Final answer:
The subject matter involves a biology experiment with Petri dishes as a part of microbiology studies, commonly conducted in high school. Students test for the effects of various substances on bacterial growth, drawing from historical examples like Fleming's discovery of Penicillin.
Step-by-step explanation:
The question relates to a biology laboratory exercise involving the use of Petri dishes containing cornstarch gel or agar, which are common mediums for culturing microorganisms such as bacteria. When students conduct experiments with bacteria, they often need to divide the Petri dish into sections to test the effects of various substances or conditions on bacterial growth.
Marking one section with a (+) usually indicates a positive control, a known condition that elicits a specific response, while the other sections are labeled with the names or abbreviations of the test substances being investigated. Sir Alexander Fleming's discovery of Penicillin serves as a historical example of this experimental approach when he noted that certain mold could inhibit bacterial growth. This type of experiment is a classic illustration of applying the scientific method in the field of biology, particularly in microbiology and antibiotic research.