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 sa id 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. To determine the total population of the culture, the number of organisms in the square-centimeter sample is multiplied by the total number of square centimeters in the culture.