How can we show that transpiration occurs mainly through the leaves?

Approach• 2 twigs will be used, one that is leafless and the other with

leaves. • Leafy twigs will have its stem completely covered in

petroleum jelly. • Leafless twig will have only its ends covered in petroleum jelly. • Twigs will be placed in separate beakers of water and covered

with bell jars and left under controlled conditions. • The evidence that transpiration takes place mainly through

leaves is shown through the presence or absence of water droplets on the bell jar at the end of the experiment.

Variables

Independent

Humidity

Presence of wind/ wind

speed

Temperature

Light

Dependent Rate of transpiration

Materials and Equipment Used

1. 2 twigs with leaves2. 2 beakers of water with oil on the surface of

the water3. A pair of Scissors4. 2 bell jars5. Petroleum jelly

PROCEDURE

Step 1

TRIM OFF THE LEAVES ON A LEAFY TWIG USING A PAIR OF SCISSORS

Step 2

APPLY PETROLEUM JELLY OVER THE WHOLE STEM OF THE LEAFY TWIG

APPLY PETROLEUM JELLY ONLY ON THE CUT ENDS OF THE PETIOLE OF THE TRIMMED TWIG

Leafy twig with

petroleum jelly on

WHOLE stem

Leafless twig with

petroleum jelly on cut

ends of petiole

Step 3PLACE THE LEAFY TWIG IN TO ONE BEAKER OF WATER THAT HAS A LAYER OF OIL OVER THE WATER SURFACE.

PLACE THE TRIMMED TWIG INTO ANOTHER BEAKER OF WATER WITH A LAYER OF OIL OVER THE WATER SURFACE.

Oil WATER

Step 4

Bell Jar

COVER BOTH SET UPS WITH A BELL JAR.

PLACE BOTH SET UPS UNDER THE SAME LIGHT SOURCE AND ALLOW IT TO STAND.

Precaution• The leafy twig’s stem is totally coated with petroleum jelly to

ensure that water is not lost through the relatively small number of stomata on the stems, and water is lost mainly through the leaves.

• The leafless twig’s petiole and ends are covered with petroleum jelly to ensure that water is not lost through the vascular bundles (xylem vessels).

• A layer of oil is applied over the water to prevent the water from evaporation, which would affect the results as evaporated water from the beaker will also condense on the bell jar and form water droplets.

• Both set ups are placed under the same light source to ensure that they receive equal amount of sunlight, so that sunlight would not be a variable affecting transpiration in this experiment.

Expected Results/ Observation

After a few hours, there will be water droplets on the inner surface of the bell jar with the leafy twig, while the other set-up remains unchanged.

WATER DROPLETS

Conclusion• Water absorbed by the twigs are released through

the stomata on the leaves as water vapour. This water vapour condenses on the cooler inner surface of the bell jar, forming water droplets.

• The presence and absence of water droplets thus indicate whether transpiration takes place mainly through the leaf or the stem.

• Water droplets are present and absent in the beaker with leafy and leafless twig respectively.

• We can thus conclude from this experiment that transpiration takes place mainly through the leaves.