Transpiration
Introduction:
The loss of water vapor from plant surfaces is called transpiration. Plants take in and transport
larger amounts of water than most animals. Up to 98% of the water absorbed is lost as water vapor.
During photosynthesis, stomata, the pores between guard cells, open to allow the diffusion of carbon
dioxide. Most water vapor is transpired through the open stomate. The cuticle, which is the waxy covering
of the leaf, prevents water vapor loss through the epidermal cells.
Transpiration rates depend on the difference in water vapor concentration between air spaces in
the leaf and air outside the leaf. Greater differences in water vapor concentration mean faster transpiration
rates; therefore transpiration occurs faster on hot sunny days than on foggy or rainy days. On sunny days a
leaf may lose all the water it contains in one hour. If water is not available to replenish the water lost
through transpiration, the plant will wilt and eventually die. Plants growing in dry environments have
adaptations that slow the rate of transpiration; among these are a reduction in the number of stomates or a
change in the location of stomates to the underside of the leaf. Also, stomates may be recessed to create
pockets of high humidity reducing the difference in water vapor concentration. In addition, some plants
have dense epidermal hair that slows transpiration by shielding stomates from drying winds. Plants in all
climates have guard cells to narrow or close stomates to prevent dehydration.
Objectives:
1.
2.
3.
4.
To calculate the total leaf area of a tree.
To determine the amount of water transpired by a tree in one day.
To calculate yearly water loss of a tree through transpiration.
To estimate the amount of water transpired by an acre of trees in one day.
Materials:
5 leaves per trial
50 ml beakers
25 ml graduated cylinder
mineral oil
dropper
distilled or spring water
graph paper
pencil
calculator
Procedure:
Day 1
1.
2.
3.
4.
5.
6.
7.
8.
9.
Select a tree. Estimate the number of leaves on the tree by counting the number of leaves of a
branchlet, the number of branchlets on the limb, and the number of limbs on the tree. Record this
number. _________________________________
Collect five leaves from five different areas on the tree.
Trace the outline of all the leaves on graph paper.
Pour 25 ml of distilled water into a beaker or graduated cylinder.
Place leaves into the beaker or graduated cylinder: be sure the petioles are under water.
Seal the surface of the water by dropping enough mineral oil to cover the water. DO NOT get any
mineral oil on the leaves. Oil on the leaves will inhibit the rate of transpiration.
Place the beaker or graduated cylinder in front of a window or under a light for 24 hours.
Next, determine the area of a leaf by finding the area of a square on the graph paper. Multiply this by
the number of squares that leaf occupies. Repeat this process for the remaining leaves.
Add the five areas to determine the total area. Record this number. __________________cm 2
Day 2
1.
2.
Remove the leaves from the beaker or graduated cylinder and measure the amount of water remaining.
Wait for the oil to rise before taking your measurement. Record this number. _________________ml
Divide the ml of water of water transpired by the total area of the leaves recorded in #9 to determine
the amount of water transpired per square centimeter of leaf in a 24 hour period. Record this number.
________________
Analysis/Conclusion:
1.
2.
3.
4.
Calculate the amount of water transpired by the tree in one day. Hint: First calculate the average area
of the 5 leaves.*
Calculate the amount of water transpired by the tree in one year. Hint: First decide how many months
of the year the tree has leaves.*
Calculate the amount of water transpired per acre of forested land (it is estimated that an acre of
forested land in this region sustains 36 trees per acre).*
Calculate the amount of water transpired per acre of tropical rain forest (it is estimated that an acre of
tropical rain forest sustains 300-400 trees per acre).*
*These “calculations” are estimates due to the many variables involved.
Last updated 8-01