President José Mujica has quietly signed into law the government’s plan to create a regulated, legal market for marijuana, the president’s spokesman said Tuesday. The presidential secretary Diego Canepa said Mr. Mujica signed the legislation on Monday night. That was the last formal step for the law to take effect. Officials now have until April 9 to write the fine print for regulating every aspect of the marijuana market, from growing to selling in a network of pharmacies. They hope to have the whole system in place by the middle of next year. But as of Tuesday, growing marijuana at home was legal, up to six plants per family and an annual harvest of 480 grams, or about one pound.