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Transpiration rates for different plants

Transpiration rates for different plants

Introduction: (Initial Observation)

Most of the water entering a plant’s root will exit the leaves by transpiration. Transpiration in plants is the passage of watery vapor through the stomata of plant tissue.
By knowing the amount of water exiting each plant trough the process of transpiration we can estimate the amount of water the plant needs. We can also compare different plants based on their need to water and select the best plants for different locations based on the availability of water.

The rate of transpiration may also change in different hours of the day, so while using automated irrigation systems, we can increase or decrease the amount of water supplied to the plant based on the time of the day.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

As you see in the title of this project guide you can perform two different studies related to the transpiration. In one study you compare the transpiration of different plants under the same environmental conditions. In the other you compare the transpiration rate of one specific plant in different times of the day. You may as well study the effect of other factors such as light and temperature on the rate of transpiration.

Information Gathering:

Find out about transpiration. Read books, magazines or ask professionals who might know in order to learn how different plants transpire at different rates. Keep track of where you got your information from.

Following are samples of information you may find.


Trees absorb water primarily through their roots. They evaporate water through openings in their leaves in a process called transpiration. As with human respiration, trees tend to transpire more with increased temperatures, sunlight intensity, water supply, and size. When it gets too hot, though, transpiration will shut down.

Many factors influence transpiration rates, including leaf shape, size, pores (stomata), and waxiness of the leaf surfaces. Where a particular tree species grows often depends upon how it has adapted its transpiration rate to a particular climate. Conifer needles are more efficient at retaining moisture than broadleaf trees because they have stiff, waxy leaves (needles) with small stomata that are recessed in the leaf surface. Because they are efficient in retaining water, conifers are found in drier and colder climates where water supplies are limited.

Plants transpire vast quantities of water – only one percent of all water a plant absorbs is used in photosynthesis; the rest is lost through transpiration. In one growing season, one corn plant transpires over 200 liters.

Transpiration, along with evaporation of moisture on land, provides almost two-thirds of the atmospheric moisture that falls as precipitation on land surfaces. The remaining one-third comes from the evaporation of the vast oceans.

In this activity, students will make a small terrarium that will allow them to observe and measure the water given off through transpiration.

Transpiration rates for different plants and conditions:

Background Information:

Naturally plants need water in order to grow, develop, and survive. Plants, unlike animals do not drink water directly. On the other hand, they attract the water through their roots. When the soil is wet, tiny little hairs on the roots soak up the water which than travels to all the parts of the plant. This process is called osmosis. When the water is absorbed, it travels to the rest of the plant, through xylem vessels, delivering minerals and nutrients necessary for development. Water does not stay in the plant forever. Once the plant absorbs all the necessary nutrients and minerals, water actually leaves the plant, mostly through the leaves. A further look proves that leaves have small little holes called stomata which allow for the water to exit. There are several variables that affect the process of transpiration.

Gathering Information:

Key Terms:


This is the loss of water from leaves by evaporation. It is much faster when stomata are open than when they are closed.


Water moves up plants (through their xylem vessels) by capillary action, which is the effect of water molecules clinging to each other and clinging to the sides of the tube.

transpiration stream

Water travels up xylem vessels from the roots to the leaves in the transpiration stream.

suction pressure

Leaves suck water out of xylem vessels to replace water lost by transpiration. It is caused by osmosis.

root hair cells

These are tiny hairs covering the ends of the smallest roots. They have a very large surface area. They absorb water and mineral salts from the soil.

xylem vessels

These are microscopic capillary tubes found in a plant’s veins. They carry water and mineral salts up a plant from its roots to its leaves.


Stomata are tiny holes in the epidermis (skin) of a leaf. They are usually found on the undersides of leaves. Each stoma is surrounded by two guard cells.

guard cells

Two guard cells surround each stoma. They can open and close the stoma.


Transpiration is the process by which water vapor escapes from living plants and enters the atmosphere. It includes water which has transpired through leaf stomata, as well as intercepted water which has re-evaporated. When the soil is covered by a growing crop, transpiration greatly exceeds evaporation.

Evapotranspiration refers to the combined effect of evaporation and transpiration.

Pan evaporation, the water loss per unit time from a standard container, is often measured at weather stations. It is not necessarily equal to actual soil evaporation or evapotranspiration. When soils are wet, actual soil evaporation and pan evaporation are nearly equal. When soils are dry, soil evaporation rates are typically lower than pan evaporation rates because water is less available at the soil surface.

Evapotranspiration rates may be higher than pan evaporation rates if soil moisture is not limiting. This is because plants (depending on canopy architecture, Leaf Area Index, and maturity, among other factors) have a greater evaporative surface area per unit area of ground, compared to the pan.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to learn about transpiration as the main form of loosing water by plants, and to learn about measuring transpiration.

Any of the following questions may be taken as the main question for this project:

  1. How does the transpiration rate vary in different plants?
  2. How does the transpiration rate change in different times of a day?
  3. How does the transpiration rate change in different light conditions?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

This is how you may define variables for question/ experiment 1

  • Independent variable is the plant type/ species.
  • Dependent variable is the daily transpiration
  • Control variables are the light and temperature
  • Constants are the type and size of water collection bags used in the experiment.

This is how you may define variables for question/ experiment 2

  • Independent variable is the time of the day.
  • Dependent variable is the hourly transpiration
  • Constants are the type and size of water collection bags used in the experiment.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
This is a sample hypothesis for question/ experiment number 1:

Among Oak, Maple, Cherry and Apple tree that I am comparing, I hypothesize that Cherry tree will have the highest rate of transpiration.

This is a sample hypothesis for question/ experiment number 2:

I hypothesize that the transpiration will be maximized during the mid day when the weather is warmer and the sunlight is shining stronger.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1:

How does the transpiration rate vary in different plants?



In a closed space, transpired vapors will condense in the form of water drops that can be collected and measured. In this experiment we will collect the transpired water in clear plastic bags.


  1. Get 4 same size clear plastic bags
  2. In each tree you are testing, find a small hanging branch that is in reach.
  3. Very early in the morning insert each identified branch in the plastic bag and wrap the opening of the bag tightly round the stem so that the moisture will get trapped inside the bag.
  4. Label each bag with the name of the tree it is attached to.
  5. Next day very early in the morning carefully remove the bags and measure the amount of water condensed in the bag.
  6. Record your results in a table like this:
    Plant or Tree Branch transpiration Branch square foot Daily transpiration per square foot

* The type of trees or plants you test mainly depends on which plants or trees are easily accessible to you.

How to measure?

You can collect the water and then measure its volume using a small graduated cylinder or a graduated pipette. Small part of the collected water may remain in the bag and reduce the accuracy of your results.

Another method is using a high precision scale. Simply weigh the empty bags before the experiment. Also weigh the bas with condensed water in it after the experiment. The difference will be the mass of transpired water.

Now you need to cut the branch and measure the total surface area of leaves and stems that has been inside each bag. (Also count the leaves. You may need that if you want to calculate the transpiration per leaf.) To do that you may separate the leaves and paste them side by side on a pieces of paper. Then you measure the area of papers. It is ok if some leaves partially overlap and some small gaps remain between the leaves. These two will compensate each other. In calculating the surface area consider both sides of the leaves.

Experiment 2:

How does the transpiration rate change in different times of a day?

Introduction: The amount of hourly transpiration is not high enough to be collected in a bag for measurement. In this experiment we measure the amount of water absorbed by the plant instead.


  1. Start your experiment early morning before sunrise.
  2. Insert the plant roots or cut stem in a tall narrow graduated cylinder.
  3. Fill up the cylinder with water to its highest graduation line.
  4. Cover the top of the cylinder with food wrap plastic, sponge or with wax to prevent the evaporation of water.
  5. Place your setup outside to get enough light and air. Secure it so it will not fall by wind.
  6. Every one hour visit the plant and refill the water using a 1 ml graduated pipette. Record the amount of water used to refill.
  7. Enter your results in a table like this:
Time Water used to refill = transpired water
6:00 a.m. 0 ml
7:00 a.m.
8:00 a.m.
9:00 a.m.
10:00 a.m.
11:00 a.m.
12:00 a.m.
1:00 p.m.
2:00 p.m.
3:00 p.m.
4:00 p.m.
5:00 p.m.
6:00 p.m.
7:00 p.m.

You may continue your observations, refill and recording up to a few hours after sunset or up to the next morning if you want. If you miss observations in the mid night, you can estimate them based on your observation in the next morning.

Materials and Equipment:

This is a sample list of materials. Your actual/ final list of materials may vary.

  • Clear plastic bag
  • 10 ml graduated cylinder
  • 1-ml graduated pipette
  • Trees or house plants
  • Small cutting of a house plant
  • Petroleum jelly/ wax or plastic food wrap
  • Lamp or source of sunlight (lamp is a substitute for sunlight while doing experiments indoor in winter)
  • Water
  • Scissors

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


If you do any calculations, write them in this section of your report.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List your references in this part of your report.