Transpiration Lab Book Report

High School ・Biology ・MLA ・5 Sources

After the reduction in the water potential gradient between the atmosphere and the leaf surface, plants transport water from their roots to their leaves by a biological process known as transpiration. According to Taiz (2015), losing water from the surface of the leave to the atmosphere during transpiration helps in creating a lower osmotic potential of the leaf. For this reason, the transpirational pull serves a very vital function in moving water from the xylem to the mesophyll cells and into the air spaces of the leaves (Graham, 2006). The rate at which the water evaporates from the air spaces of the leaf to the atmosphere depdends on the water potential gradient between the outside air or atmosphere and the leaf (Freeman, 2014)

The aim of the experiment was to carry out transpiration rates measurements under conditions of light intensity. The collection of data was done by measuring changes in pressure as water is taken up by the plant into the stem, and then subsequently lost into the atmosphere through the process of transpiration. Transpiration has both merits and demerits, which should be stringently controlled and balanced through physiological regulatory mechanisms of the plant to ensure optimal growth under varied conditions (Freeman, 2014). For instance, transpiration helps in creating a lower leaf’s osmotic potential resulting to transpirational pull, which plays an imperative role in the movement of water to the mesophyll cells from the xylem (Taiz, 2015). As water moves through the stem and leaves, the plant is nourished with nutrients absorbed from the soil. Alternatively, transpiration also causes loss of water by the plant which may not be desirable, especially during water scarcity (Jaivime, Jasechko, and McDonnell, 2015).

Research Question:

What is the effect of light intensity on the rate of transpiration of a plant cutting?


  • Observe how transpiration relates to the overall process of water transport in plants.
  • Use computer interface and a Gas Pressure Sensor to measure the rate of transpiration.
  • Determine the effect of light intensity on the rate of transpiration of a plant cutting.


    • Independent Variable : Time
    • Control Variable: No
    • Dependent: Change of the Pressure Over Time


  • Computer
  • Vernier computer interface
  • Logger pro
  • Vernier gas pressure sensor
  • Utility clamps
  • Ring stand
  • Plant cutting
  • Plastic tubing clamps
  • Parafilm
  • 100 watt light source plastic syringe

Processing the data:

⇒ Raw data:

The raw data collected from the experiment was based on the both the independent variable and the dependent variable. The independent variable was measured in terms of time take in minutes, while the dependent variable was measured in terms of change of pressure over time and expressed as the rate of transpiration or slope (kPa/min). The raw data that was collected from this experiment was quantitative data.

⇒Procedure/Processing the data:

The procedure of processing the data involves calculating or graphing slopes from the experiment observations subsequent to calculations of the personal trial average slope as well as average slope among all 5 trials.


The illustration of the experiment data will involve calculating or graphing slopes from the experiment observations subsequent to calculations of the personal trial average slope as well as average slope among all 5 trials. However, for every graph error bars with standard deviation are included in order to convey margins of error and standard deviations.

  1. For personal results:

Table 1: A Table of Personal Trial Results

Figure 1: A Line Graph of Personal Trial Results with Error Bars

The slope is calculated by dividing the change in y-axis by the change in x-axis, and these changes are determined by picking two points on the same axis and subtracting the larger values from the smaller values for each axis.

Based on the linear function of the form:

Therefore, from the linear function presented in the graph, the average slope of the personal trial results is 0.000.

  1. From all groups’ trials:

Table 2: A Table of All Groups’ Trials Results and Average

Figure 2: A Line Graph of Average of All Groups’ Trials Results with Error Bars

From the average slopes for all five groups’ trials results presented in the above graph show that the average slope is 0.002.

  1. Comparing personal trial average slope to the Average slope among all 5 trials

There are differences between personal trial average slope and the average slope obtained from all 5 trials. For instance, in the personal trial the average slope is 0.000; whereas, the average slope among all 5 trials is 0.002. This indicates that there were slight changes in gas pressure as well as the rate of transpiration over time. On the other hand, the average slope of 0.002 among all 5 trials indicates that there were variations in the rate of transpiration. The reason why the observed differences take place in this experiment is because personal trial results only presents data from one individual, while the average of the 5 trials presents data from different sources and is envisaged to have some obvious variations.

The total leaf surface area is estimated in for the plant used in the experiment by cutting out a section of leaf 2 cm × 2 cm. this was followed by determining the mass for this leaf section and dividing by 25 to find the mass of l cm of leaf. Moreover, dividing the total mass of the leaves by the mass of 1 can be carried out in order to find the total leaf surface area, which was recorded in Table 1 below. In addition, the rate of transpiration/surface area was calculated by dividing the rate of transpiration by the surface area for each plant. The obtained rate values can be expressed as kPa/min/. The obtained ate/area value was recorded in Table 1 below.

Raw Data:

Table 1


Slope (kPA/min)

Surface Area

Rate/Area (kPA/min/

Experimental ...

0.002 kPA/min


0.0005 kPA/min/






  1. The rate of transpiration was affected in each of the experimental situations including varied light intensity as well as changes of gas pressure over time as compared to the control. The experimental situations are expected to vary because they take place naturally and the collected data is envisaged to fluctuate compared to the control where conditions are strictly regulated.
  2. Light intensity is the variable which resulted in the greatest rate of water loss because light triggers the stomata to open so that carbon dioxide is take up by the plant for the photosynthesis process which is light-dependent. In most plants, stomata remain closed in the dark. The reason light intensity might increase water loss when compared to the other factors is because the experiment was carried out under condition of light intensity in order to measure transpiration rates. Other factors such as relative humidity, temperature, wind, and soil water are not considered in this experiment.
  3. There are various adaptations that enable plants increase or decrease water loss, and each is likely to affect the rate of transpiration. For instance, plant leaves contain specialized cells referred to as guard cells which control each stoma pore’s closing and opening. Transpiration rates increase when stomata are open via the guard cells; and transpiration rates decrease when they are closed through these special cells. Plants also have a boundary layer which is a thin layer of motionless air that hugs the leaf’s surface. This layer of air is still and not moving. In order to ensure that transpiration takes place, water vapor leaving the leaf stomata is required to diffuse past the boundary layer to enable it in reaching the atmosphere upon which removal of water vapor by moving air occurs. Plants with large boundary layers have slow rates of transpiration and vice versa. Furthermore, the cuticle is also an adaptation towards increasing or decreasing water loss in plants through transpiration. The cuticle is composed of a waxy layer present in a plant’s entire above-ground tissue and plays an imperative role as a barrier to movement of water from a plant’s leaf. Plants whose leaf surfaces have thicker cuticle layer experience slower rates of transpiration and vice versa.


To sum up, the lab experiment results show that it is possible to investigate how different conditions affect the rates of transpiration. A comparison of the personal trial average slope and the average slope of five trials indicate a considerable difference, which may be attributed to experimental errors. For instance, in this experiment an error that may have occurred include presence of air bubbles in the tude which could have considerably affected the rate of pressure change over time. However, considering that only one plant cutting was used in the experiment, it becomes a hard to determine how these errors may have impacted the rate of transpiration. Moreover, considering that I never calculated anything else from the transpiration conditions, little information was obtained but if calculations on more conditions such as temperature, wind, and soil water would have given more details.

Works Cited

Evaristo, Jaivime, Scott Jasechko, and Jeffrey J. McDonnell. “Global separation of plant transpiration from groundwater and streamflow.” Nature, vol. 525, no. 7567, 2015, pp. 91-94. doi:10.1038/nature14983.

Freeman, Scott. Biological Sciences. Upper Saddle River, NJ: Pearson Education, Inc., 2014, pp. 765-766.

Graham, Linda E. Plant Biology. Upper Saddle River, NJ: Pearson Education, Inc., 2006, pp. 200-202.

Jasechko, Scott, Zachary D. Sharp, John J. Gibson, Birks S. Jean, Yi Yi, and Peter J. Fawcett. “Terrestrial water fluxes dominated by transpiration.” Nature, vol. 496, no. 7445, 2013, pp. 347-50. doi:10.1038/nature11983.

Taiz, Lincoln. Plant Physiology and Development. Sunderland, MA: Sinauer Associates, Inc., 2015, p. 101.

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