The authorities discovered and destroyed more than 1,600 coca plants this week near the Guatemala border, the first such find in Mexico in recent memory. Coca, from which cocaine is produced, is grown and processed primarily in Peru and Colombia, because the climate and soil are thought to be ideal there. Traffickers based in Mexico control much of the flow of the drug to the United States and around the world. But the authorities, drawn Monday to the hidden farm by a tip, were not sure if the coca plants were an experiment or a sign that drug lords are developing fields closer to their markets.