National Geographic: By now, well into the 21st century, at least some U.S. cars were supposed to be running on an exciting new power source–clean fuels refined from corn husks, timber waste and tall, fast-growing grasses. But the U.S. Environmental Protection Agency, acknowledging that not a single facility is yet producing this advanced “cellulosic” ethanol, has proposed dramatically scaling back a federal program to promote the fuel for the second straight year. Instead of requiring that the …