Why do only some leaves turn red in the fall? Scientists can't agree on an answer

This is the time of year when Nicole Hughes gets constantly distracted by the fall foliage as she drives around North Carolina.

"I'm always looking," says Hughes, a biologist at High Point University. "You know, just seeing what's turned colors when, and what hasn't turned."

Part of her brain is endlessly searching for clues about one of the biggest mysteries of autumn leaves: why the leaves of only some trees turn red.

"We still don't really know why some species turn red and others don't," she says.

She's made a career out of studying this, even though her interest in red leaves goes back even further. Growing up, she says, "my dad was always talking about his quest for the perfect red leaf."

Every autumn, trees break down the green chlorophyll in their leaves, so that they can