Many teachers have asked for further suggestions on measuring plant growth and vigour, especially teachers of students in grades 6 and beyond. Teachers have provided anecdotal comments related to the different types of seeds in past years, but have asked for a more quantitative method for expressing ideas in this area.
Even for professional researchers, the measurement of plant growth and vigour is a difficult task. Measuring leaf area is one method used by researchers, but it is a tedious procedure.
The Tomatosphere Project Team offers the following suggestions for measurement of plant growth and vigour.
When the plants are approximately 3-4 weeks old (or whenever it is convenient to execute this part of the experiment), randomly choose 10 plants from each of the two groups of seeds planted. It is important that there be 10 from EACH group in order to properly compare the measurements from each of the treatments.
Prepare 20 small (1/4 to 1/2 litre) brown paper bags by weighing them to the nearest 0.001g or milligram and write the weight of the empty bag on the outside of each bag.
Cut each plant stem off at soil level. Carefully place each of the plants in one of the paper bags. Ensure that no leaves are lost in the process. If leaves break off, include them in the weighing process, even if detached. Make sure the bags are also labelled with which seed treatment the plants represent (A or B).
Dry the plant tissue in the bags by leaving the bags open in a warm dry place for about a week. Alternatively, place the closed in a drying oven at 60-70°C for two days. Weigh the plants periodically. When they do not experience any further weight loss, they can be determined to be at their "dry weight".
The dry weight of a plant is made up mainly of carbon that the plant takes from the atmosphere in the form of carbon dioxide through the process of photosynthesis. If the seed treatments resulted in any differences in the way the plants carry out the process of photosynthesis, the measurements will show these differences.
Use the following chart to summarize the weight of the plants. When reporting on your germination rates, you will be asked to provide the summary (average weight) for the two groups. This information is an OPTIONAL component of the reporting process.
| Seedling # | Group A - Dry Wt. | Group B - Dry Wt. |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | ||
| 5 | ||
| 6 | ||
| 7 | ||
| 8 | ||
| 9 | ||
| 10 | ||
| Total Dry Weight | ||
| Average (mean) weight of seedlings (total weight of all seedlings dried divided by the number of seedlings dried)* |
*This is the weight information which will be reported when you send in your germination results.
Teachers may wish to examine the variability in the results achieved. This can be done by calculating the standard deviation, an exercise that is easily accomplished using a calculator or Microsfot Excel. Standard deviation is a common method of statistical dispersion. It measures how "spread out" the values are from the mean. It is useful to express the "variability" of this measurement in each treatment to determine if the differences between A and B are truly "significant". This requires calculating the average (or mean) dry weight in each treatment as well as the corresponding standard deviation.
This information is for your own use... it will not be reported to the Project Team
For the calculation of standard deviation, teachers can use standard mathematics texts on statistics and probability, calculators or computer programs, or one of several web sites which provide a reasonably simple explanation of the concept. These include:
Standard Deviation can also be calculated using Microsoft Excel. For an explanation, please use the web site http://www.beyondtechnology.com/tips016.shtml.
The alternative suggestion involves much more input on the part of your students. This activity would best suit students in grades 6 and up.
Present the students with the following problem:
As a class, we have been asked to answer the question "Are there any differences in growth and vigour between the plants from the two seed treatments?" The task is to devise an experiment to answer the question and to create a method of reporting the results to the Tomatosphere Project Team. The method of reporting can be tables, pictorial and/or graphic (e.g. growth curves over time) and must be able to be sent in to the Project Team via the website.
To be valid, the comparison of the two groups of plants must include at least ten plants and the plants should be a random sample of the total seeds germinated. The type of measurements you decide to make are completely up to you and should represent an objective response to the question above.
If you choose this problem-solving activity, you will be asked to send in some samples of the results. These samples should be pictorial and/or graphic in nature and must be sent in using the web site.