A group of scientists from Stanford extended Turesson's work, from 1930's to 1980's, by crossing populations back and forth. Dr. Anthony Bradshaw and his students in Wales have conducted extensive research on the evolution of heavy metal tolerance in plants, over a time frame of less than a century. When this work was begun in the 1950's the oldest mines were about 100 years old, and known to have surface soils contaminated with very high levels of heavy metals such as iron, copper, zinc and lead. Abandoned strip mines were highly localized environments sometimes only 100 m across. Metal concentrations were as high as 2000 ppm on mine spoils, versus the normal concentration of 10 ppm away from the mine.
Heavy metals cause proteins to precipitate, causing depth.
Only a few plants grew on mine, and bent grass was one of few. Bradshaw developed an assay of metal tolerance in test-tube culture. He placed individual plants in two solutions, containing 20 or 2000 ppm of heavy metal concentration, and compared the growth of grass. This simple assay permitted lots of experimental replication. Plants from off the mine site soon died in high concentrations of heavy metals they had zero tolerance.