Effect of Surface Area On The Rates Of Transpiration

David Johnson Coursework Biology

Aim...

Compare the effects of a named factor (surface area) on the rates of Transpiration of two different plant species.

Transpiration is the evaporation of water vapour from plants; it takes place at three sites.

Stomata: Occurs in leaves and herbaceous stems, and amount for 90% of all water loss.
Cuticle: A wavy external layer on plant surfaces which serves to limit water loss through cell walls, although up to 10% of water nevertheless escapes by this route
Lenticels: Areas of loosely packed cells on the surface of woody stems through which gas exchange, and therefore water loss, takes place.

Most of the water entering a plant does so via the root hairs. It travels across the root cortex to the xylem, ascends in the xylem to the leaves and is lost by evaporation from the surface of the mesophyll cells before diffusing out through the stomata. This process is called transpiration.

Apparatus...

Potometer

Short Rubber Tubing

Calibrated Capillary Tube

Gas Syringe

Leafy shoots X2 (Specie living in wet environment, and a Specie living in dry environment)

Petroleum Jelly

Method...

A leafy shoot is cut under water to prevent air entering the xylem. Care taken not to get water on the leaves.
The potometer is filled completely with water, making sure there are no air bubbles.
Using a rubber tube the leafy shoot is fitted to the potometer under water.
The potometer is removed from under the water and all joints are sealed with petroleum jelly.
The distance moved by the air/water meniscus in a given time is measured, and from this volume the water loss can be calculated (Area of a circle*Length)
The volume of water lost against the time in minutes can be plotted on a graph.
Once the air/water meniscus nears the junction of the reservoir tube and capillary tube, the tap on the reservoir is opened and the syringe is pushed down until the meniscus is pushed back to the start of the calibration on the capillary tube. Measurements then continue as before.

Safety Precautions/Risk Assessment...

The leafy shoot is cut under water to prevent air-bubbles being drawn into the xylem (which is under tension) and blocking the flow of water through the shoot.
The end of the leafy shoot should be cut at the start to prevent any dissolved air from the water collecting at the end of the shoot.
All joints must be watertight to prevent air breaking the continuous columns of water. Tight-fitting rubber tubing and/or petroleum jelly is used at all joints.
When making comparisons, e.g. the rate of transpiration at different temperatures, all other ...

This is a preview of the whole essay

Safety Precautions/Risk Assessment...

The leafy shoot is cut under water to prevent air-bubbles being drawn into the xylem (which is under tension) and blocking the flow of water through the shoot.
The end of the leafy shoot should be cut at the start to prevent any dissolved air from the water collecting at the end of the shoot.
All joints must be watertight to prevent air breaking the continuous columns of water. Tight-fitting rubber tubing and/or petroleum jelly is used at all joints.
When making comparisons, e.g. the rate of transpiration at different temperatures, all other factors, e.g. humidity, wind speed etc. Must be kept constant.

Effects of Environmental Factors...

Light: Effects transpiration because stomata usually opens in light and closes in darkness. At night therefore, and only small amounts of water are lost through the waxy cuticle. As stomata opens in the morning, transpiration rates increase.

Temperature: The higher the temperature, the greater the rate of evaporation of water from mesophyll cells. It increases the capacity of air to absorb water from leaves and it warms the water inside leaves making it evaporate more quickly. Direct sunlight has the same effect since it warms leaves to a higher temperature than the atmosphere. Transpiration is generally faster on warm, sunny days than on cold, wet days.

Humidity: generally, transpiration only occurs when there is a lower humidity level (concentration of water vapour) in the atmosphere than exists in the air spaces inside the leaves. Transpiration stops when the atmosphere is saturated with water vapour, and resumes when the air becomes drier.

Wind: Air movements carry away water vapour from leaves and this prevents air around them from becoming saturated with water vapour. Consequently, depending upon temperature and humidity, transpiration is faster on a windy day than in still air.

The best conditions for a higher rate of transpiration are the same as those needed for drying washing on a line: a warm, dry, sunny, windy day.

Fair Test

A fan will be set roughly 1 meter away from the plant, and will provide the heat and wind essential to cause the plant to transpire, thus causing water uptake by the xylem of the stalk.

The potency of the fan will remain constant as will the distance that it is set separately from the plant. The light intensity and the humidity of the room will be kept constant as much as possible.

The unfixed factor that will be changed is the surface area of the plant. This will be done by starting with 8-10 leaves on the plant, and then after each minute two leaves will be required to be removed.

The water uptake for each number of leaves will be recorded, and two will be taken away every minute and the results recorded until there are no leaves left. 5 results will be taken for each number of leaves and an average will be made. All the other factors which alter the transpiration rate will be kept constant. I believe this will guarantee a fair test.

Obtaining of Results

The experimentation was performed with meticulousness and proficiency as 5 results were taken for each number of leaves, and an average was made. The amount of water uptake is recorded in cm as read of the capillary tube on the potometer.

After the experiment the leaves were put onto graph paper, and the surface areas were recorded in mm².

Water uptake

Prediction...

I envisage that the higher the surface area of the leaves, the more water uptake will occur.

This is because; greater surface is accompanied with greater quantity of stomata.

More stomata means greater value of water vapour can be lost by evaporation from the surface of the mesophyll cells before diffusing out through the stomata.

The graph of water uptake against surface area should therefore be a realistically straight line.