TEACHER GUIDE

TOPICS:  Plant structure and function, transpiration, ecology

LEVEL:  Middle or high school

TIME:  20 min to  make predictions and set up experiment; 20 min
on the next day for student observations, conclusions, and
discussion.

ADVANCE PREPARATION:  Use freshly cut evergreen and deciduous
branches, and keep the cut ends in water.  If the branches are
collected before class, keep the cut ends in water and store in
the refrigerator to prevent drying.

MATERIALS:  The experiment can be done as a teacher demonstration
or as a student activity (see Procedure note below); adjust the
quantities of materials accordingly.  Gooseneck lamps with 200
watt bulbs, or fluorescent lamps can be used for the light
source.


PREDICTIONS:  In class discussion, encourage creative, 
logical, and critical thinking.  We recommend that you introduce the
following ideas if they are not suggested by students: 
Transpiration by the actively growing and photosynthesizing
geranium plant should result in fogging as the water vapor
condenses on the inside of the plastic bag.  In contrast, little
or no fogging should occur in the bags with the deciduous or
evergreen twigs even after exposure to strong light; the dormant
deciduous twig lacks leaves, and the changes in the evergreen
needles which accompany winter dormancy greatly reduce
transpiration.  See Plant Water and Energy Relations Teacher
Background for more information.

_STEPS A and B can be done by the teacher as a class
demonstration, or by students working in small groups.

STEP C:  You may wish to have additional evergreen and deciduous
twigs available for student observations.

STEP D:  You may wish to expose the twigs to natural light for 24
hours.  If you get no observable results, then use a continuous
light source for an additional 24 hours and observe again.

QUESTIONS 6-12 require higher order thinking skills.  We
recommend that you discuss these in class before students write
out their answers.

3.  Fogging of the plastic bag is evidence of transpiration.

4.  Students may have correctly predicted that the geranium gives
off the most water vapor and the deciduous twig gives off the
least.  They may be surprised to find out that the winter
evergreen does not give off enough water vapor to cause fogging.
 
5.  Most transpiration occurs through the stomates in the leaves
and the lenticels in the stems.  A small amount may be lost
through the cuticle.

6.  Both the geranium and the evergreen twig have leaves with
stomates, but the total leaf surface area may be different.  In
addition, available evidence suggests that dormant winter
evergreens do not photosynthesize and their stomates are probably
closed thus limiting transpiration.  Other possibilities include
the presence of sunken stomates or higher osmotic potential in
the cells of the evergreen.

7.  The geranium plant gives off more water than the leafless
deciduous twig because the geranium has a large leaf surface with
many stomates.

8.  Evergreen twigs and leafy deciduous twigs would be expected
to give off enough water vapor to fog the plastic bags if they
were tested in summer.

9.  The water vapor, an invisible gas, becomes a visible liquid
when it condenses on the inside of the plastic bag.  This occurs
when the air in the bag reaches 100% relative humidity.  The
temperature of the leaves under lights is greater than the
surrounding air maintaining the diffusion gradient.

10.  In the presence of light, a summer-active plant like
geranium carries on photosynthesis, the stomates are open, and
more water is lost by transpiration.

11.  Lack of evidence of transpiration by the evergreen twig
indicates that winter evergreens are probably dormant like
deciduous trees.

12.  Evergreens probably do not carry on photosynthesis during
the winter.  If the stomates are closed as suggested by the
relatively low amount of transpiration, the amount of
photosynthesis may also be very low.  (See Plant Water and Energy
Relations Teacher Background.)