To test the hypothesis that the fungus chemicals selectively harm the cancer cells, we need to perform the same test on the normal cells also, as a negative control of the experiment.
The observation that if the chemicals from the fungus harm or cause a decrease in the number of the cancer cells will support the hypothesis. The normal cells however are the experimental negative control. If the negative control also shows up harm due to the chemicals from the fungus, then the hypothesis will be considered false.