5.1 2.1.2 transpiration  (Page 2/2)

This diagram shows a potometer which is used to measure the rate of transpiration. As the leafy twig transpires, the air bubble moves to the left.The quicker the air bubble moves the faster the leafy twig is transpiring.

Suggestion: Can we have a

graph relating to

transpiration rate over time?

Also add an arrow to show

movement of the bubble

TOWARDS the plant for

greater clarity. [clark]

The diagram below shows a summary of the movement of water from the roots to the leaf.

Why do plants need water?

Plants need water to maintain turgor pressure. This helps provide support forthe plant as when a cell absorbs water the cell membrane pushes against the cell wall. The cell is now turgid. If there isn’t enough water in theplant the membrane moves away from the cell wall and the cell is now flaccid. This is when a plant begins to wilt and it will eventually die. Wilting is theloss of rigidity of the non-woody parts of plants and occurs when the turgor pressure falls towards zero. Lower water availability can results from droughtconditions, high salinity, saturated soil conditions, low temperatures or bacterial or fungal infections that affect the vascular system of the plant,

When the environment is extremely humid (moist) the rate of transpiration is very low. In some plants, the leaves secrete water onto the surface of theleaves through specialised pores called hydathodes. Hydathodes are the open ends of xylem vessels at the edges of the leaves in certain species of plantssuch as strawberries and some grasses.

Investigation – the effect of environmental conditions on transpiration rate (using a simple potometer)

A potometer measures the rate of transpiration by measuring the movement of water into a plant. The following experiment uses a simple hand madephotometer.

Aim: To assess the effect of different environmental conditions (e.g. temperature) on transpiration rate.

Apparatus

• a drinking straw
• a soft green leafy shoot
• Vaseline
• Marking pen
• Play dough / putti
• Plastic bag
• Elastic band
• Ruler

Method

Perform the following steps under water

1. Cut the stem of the leafy shoot under water .
2. Test to make sure the stem of the leafy twig will fit snug tightly into the top of the straw.
3. Remove the leafy shoot from the straw and set aside.
4. Fill the straw with water. Place your finger over one end of the straw to stop the water from running out.
5. Put the leafy shoot into the open end and seal it with play dough while removing it from water (KEEP FINGER ON THE STRAW!)

Perform the following steps above water

1. Seal with Vaseline. Make sure it is air tight and water tight – if not, all the water will run out when you take your finger off the straw.
2. Mark the water level on the straw.
3. Place your photometer under one of the following conditions for one hour:

• as is, in a warm, sunny place (no wind)
• as is, in a warm, windy place
• with a plastic bag tied around the leaf, in a warm, sunny place.
• A shady place

1. After an hour: use the marking pen to mark the change in water level on the straw.
2. Measure the distance the water moves.

Results

• Draw a table and record the class’ results.
• Plot a bar graph to compare the distances the water moved in the different straws.

Discussion

• Why is it important to cut the stem under water?
• What does the water movement in the straw indicate?
• Which four external environmental factors are you investigating?
• Under which condition is water loss from the leaf the greatest?

Conclusion

• What can you conclude from this investigation?
• Give two ways in which you can improve your experimental results.

More information about potometer experiments can be found on the following websites:

(External Link)using-a-potometer,62,EXP.html

(External Link)