1. The Potometer

2. A potometer measures the rate of uptake of water from a plant.
This is assumed to be approximately equal to the rate of transpiration. This may not quite be the case because:
Water is used to provide turgidity
Water is used for photosynthesis.
Water is produced in respiration
The apparatus is not sealed.

3. Why do you do that? Plant stem cut under water…
The stem was cut diagonal and slanting…
Any drops of water on the leaves were removed with absorbant paper…
…to prevent air entering / keep a continuous column of water …to make it easier to inset into the rubber tubing and to maximise the SA for water uptake …to prevent it affecting rate of transpiration

4. Conditions that affect the rate of water loss from a plant:
Temperature
Light
Humidity
Air movements

5. Measuring Rate
Measure distance moved by the air bubble (h) in a certain time (t) and the diameter of the capillary tube:
Radius = diameter / 2
Volume of water lost = πr²h
Rate of water loss= Volume / t
The reservoir allows repeat readings to be taken by returning the bubble to the start of the scale.

6. Most rapid; warm, moving air in the light because:
Increased temp increases the kinetic energy of water molecules so they are lost more rapidly from the leaf by diffusion.
Hot air is also drier so there is an increased concentration gradient for water from the leaf into the air.
Stomata open in the light so photosynthesis can occur. Water is lost through the open stomata by transpiration

7. …continued
Air movements remove the layer of humid air around the surface of the leaf and so decrease the water potential found here. This increases the water potential gradient between the leaf and surrounding air this increasing the rate of transpiration.

8. Least rapid: Warm, still air in darkness
Stomata are closed in the dark so water cannot be lost through the stomata.
Without any air movements humid air accumulates around the surface of the leaf reducing the water potential gradient between the leaf and the surrounding air so rate of transpiration is lower

9. Why water movement is helpful:
Transports water through the plant
Transports mineral ions, sugars and hormones dissolved in the water
Cools the plant