From the delicate new growth that emerges in springtime to jewel-toned fall foliage to winter's brown husks, leaves unfurl, mature and wither in a life cycle that evokes our own. But a tree's leaves and the protective pods that cradle its seeds not only serve as metaphors for our lives: they actually sustain us. Each leaf plays an essential role, gathering nutrients for the tree and producing oxygen for the planet. Josie Iselin celebrates the diversity...