Medical marijuana is now legal in 20 states, and legislation is pending in 13 others. It's become a $1.5-billion-a-year industry, and it's expected to triple in just a few years. With legal cannabis one of the world's fastest growing market sectors, investors are seeing green. Everyone from mom-and-pop operations to venture capitalists are rushing to get in on the ground floor, Bruce Barcott tells NPR's Rachel Martin. Barcott is a journalist who covers the cannabis industry in Washington and Colorado, the two states that have legalized recreational marijuana.