Mostly the physics of leaves, not their molecular biology. I learned many interesting things - that you can float using a wet pillow-case, but not a dry one, why your gas mileage goes down so quickly with increased speed, and a review of the peculiarities of water (the way it adheres to itself, the way its density changes with temperature) that permits life as we know it. Also, why you have to use a dish cloth when you clean dishes; the velocity of a viscous fluid at the luminal surface is zero.