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Sunday, April 29, 2012

Transpiration Made Simple!



The plants always seem to get left out!

It is nearing the end of the school year, standardized testing is all anyone is talking about, the students are getting antsy to be out for the summer, and the biology teachers have to decide "what not to cover this year."  This always happens at the end of the year as we begin to run out of time.  For many biology teachers, a unit on plants is the first to go.

Teaching about plants is actually one of my favorite units, and I always find that my students are woefully lacking in knowledge about the plants.  I make it a priority each year to leave enough time at the end of the school year for my plant unit.  I like teaching plants at the end of the year because everything in our area is in full bloom!  Among the many activities I do with plants, I always have students germinate seeds.  It is mind-boggling to me how many of my students have never planted a seed! They love taking care of them in the lab and they get so excited to watch them grow.  Since the weather is nice, the students then take their seedlings home and plant them in their yards.  Many will report back to me at the start of the following year with news of their plant.

One of the labs I do that is so very easy to setup and clean up, is a transpiration lab. Transpiration is the loss of water through the leaves.  As the stomata open to allow carbon dioxide to enter the leaf for photosynthesis, water vapor escapes the leaf and enters the atmosphere.  For trees and other tall plants, it is tricky business to get the water up to the top of a very tall tree.  The major force in water transport is provided by the evaporation of water from the leaves during transpiration.  As water is lost, the movement of the water out of the leaf "pulls" water upward through the xylem all the way down to the roots.  This is called transpiration pull.

Measuring transpiration from the leaf is a very simple thing!  I went to Wal Mart and bought begonia plants.  The one tray contained 6 plants.  The begonia is a good choice because it has a thick and fleshy leaf.  Once back in my school lab, I placed plants in small (250 mL) 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 testable variables!  What factors will cause an increase or decrease in the rate of transpiration?  We decided to test the following factors:

  • One plant will be placed in continuous light for 24 hours. 
  • One plant will be placed in front of a blowing fan for 24 hours.
  • One plant will be placed inside a plastic bag that had previously been spritzed with water (high humidity inside the bag). 
  • 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 additional influence of light, a blowing 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 carry on 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.  The heat from the light also plays a role.  The increase in heat will cause a higher rate of evaporation from the leaf.

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.

Try this...It works like a charm.  And have fun teaching plants!

Here are some additional resources:

Look for video segments from "The Private Life of Plants"by David Attenborough.  There are quite a few of these on YouTube.  Each is very short - about 3 to 5 minutes - and my students really enjoy them.

Lab: Leaf Structure and Function
Lab:  Living Flowers
Photosynthesis: Complete Unit Plan/Bundle
Lab: Pigment Chromatography 
Lab:  Stomata - Gateways into the Leaf

2 comments:

Amy said...

Thank you so much for this post. I did a recent transpiration experiment with my kids at home and couldn't have explained the science behind it better than what you have provided here! They loved it. We posted on our blog and linked back to your post here.

Anonymous said...

Thank you. I homeschool my eight year old son, who can never get enough science! This will be a wonderful activity to encourage his latest fascination with plants and how they function.