In the early days of germ theory, contagious diseases were thought to be caused only by fungi or bacteria. In the 1890s, Dmitri Ivanovski filtered extracts from diseased tobacco plants and discovered that the disease could be transmitted to new plants through the filtrate. He hypothesized that something smaller than bacteria or fungi was responsible for the transmission of the disease. Which best explains how Ivanovski’s work led to a change in the germ theory?
Answer
A
He tried to promote his hypothesis as a law.
B
He used a new experimental method to test his hypothesis.
C
He used a more powerful bacterial strain than other scientists had.
D
He obtained results that confirmed what other scientists were thinking.
Explanation
Ivanovski passed material from diseased plants through a filter that trapped bacteria and fungi, then found that the filtered material still caused disease in new plants. This new experimental method provided evidence for an infectious agent smaller than those known organisms, changing ideas about what could cause disease. The best choice is “He used a new experimental method to test his hypothesis.”
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