use the cuticle, a wax coating on their leaves, to prevent water from being evaporated.

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