In the mountains of Brazil, Barbacenia momentosa and Barbacenia macrantha_--two plants in the Velloziaceae family--establish themselves on soiless, nutrient-poor patches of quartzite rock. Plant ecologists Anna Abrahao and Patricia de Britto Costa used microscopic analysis to determine that the roots of B. tomentosa and B. macrantha, which grow directly into the quartzite, have clusters of fine hairs near the root tip; further analysis indicated that these hairs secrete both malic and citric acids. The researchers hypothesize that the plants depend on dissolving underlying rock with these acids, as the process not only creates channels for continued growth but also releases phosphates that provide the vital nutrient phosphorus.

Which finding, if true, would most directly support the researchers' hypothesis?