Final answer:
The mark and recapture method is the best experimental method to measure both the size and density of a plant population.
Step-by-step explanation:
The experimental method that can best provide data on both the size and density of a plant population is mark and recapture. This method involves marking a sample of captured plants in some way and then releasing them back into the environment. Later, researchers collect a new sample, including some marked plants (recaptures) and some unmarked plants, to estimate the population size and density.