Each leaf lasts up to four years and often has a dark, waxy skin. Some trees, such as horse chestnuts, have big, showy flowers, but the flower of oaks and ashes are so tiny you would hardly notice them. Trees pollinated by animals tend to have large flowers to attract the animals. Most trees pollinated by the wind have much smaller flowers. Many wind-pollinated trees have long clusters of flowers called catkins.
Catkins have almost no petals, so the pollen sacs on the anthers are exposed to the wind.
So too are the sticky stigmas, ready to catch pollen carried to them from the anthers by the slightest breeze. Once the flowers are pollinated, the seed grows inside a fruit, which can be soft like a plum or hard like a nut. Elms have seeds like flying saucers, maples have seeds with wings and willows have seeds like parachutes. Inside soft fruits there is usually a hard seed. When a bird eats the fruit, the seed inside will pass right through the bird unharmed, ready to grow wherever it is dropped.