Brent Cornell

    
The movement of water up the length of the xylem can be modelled using a number of simple apparatus

• These include capillary tubing, filter or blotting paper and porous pots

Capillary Tubing:

• Water has the capacity to flow along narrow spaces in opposition to external forces like gravity (capillary action)
• This is due to a combination of surface tension (cohesive forces) and adhesion with the walls of the tube surface
• The thinner the tube or the less dense the fluid, the higher the liquid will rise (xylem vessels are thin: 20 – 200 µm) 

Filter Paper:

• Filter paper (or blotting paper) will absorb water due to both adhesive and cohesive properties
• When placed perpendicular to a water source, the water will hence rise up along the length of the paper
• This is comparable to the movement of water up a xylem (the paper and the xylem wall are both composed of cellulose)

Porous Pots:

• Porous pots are semi-permeable containers that allow for the free passage of certain small materials through pores
• The loss of water from the pot is similar to the evaporative water loss that occurs in the leaves of plants
• If the porous pot is attached by an airtight seal to a tube, the water loss creates a negative pressure that draws more liquid

Models of Water Transport

    
A potometer is a device that is used to estimate transpiration rates by measuring the rate of water loss / uptake  

• When a plant is affixed to the potometer, transpiration can be indirectly identified by the movement of water towards the plant
• This water movement can be assessed as a change in meniscus level or by the movement of an air bubble towards the plant
• The initial starting position of the meniscus or air bubble can be adjusted by introducing additional water from a reservoir 

When measuring transpiration rates with a potometer, it is important to remember that not all water is lost to transpiration

• A small amount of water (~2%) is used in photosynthesis and to maintain the viable turgidity of plant cells

Measuring Transpiration Rate with a Potometer

    
Potometers can be used to test a number of variables that may affect the rate of transpiration in plants

• These variables include temperature, humidity, light intensity and wind exposure

Temperature:

• Increasing the ambient temperature is predicted to cause an increase in the rate of transpiration
• Higher temperatures lead to an increase in the rate of water vaporisation within the mesophyll, leading to more evaporation
• The effect of temperature variation can be tested experimentally by using heaters or submerging in heated water baths

Humidity:

• Increasing the humidity is predicted to cause a decrease in the rate of transpiration
• Humidity is the amount of water vapour in the air – less vapour will diffuse from the leaf if there is more vapour in the air
• The effect of humidity can be tested experimentally by encasing the plant in a plastic bag with variable levels of vapour

Light Intensity:

• Increasing the light intensity to which a plant is exposed is predicted to cause an increase in the rate of transpiration
• Increasing light exposure will cause more stomata to open in order to facilitate photosynthetic gas exchange
• The effect of light intensity can be tested experimentally by placing the plant at variable distances from a lamp

Wind Exposure:

• Increasing the level of wind exposure is predicted to cause an increase in the rate of transpiration
• Wind / air circulation will function to remove water vapour from near the leaf, effectively reducing proximal humidity
• The effect of wind can be tested experimentally by using fans to circulate the air around a plant