Saplings are a Decoration Block. In Creative mode, they can be placed freely. Saplings will grow into a full tree if the server has "grow-trees" set to "true" in the "server.properties" file. In Survival, Indev and Infdev, saplings can be obtained from busting leaves. Their process of destroying themselves once the tree has been cut down also causes saplings to fall. The saplings can be replanted to make a new tree, but will only grow if they're on a lit dirt tile and have at least 5 blocks of "headroom" above them.