Transpiration and water movement through plants play an important role in the hydrologic cycle. The activity shows how transpiration relates to weather conditions.
Instructional Method: Experiment
Goal: To show students how plants perform a vital part in the hydrologic cycle.
Objectives: Students will be able to:
Identify the source of water collected from a tree leaf.
Describe how water moves through a plant.
Label transpiration on the hydrologic cycle chart.
Set up: 5 minutes
Observation: one week
Tree with low branches, or house plant
Student handout of parts of a tree
PDF Hydrology Worksheet
Surface water such as lakes and oceans is not the only place from where water vapor comes. Plants are instrumental in moving water within the ground up to the surface. Transpiration is the act of liquid water changing into water vapor on the surface of a plant. It is similar to evaporation, only more difficult to observe, and takes place ONLY on plants. Plants move water through small "vessels" that are stacked one on top of the other, like a giant soda straw. As water is removed from the top of the straw through transpiration, more water must move up the straw to take its place.
As the sun transpires water from the leaves of a plant, more water must move from the branches into the leaves to take its place. As this water moves onto the leaf, more water must move from the roots, to the trunk, to the branches. The source for water in plants is moisture in the soil. Thus, plants are able to bring water from the ground into the atmosphere.
Water is released from a plant during photosynthesis (a plant's process for making food, utilizing the sun's energy and carbon dioxide). The water is released through openings on the underside of the plant called stomata. These stomata are only open during photosynthesis. Therefore, stomata are only open during the day while the sun is shining and evaporation of water from the surface of the leaf can take place. If the surrounding air is dry, more water evaporates from the open stomata than if the air is humid.
In this activity we will see water transpiring off of a plant and explore its relationship with other weather conditions.
Select two or more lower branches on a tree outside. At least one branch should be in full sun most of the day and another should be in the shade. Tie a baggie around each branch, and secure with a twist tie according to the diagram.
Once a day for a week, go to the tree and carefully remove the bags. Do not spill the water while removing them.
Pour the collected water from one branch into a graduated cylinder and mark the amount on a chart. Place a plug on the cylinder. Repeat this step for each branch that is part of the activity.
Complete the rest of the information on the chart as well, including time collected, temperature, amount of sun on the branch, current weather, etc.
Repeat the observation and data recording once a day at the same time each day for at least one week.
How much water was collected? From where did the water come? Which branch produced the most water? Why? Does the amount of sun make a difference? Was it cloudy on any of the days we collected water? What happened when it was cloudy? If we wouldn't have collected the water, where would it have gone? How is transpiration different from evaporation? What happens to the water that is in the air?
If you are short on time, or your students want to explore the concept further, check your tree bags three times a day, early morning, at noon and late in the afternoon. Explore whether the time of day creates a difference in water production and why.