Experiment Notes
These guidelines have been compiled by David Prescott, the Head of the Science Faculty at St. John's-Ravenscourt, a private school in Winnipeg, Manitoba. They have been compiled based on years of working with growth chambers in the classroom and provide commentary that correlates with the basic science labs posted in the Experiment Exchange.
Seeds
In general, it’s a good idea to plant more seeds than you think you will need. While the germination rate for WFP is high, you can only pollinate plants that flower at the same time. As well, the germination rate tends to decrease with successive generations. Extra seeds can be frozen and used in future experiments.
Artificial Selection
The two best traits to use are the number of trichomes (hairs) and the amount of purple colour. Both of these are polygenic and can be successfully selected. One generation is usually enough to show a change in frequency that is statistically significant.
Both characteristics can be analyzed statistically. The average number of trichomes per plant can be analyzed with the t-test, or, the number of plants with a specific number of trichomes can be analyzed with a chi-square test. The number of plants with a significant amount of purple colour (I use colour on stem and leaves) can be analyzed with chi-square.
Replicating Mendel
This lab takes close to three months to complete from the Parent generation. It’s important that the students keep accurate records, difficult, considering the length of time involved.
I usually have the students plant one variety per tray. I have them use plastic numbered stakes to keep track of the experimental crosses.
Introduction to Plants and Plant Life Cycles
This lab can be used at all levels by varying student expectations. Younger students will make more general observations while older students would be expected to provide specific detail and use anatomical vocabulary. As well, older students could use mass increase as an indirect measure of photosynthesis.
It’s not essential to use the fast plants for this lab unless you want to include pollination and seed development.
If you choose to use local plant seeds you should set the chamber to mimic the environmental conditions in your area.
Factors Affecting Plant Growth
It is not essential to use Wisconsin Fast Plants for this lab. Students will not be pollinating or growing successive generations so the life cycle time is not important.
Again if you choose to use local plant seeds you should set the chamber to mimic the environmental conditions in your area.
This lab can be run in a number of different ways. Students can brainstorm the factors individually or in groups and then design the lab using the guide below, or some other format.
Since it can take up to two weeks for many common garden plants to germinate I usually start them ahead of time, using large germination trays. This way I can have all the seeds germinated and ready at the same time. The students then choose the type of plant(s) for their experiment. The students transfer the plants to 4 or 6 spot bedding plant trays for the experiment. Small plastic food storage containers can be used for watering trays.
For all experiments, fertilizer should be added to the watering tray. It’s very easy to burn the plans with direct application. The specific fertilizer depends on the type of plants used. Comparing fertilizers is a good experimental factor to choose.
Other factors include intensity/type/length of light, type of growth medium, type/brand/amount of fertilizer, temperature, amount of water/humidity, the effect of wind. Students can also test the effect of any of the previous factors on different types of plants.
Analysis of the data will depend on the level of the students. Middle school students may use qualitative observations only. General Science students should be able to determine average growth (mean, median, mode). AP Biology students should be able to use a t-test to analyze the averages, as well as graphing the data with a reliability of the mean calculation.
Experimental Design Guide
The experimental design guide below was set up to include all of the information required in AP Biology for a student-designed lab.
Title:
Accurately describes the experiment, including independent and dependent variables.
Hypothesis:
Explains observations used in the design. Often includes a prediction about the relationship between independent and dependent variables.
Independent Variable:
The variable the experimenter is investigating. This variable is changed/determined by the experimenter. Include units and how it is measured.
Dependent Variable:
The variable used to determine the effect of the independent variable. This variable is the one that is measured/observed by the experimenter. Include units and how it is measured.
Control:
Normal or standard value of the independent variable. These are the trials that give you a point of comparison.
Controlled Variables:
These are variables kept constant, or monitored, so that they do not affect the results. These are often other potential independent variables.
Summary:
A brief summary of your experiment. This should be an overview of what you intend to do. It should include the number of test points (minimum 3) and the number of trials at each test point (minimum 3).
