If a scientist had only one plant in each group, and one of the plants died, there probably would be no way of determining if the cause of death was related to the experiment being conducted. The experimenter gathers actual, quantitative data from the subjects. Rather, in that experiment, the scientist might have a list of certain behaviors, and record how often the dog exhibits each of those pre-defined behavior patterns. Data for each of the groups are then averaged and compared statistically. The scientist must also calculate the standard deviation or some other statistical analysis to document that any difference is statistically significant.
Research is cumulative and progressive.
Scientists build on the work of previous researchers, and one important part of any good research is to first do a literature review to find out what previous research has already been done in the field. Remember four years of good friends, bad clothes, explosive chemistry experiments.