A certain cultivated herb is one of a group of closely related plants that thrive in soil with high concentrations of metals that are toxic to most other plants  Agronomists studying the growth of this herb have discovered that it produces large amounts of histidine, an amino acid that, in test-tube solutions, renders these metals chemically inert
Hence, the herb's high histidine production must be the key feature that allows it to grow in metal-rich soils
Whether the herb's high level of histidine production is associated with an unusually low level of production of some other amino acid