Sample Hypothesis: Let's use the question, “Why do plants of the same species vary in the number of their leaves?” Our hypothesis might be, “Some plants have more leaves because they have more resources available.” Or, “A reduction in an essential resource will cause plants to allocate the available resource into growth of the most essential plant structures, depending on the life stage of the plant.”

Sample Experiment Design: Grow plants from seed or start with seedlings purchased from a nursery. Fast-growing annual plants such as wildflowers would work well, or a shrub such as brittlebush, also a fairly fast grower. The independent variable is the resource to be tested. One possible resource to test is the amount of water. The dependent variable will be the measurements taken on the plants. There are several possibilities. For this example, we will use the number of leaves produced. If you work with purchased plant seedlings, be sure to count and record the number of leaves on each plant at the time of purchase, as you will have to subtract that number from the total number of leaves on each plant at the end of the experiment to get an accurate result.

Use at least eight plants of the same species for replication. Each plant is an experimental unit. Control variables such as soil, temperature, and light by planting all plants in either the same place in the habitat, or in the same size pots in the same place. If you plant in the habitat, the plants will need to be far enough apart, approximately one-half to one meter, to avoid overlap of plant growth The treatment will be two levels of the resource, water. Half of the plants will receive one and one-half times the amount of water given to the other half. How much water you give each treatment group will depend upon what you select to plant, where you plant it, and what time of year it is. Plants placed in soil outdoors in the habitat will require more water than those planted in pots. Plants grown outdoors, whether in soil or in the ground, will require more water than those grown indoors. Plants will require more water during hot months and less during cool months.

The experiment will run until the plants have grown some leaves, then continue letting them grow for another three to six weeks. The total count of leaves will be the number of leaves present at the end of the experimental time period. Time until leaves appear will depend on the species chosen and the weather. Most Sonoran Desert wildflowers need to be planted in the late summer/early fall and will bloom in the spring (February-April). If you use seedling for this experiment, the experiment can begin as soon as seedlings are purchased and run for three to six weeks.

Sample Prediction: Plants receiving more water will produce more leaves than those receiving less water. You might want to make this experiment more interesting by adding the factor of time and keeping track of when new leaves appear on each plant. You then can add to your prediction that plants receiving more water will produce more leaves, and the leaves will appear sooner, than those plants receiving less water.

Record Results: Tag each leaf with a little bread bag tag with the date written on it to keep track of the leaf’s appearance date. Record when the leaves appear as well as how many.

Sample Analysis of Data and Presentation: Make a bar graph with the treatment (low, high water amounts) on the horizontal axis and total number of leaves on each plant on the vertical axis. Calculate the average number of leaves for each treatment group. Graph the mean number on the vertical axis. Create line graphs of the number of leaves produced over time.

Discussion: Was your hypothesis supported? If yes, go on to test other hypotheses. If not, why not? What did happen? Why? This is a great opportunity to revise your hypothesis and do another test.