Biodeg. Pack. Material

William Tivol tivol at news.wadsworth.org
Mon Jun 19 16:11:07 EST 1995


Robbie Byers (VFWN86B at prodigy.com) wrote:
:      I conclusion, I would appreciate any and all comments, suggestions, 
: criticisms, and information about my project.  

Dear Robbie,

: Title of Research:   The Effect of Renature Contaminated Water on Radish 
: Seedlings


: Purpose:   The experimenter wanted to find how Renature contaminated 
: water affected the environment.  In this experiment, the environment was 
: represented by the radish seedlings.


: Hypothesis:   The experimenter believed that the higher amount of 
: Renature solution given to each radish seedling would affect its growth, 
: and that the Control group would grow more than any other group, 
: especially the 30g group.

	The first part of the hypothesis is OK; the part about which group
would grow more is not.  Do not assume results before beginning the experiment.
Although you seem not to have been biased by your expectations, this can be a
great danger in science--one always seems to find what one looks for, and there
may be unintended bias (putting the expected "better" plants closer to the
light, stretching them out more when measuring, etc.).

: Procedure:   One 14.175g package of White Tipped Scarlet Globe Radishes, 
: four newspapers to place under the four pans to keep excess water from 
: leaking and from mixing with other groups, one four-beam balance, 60g of 
: Renature, 18.145 kilograms of sterile potting soil, and three 3.79 liter 
: jugs of distilled water were needed for the experiment.

	Good planning, but perhaps tap water would have been better than dis-
tilled water.  First, there are micronutrents in tap water (and maybe in Rena-
ture) which are not in distilled water (which can leach these from the soil,
thus making the plant growth slow and then stop), and, second, no plants in
the ecosystem are watered with distilled water, so going from your experiment
to any conclusions about the environment is dangerous.  Rain water would be a
good alternative, and should be easy to collect (I didn't check to see where
you live, so possibly rain water might not be plentiful).

: The plants were measured in 
: centimeters after they germinated.

	May not be the best measure of environmental pollution.  Lack of light
will cause some (most?) plants to grow taller.  Other conditions can lead to
the same thing, and think of a taller lettuce plant to get an idea that taller
may not be better.  Plant mass might be a better measure, and root mass (the
edible part of the radish) is perhaps the best measure.  Of course, both of 
these are harder to measure repeatedly than is height.

: Discussion:   The Control group and the 30g group grew taller than the 
: 10g group and 20g group. The experimenter did not understand this.

	A good reason not to presuppose the results!

: One 
: reason this may have happened was that the starch in Renature could have 
: stimulated the growth of the radish plants. Or, there may have been 
: another chemical in Renature that could have stimulated the growth. 

	See above re micronutrents.

: Possibly even the polyvinyl alcohol portion of the Renature (a water 
: absorbent used in baby diapers) could have helped maintain constant soil 
: moisture which may have stimulated plant growth.

	Another possibility.

: It seems probable that further testing would still not 
: produce the same data which would support the research hypothesis.

	It *does*, however, support the first part of the hypothesis.

: Conclusion and Results:   The research hypothesis was only partly 
: supported by the data.  It was found that the Control group average was 
: 11.58 cm, the 10g group average was 10.37 cm, the 20g group average was 
: 10.71 cm, and the 30g group average was 10.82 cm.

	A very important datum is missing.  You need to know the standard dev-
iations of these measurements.  If the measurements are assumed accurate for
each plant (it is not easy to measure plants to 0.01 cm!) the range of heights
within a group will tell you whether the groups are significantly different.
After all, the 20g and 30g groups differ by ~1 mm.  If the within-group vari-
ability (standard deviation) is 1 cm, then for 25 measurements, the standard
deviation would be 0.2 cm (it goes as the square root).  The proper conclusion
in this case is that there is no significant difference between these groups.

:  The experimenter 
: concluded that the Control group grew taller than all the other groups, 
: but the 30g group grew taller than the 10g group and the 20g group.  

	These are not conclusions; they're observations.  They are correct,
but may not be significant.  That is 11.58 is always larger than 10.82, etc.,
but if the standard deviations are ~1 cm for the groups, there is no signifi-
cant difference.  The appropriate conclusions would be "x grams of Renature,
when added to a liter of water did [not] significantly affect the growth of
radish seedlings."

: Perhaps the starch in Renature was able to stimulate growth, or maybe it 
: was another chemical.

	Another testible hypothesis--add starch to distilled water and see
what happens (use the same starch as is present in Renature, of course).

:  It was surprisingly noted that 10g of Renature 
: seemed to slow the growth of the radish seedlings more than the increased 
: dosage.
:  
	Might not be so surprising.  If there are opposing effects, different
dose-dependence can account for this kind of behavior.
	Altogether a good effort.
				Yours,
				Bill Tivol




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