Plants living on toxic, serpentine soils would face significant selection pressure due to the nature of these soils. Only those plants with some kind of mechanism to detoxify the soil around their roots or to somehow tolerate the toxic metals in them, perhaps due to beneficial mutations in genes for these mechanisms, would have been been able to survive and reproduce. These adapted plants would form the next generation, and the progeny of those survivors with better mechanisms for dealing with these soils would survive better than their siblings. This would continue for generations until the tolerance to these soil conditions was an established trait.
Work Step by Step
The key here is to review the concepts behind natural selection and to ask yourself what would be the selector in this environment. Here, it would be the toxic nature of the soil. Since the best adapted organisms are selected to contribute more to the next generation, imagine how plants might be adapted to the soil so that they could survive and reproduce.