It would be impossible to test the entire population of black-eyed Susans, so you would select part of the population for your experiment. Through careful experimentation and observation on a sample of the population, you could generalize the effect of the chemical on the entire population. Here is a more familiar example. If you did, you were estimating. If you said about 300 beans, your estimate would be close to the actual number of beans.
Scientists use a similar process to estimate populations of organisms from bacteria to buffalo.
Scientists count the actual number of organisms in a small sample and then estimate the number of organisms in a larger area. For example, if a scientist wanted to count the number of microorganisms in a petri dish, a microscope could be used to count the number of organisms in a one square millimeter sample. To determine the total population of the culture, the number of organisms in the square millimeter sample is multiplied by the total number of millimeters in the culture.