Widespread loss of plant species and crop varieties could undermine agricultural productivity, according to a new report by the Worldwatch Institute, which found that more than 80 percent of seed varieties sold in the U.S. a century ago are no longer available. The report says that 29 percent of plant species in the U.S. are threatened, more species in total than in any other country, but the problem is worldwide in scope. Fifty years ago, Chinese farmers were estimated to have been growing 10,000 varieties of wheat, but by the 1970s that number had fallen to about 1,000. In Mexico, farmers are now growing only 20 percent of the corn varieties cultivated in the 1930s. Worldwide, more than 30,000 plant species are threatened.