The experiment involved 24 bean seedlings, 12 in each group. The pots were all in the same room and contained the same soil. The pots were given 40 ml of water every other day. The only intended difference between the two groups of plants was the amount of time they were exposed to light each day (12 hours and 8 hours for the two groups). The plants were monitored for 14 days, and data on plant height and leaf colour was collected.
After the 14 days, the average plant height for the group exposed to 12 hours of light per day was 13.6 cm, while the average height of the group exposed to 8 hours of light was 11.2 cm, representing a difference of 2.4 cm. Also, two of the plants exposed to 8 hours of light had developed yellow leaves, while only one of the plants exposed to 12 hours of light had developed yellow leaves. No plants died.
There are a number of factors that limited this experiment. The plants were not randomly allocated to groups, and instead their treatment was determined by which tray they were in. The plants’ starting height was not measured, and so it is possible that the plants were already different sizes prior to the beginning of the experiment. The intensity of light the plants were exposed to was not measured, and so we do not know if the light intensity was the same in both groups (although they were both in the same room, the trays were not necessarily equidistant from the light source).
If we were to repeat this experiment, there are a number of things we could do to improve it. Randomly allocating the plants to the two treatments would help make the groups as similar as possible prior to the beginning of the experiment. Measuring the plants’ heights before commencing the experiment would allow us to quantify any differences between groups. Measuring the light intensity at each pot would allow us to quantify this variable. Using more than one tray per treatment would allow replication within treatments.
Although there are a number of factors that limited this experiment, this does not mean that the data collected during this experiment should be thrown away! Instead, these factors should be noted when interpreting and reporting the data. For example, we cannot say definitively that exposing plants to 12 hours of light per day will result in taller plants than exposing them to only 8 hours of light per day. However, based on these results, we could hypothesise that there is some relationship between light exposure and plant growth, and design further experiments to investigate this relationship further.