EXERCISE 128 



Problem. What is the relation of light to the growth of living things ? 



What to use. Sterilized petri dishes with culture media ; black paper. 



What to do. Expose two petri dishes to infection by dust from the air. Cut a hole of a distinc- 

 tive form in a piece of black paper. Cover one dish with the perforated paper and one with whole 

 paper. Expose both to bright sunshine at room temperature or warmer for twenty-four hours. 



Record. Examine petri dishes. Make diagram showing numbers and distribution of colonies in 

 both. Tell what effect the sunshine had on the growth of the bacteria. 



Questions. 1. What practical use can be made of the fact brought out by this experiment? 



2. How do these results harmonize with what we know about the need that plants have of sunlight? 



3. What is there in your experience with animals that corresponds to these results ? 



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