Potometers come in a variety of designs, but all follow the same basic principles and usually consist of:
-a length of capillary tube. A bubble is introduced to the capillary. As water is taken up by the plant, the bubble moves. The distance the bubble travels in a given time is determined by the rate of water uptake by the plant.
-a reservoir. By turning the tap on the reservoir, the position of the bubble can be set at the start of the experiment. Some designs of potometer use a syringe instead of a funnel with a tap.
-A tube for holding the shoot. the hole in the bung can also be greased with petroleum jelly to keep it airtight
Here's how the experiment is carried out:
1. Cut a shoot underwater to prevent air from entering the xylem. Cut it at a SLANT to increase the surface area available for water uptake.
2. Assemble potometer in water and insert shoot under water so no air can enter.
3. Remove the apparatus from the water but keep the end of a capillary tube submerged in a beaker of water.
4. Check that the apparatus is watertight and airtight.
5. Dry the leaves, allow time for the shoot to acclimatise and then shut the tap.
6. Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into water.
7. Record starting position of air bubble.
8. Start stopwatch and record the distance moved by the bubble per unit time, e.g. per hour.
9. Keep the conditions constant and controlled throughout experiment, e.g temperature and air humidity.
You can select independent variables to investigate how this affects the rate of transpiration. e.g. changing wind speed during the investigation using a fan to investigate transpiration rate and if there's been a change in transpiration rate.
small digestive tract is fixed with villi and microvilli, collapsed
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