Learning Lesson: Leaf it to Me

 Overview

There are two ways in which water moves from the ground to the atmosphere as part of the hydrologic cycle: evaporation and transpiration. Transpiration is basically evaporation of water from inside plant leaves. Studies have revealed that transpiration accounts for about 10% of the moisture in the atmosphere, with oceans, seas, and other bodies of water (lakes, rivers, streams) providing nearly all of the remaining amount.

The student will observe the effect of transpiration as water is moved from the ground to the atmosphere.

Procedure

1. Place a large plastic bag over a living limb of a tree or large bush. (The limb should not touch the ground.)
2. Tie the open end of the bag around the tree or bush. Make sure there are no air leaks.
3. At the closed end of the bag, tie a rock to the bag so the bag is weighted and forms a collection point for the water.
4. After a predetermined time of your choosing (at least 2 hours after "bagging" the branch), poke a hole in the bag and collect and measure the water. Then remove the bag and rock from the branch.

Discussion

Plant transpiration is generally an invisible process as the water from the exiting the leaves evaporates quickly. The temperature inside the bag will increase with heating from the sun. However, the water vapor will condense back into water as it comes in contact with the bag’s surface.

During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year.

The amount of water that plants transpire varies greatly geographically and over time. There are a number of factors that determine transpiration rates:

• The transpiration rate goes up as the temperature goes up, especially during the growing season.
• As the relative humidity of the air surrounding the plant increases, the transpiration rate decreases. It is easier for water to evaporate into dryer air than into more saturated air.
• Increased movement of the air around a plant will result in a higher transpiration rate. If there is no wind, the air around the leaf is still and transpiration raises the humidity around each leaf. Wind moves this saturated air away from the leave, replacing it with less saturated air.
• Plants transpire water at different rates. Some plants which grow in arid regions (e.g. cacti and succulents) transpire less water than other plants as a measure to conserve water.

While the transpired water may have a bitter or harsh taste (depending on the type of tree or bush selected) it is safe to drink and can provide much needed drinking water in survival situations.