Answers to Hands-on Pg 8-10

Aim : To study the effect of light intensity on the rate of transpiration.

Hypothesis : The higher the light intensity, the higher the rate of transpiration.

MV : Light intensity

RV : Distance traveled by air bubble in 10 minutes

FV : Air movement/temperature/air humidity/type and size of plant

Method :

1. Setup the apparatus as shown in diagram above.

2. Choose a leafy shoot and cut the shoot end in a basin of water.

3. Insert cut stem in the potometer (capillary tube fitted with rubber tubing).

4. Lift the capillary tube to insert an air bubble.

5. Adjust and mark the initial position of the air bubble with a thread.

6. Dry the surface of the leaves before the start of the experiment.

7. Make sure the apparatus is airtight by smearing the joints with vaseline.

8. The air movement / temperature /humidity of the surrounding/type and size of plant must be maintained throughout the experiment.

9. A table lamp was placed 50cm away from the potometer.

10. Measure and record the distance traveled by the air bubble in 10 minutes using a stopwatch.

11. Repeat the experiment by placing the lamp at different distances of 40cm, 30cm and 20 cm from the potometer.

12. Calculate the rate of transpiration using the formula : Distance traveled by air bubble/time.

13. Tabulate the results in a table.

Distance of light bulb (cm)	50	40	30	20
Distance traveled by air bubble in 10 min (cm)	4	8	11	15
Rate of transpiration (cm/s)	4/10 = 0.4	8/10 = 0.8	11/10 = 1.1	15/10 = 1.5

Discussion :

1. When the distance between the light source and the leafy shoot decreases, the light intensity increases.

2. Rate of transpiration is highest when the light source is 20cm from the leafy shoot.

3. Rate of transpiration is lowest when the light source is 50cm from the leafy shoot.

4. When light intensity increases, the stomata open more as photosynthesis increases. Therefore more water vapour is lost through the stomata. Hence the rate of transpiration increases.

Conclusion : The hypothesis is accepted. When light intensity increases, the rate of transpiration increases.