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Plant Transpiration - Great for All Ages!

I really enjoy teaching plants to my biology students.  I did this activity a few days ago on plant transpiration.  It is so simple, easy to set up, doesn't require any fancy lab equipment, and best of all, it is adaptable to all age groups, K - 12!

Transpiration is the loss of water from the leaves of a plant.  The leaves have small pores called stomata.  The stomata must open to allow carbon dioxide to enter the leaf for photosynthesis.  While the stomata are open, however, water vapor is lost from the leaf.  If too much water is lost from the leaf, the plant will lose turgor pressure and the plant will wilt.

Measuring transpiration from the leaf is a very simple thing!  I went to Wal Mart and bought one tray of Vincas (aka Periwinkles).  The one tray contained 6 plants.  Once back in my school lab, I placed plants in small (100mL) beakers.  I watered them thoroughly.  Since I wanted to measure the amount of water lost from the leaves, I took a plastic sandwich bag and wrapped it very tightly around the beaker, and around the stem of the plant.  It is necessary to insure that no water evaporates from the dirt itself.


1)  Place plant in beaker.
2)  Wrap beaker tightly around plant, and tightly around the stem.  Only the leaf should be sticking out of the plastic bag.

The only way water is getting out of this beaker is through the leaves of this plant!

3)  Find the initial mass of the entire set-up and write the mass on a piece of masking tape which is then placed on the beaker.


Now for the variables!  What factors will cause an increase or decrease in the rate of transpiration?  We decided to test the following factors:  (1) One plant will be placed in continuous light.  (2) One plant will be placed in front of a blowing fan.   (3) One plant will be placed inside a plastic bag that had previously been spritzed with water (high humidity inside the bag).  (4) The fourth plant will serve as a control.





This plant was placed in the beaker, the beaker was covered with a plastic bag, and the initial mass was written on the outside.
The plant was then placed under a lamp and left for 24 hours.





This plant was placed in front of a blowing fan for the same 24 hour period of time.



The inside of this bag was misted with water and the plant was then sealed inside the bag for 24 hours.  This simulates a high humidity environment.



This is the control.  The plant is still transpiring, but without the extra amount of light, without the fan, and without the added humidity.  A true control would be the absence of transpiration, but we decided to use a plant at normal room conditions as our control.






Let the plants sit for 24 hours (or longer if you want!).  Each plant is massed again after a 24 hour period of time.  Students will be amazed at how much water has been lost through the leaves.  The results should be as follows:

Plant in Light:  This plant loses a good amount of water.  The light causes the plant to continuously do photosynthesis.  This requires a lot of carbon dioxide, so the stomata stay open for a longer period of time to let in the carbon dioxide.  While the stomata are open, the plant will lose water.

Plant in front of fan:  The increased air movements across the surface of the leaf will cause a higher rate of evaporation from the plant.

Humidity:  Less water is lost from the plant when the humidity of the air surrounding the plant is high.

All masses should be compared to the control to see if the factor being tested causes an increase or decrease in the rate of transpiration.

Here are some ideas of how this might be used at different age levels:

Elementary:  It may be enough in the lower grades to just show that water is taken up by the roots and escapes from the leaf.  Students can be asked before the experiment to make predictions as to what they think the outcome will be.  They can form a hypothesis and go through the steps of the scientific method.

Middle Grades may add the following:  Have students calculate how much water is lost per minute in each plant.  Test an additional factor, such as complete darkness, or various temperatures.  Test different types of plants to see if the transpiration rate is the same in all types of plants.

High School Grades may add the following:  Remove the leaves and determine the surface area of each leaf.  Determine how much water is lost in a given amount of surface area.  Have students research the mechanisms of transpiration, such as cohesion, adhesion, capillary action, and transpiration pull.

These are just a few ideas that will allow this same simple concept to be used over multiple grade levels.  I hope you will give this a try in your classes.  I think many students have little experience in dealing with plants;  my students are always intrigued and interested in labs that involve plants.

I would love to read some comments if you have other ideas, or if you want to let me know how it worked for you.  Good luck...... and make science FUN!!!

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