Transpiration (Scantlin)

Research Question and Hypothesis

Question: Under which environmental condition will transpiration be greatest in bean plants?

Hypothesis: Changes in environmental factors will alter the rate of transpiration of a bean plant. The amount of water transpired by a bean plant will vary in direct proportion to increases in temperature and wind speed, and an inverse proportion to increase humidity. (All of the plants will loose water through transpiration, but the one with the fan and light will loose the most). 


Include the California and/or National science standards that this addresses. 

Life Sciences

2.Plants and animals have structures for respiration, digestion, waste disposal, and transport of materials. As a basis for understanding this concept:

a.Students know many multicellular organisms have specialized structures to sup­ port the transport of materials.

Experimental Design

Students will use potometers and bean plants to measure the amount of water loss through transpiration. The plant will pull water from the photometer by the process of transpiration. 

They need to set up four potometers and plants, each with a different environmental condition.

The control will be left alone in normal conditions. 

The other 3 plants will be: one in humid conditions with mist in a bag, one with wind coming from a fan, and one with bright light. 

Independent variable

The independent variable is time. They will read the potometers every 3 minutes for 30 minutes total. 

Dependent variables

The dependent variable is the amount of water loss measured in mL/min/m2. They will need to read the water level and find the change. 

The series are the different environmental conditions. Control, humid, wind, and bright light.

Constants and Controls

What is factors are held constant?  All plants should be the same species, similar in size, and similar amount of leaves with similar surface area. They should all be kept in the same area in a room. The time they are read at should be the same. Converting the data to mL/m2 makes all the readings have the same surface area which eliminate differences in leaf surface area.  

What serves as a control? The Control is not given any change in their environmental condition. This would be the normal room condition plant. 


Provide a complete list of all materials needed and explain where they may be obtained. 

Distilled Water -store


4 potometers you can make from ring stand, clear plastic tubing, calibrated pipette

Scissors/ scalpel

Plastic bag


Bright light/ heat lamp

container of water

spray bottle

pinto bean plants (you can grow them yourself)

1 mL pipette 


Procedure 1 – Potometer Readings (Ms G will sketch the set up before the lab begins)

  1. Place the tip of a 0.1ml pipette into a 16” piece of clear plastic tubing.
  2. Submerge the pipette and tubing and draw water through the tubing until no air bubbles are present.
  3. Carefully cut the plant stem UNDER WATER.  It is very important to make this cut under water so absolutely no air enters the xylem.
  4. While your plant and tubing are under water, insert the stem into the open end of the tubing.
  5. Bend the tubing into a U and attach it to a ring stand as shown in the diagram below.
  6. Seal the stem/tubing junction with parafilm.
  7. Allow the potometer to equilibrate for 10 min before recording your zero reading.
  8. Expose your plant to one of the following treatments (I will assign you one).
    1. Room conditions
    2. Lamp .5 meters from the plant
    3. A fan on low to create a slight breeze over the plant.
    4. Plant in the mist (spray the plant several times, cover with plastic bag in between readings
  9. Record the water level in the pipet every 3 minutes for 30 min.

Procedure 2 – Calculation of Leaf Surface Area

  1. Dismantle your potometer and cut all the leaves off the plant.
  2. Blot the leaves dry and determine the total leaf mass of your plant.
  3. Cut an exactly 1cmsection of a leaf and weigh it.
  4. Calculate the total surface area of leaves on your plant in cm2. (Hint – you need to go from g/cm2 to total cm2)

Sample data and graphs

Include raw data and graphs.  Make certain to include units in all measurements, titles on all graphs, and labels on all axes. 
 Treatment  Cumulative water loss (mL/m2) for
30 min
 A. Normal 125.46 4.18
 B. Heat 310.81 10.36
 C. Moving Air 298.12 9.34
 D. High Humidity 61.77 2.06

Table 1: potometer readings

Table 2: leaf area determination

Mass of leaves _______________________________

Mass/ cm2 __________________________________

Total cm2 __________________________________

Table 2: Normalized data (water loss per square centimeter) = ml / cm2

Table 3: Class Averages Cumulative water loss in ml/cm2

Analysis & Conclusions

1) Explain why the average rate of transpiration results make sense (or don’t) based on what you know about water potential and movement of water through plants

Heat: The absorption of light resulted in an increase in leaf temperature which increased evaporation. Evaporation increases the water loss through transpiration. 

Air: An increase in wind speed results in an increase in water loss because wind decreases the boundary layer of still air on the leaf's surface. Moving air results in evaporation and low water potential in the air immediately surrounding the leaf which increases the rates of water loss from the leaf. 

Humidity: Increase in humidity in the air surrounding the leaf decrease the water potential gradient between the saturated air the leaf air spaces and the air surrounding the leaf. This results in the decreased rate of water loss. 

2) What is the advantage of closed stomata for a plant in a dry environment?  What is the disadvantage?

 This would keep the plant from evaporating too much and drying out. The disadvantage is that photosynthesis can't happen as quickly if carbon dioxide cannot come in. 

3) Describe several adaptations that enable plants to reduce water loss from their leaves.  Include structural and physiological adaptations.

Many plants have tiny hairs on their leaves to help with more air space so that there will be an increase in surface area and less water will evaporate. Many plants will only open their stomata at night when temperatures are not as high. 

Photos & Movies

Include photos and movies of your longitudinal research.  Post your photos in the class photo album and your movies in your Youtube account.  Embed gadgets that allow you to see your movies and photos in this web page. 

YouTube Video


Include links to all relevant references.  Don't just paste the URLs here.  Give a title for the resource and then link the text to the appropriate URL.