Chapter 23

Transpiration Experiments

Bubble Potometer:

Is an instrument used to measure Rate of Water Uptake by a leafy shoot, a living surface.

This rate of water uptake is only approximately equal to transpiration since some water may be retained by the leafy shoot for other processes e.g photosynthesis.

Bubble Atmometer:

Is an instrument used to measure Rate of Evaporation from a non living surface. The rate of evaporation will equal rate of water uptake.

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Chapter 23

When a bubble potometer and a bubble atmometer are subjected to the same conditions, the changes in rate of water loss from a living and a non-living system can be compared.

In Light: both lose water rapidly

In Dark: the atmometer continues to lose water. The potometer does not since the plant has closed it’s stomata.

Therefore the use of an atmometer in transpiration experiments shows when the potometer is actring as a free evaporator and when it is affected by Physiological Factors e.g. stomatal closure.

List: the factors which will have an effect have an effect on the transpiration rates of a plant.

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Chapter 23

The Weight Potometer and Atmometer

The oil layer

At the end of the experiment, water is added to the conical flask via the syringe to restore the original water level.

NB: For water 1g – 1ml

Results

Apparatus / Original Weight / Final Weight / Amount of water needed to restore original level
Weight Potometer / 1000g / 900g / 110ml
Weight Atmometer / 1000g / 900g / 100ml

1) Why is the oil layer needed?

2) Why is a plant with it’s rooting system present preferable to a rootless plant?

3) What does

a) a weight potometer measure?

b) a weight atmometer measure?

c) Express each of these as a rate of water loss.

4) Consider the above table of results.

a) In which apparatus is the volume of water added at the end using the

syringe equal to the volume lost during the experiment?

b) In which apparatus is slightly more water required at the end to return the

water level to it’s original mark?

c) Account for the difference between a) and b).

5) For each apparatus state

a) the weight loss during the experiment

b) the volume of water needed to restore the original level.

6) Copy and complete the following conclusion using the words:

Atmometer Turgidity equal physical photosynthesis

physiological 100 Potometer 10

In the weight ______the amount of water required to restore the original level is ______to the amount lost by evaporation is an entirely ______process. In contrast the weight ______. Assuming ______ml is lost by the physical process of transpiration, the extra ______ml used must have been incorporated by ______processes into the plant tissue e.g. for ______or maintaining ______

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