Biology

Plants take up water at the roots. The water moves through the plant to the leaves. This moisture escapes through stomata, small pores on the surfaces of the leaf. The water escapes as water vapor and evaporates into the atmosphere. This process is called transpiration. It is essentially evaporation of water from plant leaves.

• As temperature increases, the rate of transpiration increases. Water evaporates more rapidly at higher temperatures.
• As relative humidity increases, the rate of transpiration decreases. It is easier for water to evaporate into dryer air than air containing a lot of water vapor.
• As wind (air movement) increases, the rate of transpiration increases. Transpiration causes the air right next to a leaf to become humid. Wind moves the humid air away from the leaf and brings in dryer air.

Note: The leaves of desert plants are distinctly different from those of plants that live in moist environments. Structurally, leaves of desert plants are long and narrow and contain the majority of their stomata on the underside of the leaf blade. Although stomata act as openings through which carbon dioxide can enter leaves, they also present a problem. Water traveling up the plant from its roots eventually reaches the stems and evaporates through the stomata. If this rate of evaporation is fast, essential water supplies can be depleted before the plant has met its photosynthetic needs. Plants with elongated, narrow leaves lose water at a slower rate than those with broad leaves, which is why many desert plants have pointy or spine-like leaves. They have adapted to transpire as little as possible. Since less transpiration means less photosynthesis, desert plants often grow quite slowly.

• light source
• clock timer
• graduated cylinders
• cellophane to cover the graduated cylinders
• broad plant leaf
• long, thin plant leaf

1. You will conduct an experiment with a broad leaf and a long, thin leaf, both with the same surface area. Predict which of the two leaves will transpire (lose) the most water in a 12-hour period.
2. Under darkness, press the start button on the clock and observe how the water level in the graduated cylinders changes. When the clock reaches 12 hours, measure the water level in each graduated cylinder and record these volumes in the data table in the column titled “final volume.” (Click the table icon in the menu bar to access the data table.)
3. Determine the amount of water that evaporated from each leaf using the following formula:
       Initial volume — Final volume = Amount evaporated
4. Record the amount evaporated in the last column of the data table.
5. Click on the light bulb to turn the lamp on. Repeat Steps 2—4.

1. From which leaf did the most water evaporate? Why do you think this was so?
2. In this experiment, why must the graduated cylinders be covered?
3. Both of these leaves have the same surface area and the same stem size. Why do you think this is an important factor in this experiment?
4. Desert plants do not grow as tall as plants in other biomes. Offer an explanation for this fact.

1. Answers may vary. The longer leaf could have fewer stomata per given amount of area. Or, the longer leaf could have more stomata on its bottom surface. The underside of the leaf would be cooler and there would be less evaporation.
2. To prevent evaporation of water from the graduated cylinders.
3. To maintain constant the variable of size.
4. Answers will vary. Growing taller leads to greater surface area for the plant and thus more water evaporation through stomata. In addition, desert plants often have reduced photosynthesis as means of conserving water, so they grow more slowly.