A plаnt prоduces trichоmes оn its leаves. Plаnts vary in their trichome production, and the variation is genetically based. The researchers create genetically identical plants (clones) from one of the individuals in the population. They assign the clones randomly to one of four herbivory treatments: none, low, moderate, or high damage. They allow the plants to grow for 3 months following damage and then measure the number of trichomes per square centimeter on newly produced leaves. Their results are shown below. What best accounts for the difference in trichome densities under the different herbivory levels?