The effect of wind speed on the rate of transpiration.

I was asked to design an experiment to investigate the effect wind or air movement on leaves on the rate of transpiration. 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, and the flow of water from the roots to the transpiring surfaces forms the transpiration stream.

Apparatus

1 plant pot with plant

1 electronic scale

1 anemometer

1 thermometer

1 hydrometer

1 Stopwatch

1 meter stick

1 fan

1 plastic bag

I will start the experiment by first wrapping the plant pot/plant roots in the plastic bag. This is to prevent water loss via evaporation, I don’t want this to happen because I am trying to measure transpiration and it will interfere with my experiment dramatically. Then I will weigh the plant pot/ plant using an electronic scale, which should be very accurate. I will record this in my table of results. I will then use a hydrometer to measure the humidity of the room and use a thermometer to measure the temperature of the room. I will take both of these figures into account when collecting my results. Both of these are dependent variables, which I need to keep the same throughout the experiment. This is going to be hard to do but I can keep an eye on them both and take them both into account. To try and keep them the same I intend on doing the experiment on the same day so it won’t change too dramatically. Both of these dependent variables effect the rate of transpiration dramatically. Next I will set up the fan then measure 20, 40, 60, 80,and 100cm away from this and mark it down on the table. I will use an anemometer to measure the wind speed of the highest setting on the fan and I will record it down. I chose to use a fan because it has a wider diameter than a hairdryer and it is also stationary so it wont move about too much. A hairdryer will move about because it is not stationary and it will give some leaves greater wind speeds than others making the experiment unfair. The small diameter would mean that the wind will not get to all the leaves. I intend to place the plant at 20, 40, 60, 80 and 100cm away from the fan thus decrease the wind by five times. Later I will measure using an anemometer the relationship between these distances and the wind speed. I will turn on the fan at full speed and time it using a stop clock for 20 minutes in this time transpiration should have occurred. I will then weigh the plant pot again using the electronic scales and record this down. I will then calculate the difference between the original weight of the plot and the new weight by taking the second result and subtracting it from the first result. This should give me the amount of water loss. I will then use this new weight that I obtained for 20cm and do the same experiment for 40cm. I will subtract the weight for 40cm from the previous weight for 20cm and so on till I get to 100cm. I am measuring the loss of mass.

Here is what my table of results will look like.

I will then repeat each experiment three times so that I can reach a good conclusion. Doing repeats enables me to identify any odd results because I am able to compare them with each other.

I will take an average so that my results will be as accurate as possible.

I have chosen to conduct my experiment in this way because it is very simple and realistic. This is because I am using a plant in a plot, which has ...

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Here is what my table of results will look like.

I will then repeat each experiment three times so that I can reach a good conclusion. Doing repeats enables me to identify any odd results because I am able to compare them with each other.

I will take an average so that my results will be as accurate as possible.

I have chosen to conduct my experiment in this way because it is very simple and realistic. This is because I am using a plant in a plot, which has taken thousands of years to adapt to that natural formation. It is also the formation that it would occur in its habitat. I think that five different wind speeds are efficient for me to make a good comparison. The plant pot also has many leaves, which offer a good sample size. The same plant will be used the same time therefore the surface area will not change only the mass will has the plant losses water.

My independent variable will be the distance from the fan, which in reality is wind speed, which I will change, each time. The wind will come from a fan and will measured using an anemometer.

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. Wind moves the air and water vapour away from the leaf, this increases the water potential gradient from the plant to the 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.

Variables that I will keep the same are:

Weber

LIGHT- effects transpiration because stomata usually open in light and close in darkness. At night therefore, and only small amounts of water are lost through the waxy cuticle: As stomata open 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 in therefore generally faster on warm, sunny days than on cold dull ones. Temperature transfers more energy to the water molecules. The more energy the water molecule has, the faster it moves and the more likely it is to escape to the outside of the leaf and away from the plant.

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. It is a measure of the number of water molecules in the air. A decrease in the humidity around the leaf would mean that there would be less water molecules around the leaf thus creating a larger water potential gradient.

The leaf

1) Stomata: by evaporation of water from cells and diffusion of the water vapour through stomata, the pores found in the epidermis of leaves and green stems (about 90%). Each pore is called a stoma when stoma open the hole that allows carbon dioxide to pass into the leaf also lets water molecules out. This is a disadvantage to the plant because it loses precious water; if the loss of water from the leaves is greater than the uptake of water by the roots, the plant wilts.

Underside of the leaves is tiny holes called which allow the plant to breathe. When the water reaches the leaves it evaporates and escapes through the stomata. This is called transpiration. As the water escapes, more water is sucked up the xylem. In other words, the leaves create suction pressure to suck water up the plant.

Mineral salts dissolved in the water, which include nitrates and phosphates, are needed for growth.

Plants need to control the water loss from transpiration. They do this by opening and closing their stomata. Two guard cells surrounding each stoma can open and close it. The holes are opened to allow gaseous exchange (breathing) and are closed to reduce loss of water. The guard cells surrounding the stoma have feature, which allows the pores to open and close. There are additional cellulose micro fibrils in the inner walls of the guard cells that line the stoma. When the guard cells take in water, they become turgid but they cannot expand in diameter because the cellulose micro fibrils will not stretch. They therefore increase in length particularly along the thinner, outer walls. The outer walls stretch more than the inner walls, causing the cells to change shape and to form a gap between them.

2) Waxy cuticles: by evaporation from the outer walls of epidermal cells through the waxy cuticle covering the epidermis of leaves and stems (about 10%, varying with thickness of cuticle).

Transpiration increases when:

The leaf stomata are open
The air is dry
It is warm
It is windy
The leaves have a large surface area

The cohesion-tension

The root creates some pressure which forces water up the stem but not enough to force it to the very top of the plant. There is also a negative pressure inside the vessel of the stem, which pulls water up. As water evaporates from the leaves it creates a negative pressure in the xylem. This pulls up water from the roots. Water molecules stick together to form a

column. The mechanism which provides the force that pulls water up the plant is called the cohesion-tension hypothesis.

Some plants such as the cactus are adapted to prevent water loss these are called xerophytes. These plants have special features that prevent them from loosing too much water. They have special features like a leaf that is rolled inwards this traps humid air around the leaf surface. Some have hairs that reduce air movement around the surface of the plant. Others have a swollen stem, which gives it a low surface to volume ratio and acts as a reservoir for water.

I will try and make it fair by sticking to the plan. I will use a stopwatch to measure the time and will stand equal distance from the leaf.

There is nothing in this experiment that I feel is of any danger to others or me. I will hang my coat up and have my bag under the table. I will use the same fan each time because different fans have different wind speeds and different diameters. I will use a meter stick to measure the distance accurately and precisely.

Once I have obtained my results I will plot them on a graph. This will enable me to see the results more accurately and I will be able to see the rate of transpiration better. The graph will look something like this.

Water loss (%)