Develop a Method for Determining a Plant's Rate of Transpiration

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Author: Janice VanCleave

What You Need to Know

Vascular plants are plants that have special tubes for transporting sap (in plants, a watery solution with dissolved nutrients). The xylem tubes transport water and nutrients taken in through the roots throughout the plant. Transpiration is the release of water from a plant's surface. Most transpiration takes place in a plant's leaves. Transpiration rate is the measure of transpiration in a given time.

How Does Transpiration Work?

Transpiration is the process in which water is taken in by plant roots, moves up through xylem tubes, passes through holes in the leaves called stomata, and then evaporates into the atmosphere as water vapor. Sap moves through the xylem tubes in the stems, as shown in the diagram on page 49. As the water evaporates from the surface of the leaves, the column of sap in the xylem rises, and more water molecules are pulled in at the roots; thus, a continuous flow of water enters the roots and moves throughout the plant.

What Does This Have to Do with Transpiration Rate?

Plants control the amount of water lost by transpiration by opening and closing their stomata. Cells surrounding the stomata become more or less stiff, which causes the stomata to open or close. In a dry atmosphere, such as in a desert, the stomata of plants stay closed much of the time. This keeps the water inside the plant. Thus, desert plants have a low transpiration rate. The opposite is true of plants that grow in tropical rain forests.

What Does This Have to Do with Transpiration Rate?

Fun Fact

The water vapor that trees transpire into the air cools the air.

Real-Life Science Challenge

Plant transpiration rate is affected by humidity (the measure of the amount of moisture in the air). If the humidity is too high, the transpiration rate will be so low that the plant cannot pass the oxygen and water it produces back into the atmosphere. When this happens, the plant's growth can be stunted or the plant may even die. This can happen in greenhouses that don't have proper ventilation or if plants are watered too much.


Now, start experimenting with transpiration rate.


  • Decide on a way to measure how much water the plant gives off.
  • Think about how to collect the water that is given off by a plant's leaves in a specific amount of time.
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