When a leaf falls off a tree what is left inside the leaf? Is it just a crisp pile of nothing?

It takes red light to make chlorophyll. The leaves absorb red and reflect green. When the red light dims in autumn, the leaves stop making chlorophyll and cellulose, and then they are some other color and they die. There are a lot of differences between leaf dropping trees and evergreen trees. Deciduous trees produce seeds inside fruits or nuts so they are called (Greek word) angiosperm. That means "clothed seed". Evergreens have needles, not leaves, and their seeds are naked, so they are called (Greek word) gymnosperm. Evergreens grow in cold areas, deciduous trees grow in areas that have distinct seasons. Before the dinosaurs were wiped out, the world grew mostly gymnosperms: evergreens and ferns. After the extinction, the world grows mostly angiosperms: deciduous trees and flowers.