Re: effect of black light on plant growth

Ian,
  The first thing you need to do is a little research in order to develop a 
good hypothesis. Look for information on plant photosynthesis and light 
wavelengths. You should have in mind what wavelengths plants use and what 
wavelengths each of your treatments provides; then you will be able to make 
a good hypothesis based on the information you find.
  It sounds like you're on the right track with the experiment itself. You 
will need to think about what kind of plant you want to use.  I would look 
for something that has a short life cycle (if you use garden seeds, the 
packet will indicate this as "days to harvest") and that will be easy to 
measure (not a vine, like pole beans or peas, or a rosette, like radish or 
cabbage). A few controls that I would suggest are:
    1. Make sure that all factors other than light are the same for all 
your plants (try to keep the temperature the same, keep them all well 
watered, etc.)
    2. Keep the duration of light the same for the sunlight and black light 
treatments; i.e., turn your black light on at sunrise and off at sunset. 
That way you know that any differences you observe are due only to 
wavelength and not to how long the plants received light.
You should also make sure that you have several replications within each 
treatment. That is, grow 5 or 10 (or as many as you have room for) plants 
in sunlight, and the same number in  black light and in the dark. When you 
are done with the experiment, you can average the results of all the plants 
in each treatment and compare the averages. Just like us, plants have 
genetic variability; some individual plants will grow better than others 
just because their genes are different. If you only have one plant in each 
treatment, you can't be sure that the differences you see aren't just 
individual differences in the plants' genes. The more plants you use, the 
more confidence you can have that your results aren't due to these 
individual differences.
  As you are setting up, think about what kinds of measurements you will be 
taking, how often, and for how long. How are you going to determine "good" 
versus "bad" responses?  Survival for some period of time? Leaf size? Leaf 
color? Number of leaves? Plant height? Development of flowers and fruit? 
Again, a little research will help you determine how you should judge your 
plants' growth.  Some plant responses to adverse conditions will make 
measurements like height unreliable by themselves (hint: look up 
etiolation), so you should consider using a group of characteristics to 
describe the condition of your plants.

  Good luck, and have fun!

Brantlee Spakes
Department of Plant Biology
Arizona State University
Box 871601
Tempe, AZ 85287-1601