Terri Ann Science Center



Transpiration is the process through which water is lost from a plant by evaporation. Water is taken into a plant through roots and root hairs by osmosis, and it exits the plant through ting openings on the underside of leaves known as stomata. Oxygen and carbon dioxide are exchanged through the stomata. Transpiration is also the major mechanism that powers the movement of water throughout a plant. This transportation of water through the plant is due to water potential. Water potential is the potential energy created by the water molecules within the plant stem. Water always flows from areas of high water potential to areas of low water potential. Gravity, pressure, and solute concentration are all factors determining water potential in a plant.

There are three main kinds of cells in plants. the most abundant is parenchyma cells. These cells are mainly unspecialized and make up the mesophyll layer in leaves. Most parenchyma cells store food such as starch to be used later in the plant. Sclerenchyma cells are lignified and dead at maturity. These cells make up fibers and have thick secondary cell walls. They serve as support in plants. Collenchyma cells can be found in young stems and leaves. They are living at maturity and have thick primary cell walls. There are also three types of tissues found in plants — xylem, phloem, and epidermal. The epidermal cells make up the outermost layer of cells on a plant and function in protecting the plant. Xylem is the water conducting tissue of the plant, while phloem is the food conducting plant tissue.

In this experiment, four bean plants will be used to test transpiration rates under different environmental conditions. The conditions included a normal room setting, exposure to a fan, heat lamp, and moist environment ( air misted and plant covered with plastic bag). Data will be obtained from each setting to determine if the various conditions affected the rate of water loss from leaves.

Under the setting in which the plant is prayed with water and then covered in a plastic bag to create a moist environment, there will be the lowest rate of transpiration.

Materials used for part A included a graduated cylinder, parafilm, distilled water, bean plant, scalpel, watch, fan, heat lamp, spray bottle, plastic bag to cover plant, and a metric scale.
Materials needed for part B included a microtome, single edge razor blade, paraffin, 50% ethanol, toluidine blue stain, distilled water, 50% glycerine, microscope slide, petri dishes, and compound microscope.

First make a potometer by filling the graduated cylinder with water and covering it securely with parafilm. Poke a hole in the parafilm. Remove the root from the rest of the plant and insert the plant into the parafilm hole so that the end of the stem is below the water level in the graduated cylinder. Record the initial water level in the potometer. Weigh the potometer with the plant and record the initial mass. Expose this plant to one of the four conditions (misted plant), and take readings of the potometer mass every 10 minutes for a total of 30 minutes. Record this data in your data table.

In this part of the experiment, a cross-section of a leaf will be observed. Cut the stem of a non-woody plant about 5mm longer than the depth of the microtome. Hold the stem vertically in the microtome and pour melted paraffin around it. Allow the paraffin to cool and harden around the stem. Use a razor blade to cut off the excess stem above the paraffin. Slightly turn the microtome to expose a thin layer of the stem. Slice several thin layers of the stem from the microtome and place these slices in a petri dish containing 50% ethanol for 5 minutes. Move the slices to another petri dish containing toluidine blue stain for 1-2 minutes. Rinse the slices and then mount each section on a microscope slide in a drop of 50% glycerine. Add a cover slip and observe under a compound microscope. Draw the stem cross-section.

Calculating Leaf Surface Area

Data 9A

Transpiration Rate


Water Level in Millimeters

0 minutes

10 minutes

20 minutes

30 minutes




















Date 9A

Leaf Cross Section


1. Calculate the average rate of water loss per minute for each of the following treatments:
Room: 0 ml/min
Fan: .53 ml/min
Light: .367 ml/min
Mist: .23 ml/min


Condition Effect Reason
Room No change No factor promoted water loss
Fan Much water loss Fan provided air currents that increased
Light More water loss Heat from light sped up transpiration
Mist Little change Saturated atmosphere decreased amount of water loss
4. A plant with its stomata closed prevents water that is needed by the plant from escaping.

5. Some plants, such as CAM plants, have adaptations to prevent water loss. These plants have their stomata closed during the day (hottest part of the day when water loss would be greatest) and their stomata open during the night when its cooler to carry out photosynthetic reactions. This reduces water loss from leaves.

Error Analysis
During this experiment, there were many complications that arose. When using potometers, sealing the potometers was difficult affecting the rate of water loss. By changing the procedure and massing the graduated cylinders at timed intervals, more accurate data was obtained.

Although the misted plant did have a low rate of water loss, it was not the lowest transpiration rate observed. The lowest transpiration rate came from the plant at room temperature, the control plant. The plants exposed to the heat lamp and fan showed the highest rate of water loss as expected

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