Collecting and Interpreting Lettuce
Seed Bioassay Data
Before reading this page, you should first follow
the steps on the "Conducting Reference Toxicity Tests"
end of the 5-day growth period, count and record how many seeds in each dish
For each sprout, measure the radicle length to the nearest
mm. (The radicle is the embryonic root).
Look carefully at the plants to make sure you are measuring
just the radicle, not the shoot as well. For example, in the
picture below, you would measure just the part between the two
arrows, not the shoot and cotelydons to the left.
How Good are Your Data?
Once you have counted how many seeds germinated, and measured
their radicle lengths, then what? How can you interpret these
Comparison to the Control
The first thing to check is your control (the dishes that
contain deionized or distilled water rather than a sample). The
purpose of the control is to identify how well the seeds will
grow without any added contaminants. Would you expect all of
the seeds in your control dishes to germinate? Probably not,
just like a gardener does not expect all the seeds in a garden
If fewer than 80% of the seeds in your control dishes sprouted,
something may have gone wrong in your experiment. Perhaps the seeds were too
old or stored improperly, so they were no longer viable. Or maybe something
went wrong with the conditions for growth. Did the dishes get too hot, too
dry, or contaminated in some way? Did you use tap water for your control, rather
than deionized or distilled water? In many cases this works fine, but since
tap water is highly variable from source to source, it gives less predictable
A Look at Variability
Within each treatment, how much
variability did you find in your results? Did the replicate dishes show similar
numbers of seeds sprouting, and similar average radicle lengths? If you think
the data are much more variable than you would expect, you might want to explore
the potential sources of variability
for this type of experiment.