Plant fossils are rare compared to fossils of bones, teeth, and shells. The soft tissues of leaves are usually destroyed long before fossilization can take place. Only when conditions are just right can leaves be preserved. The Goldilocks conditions needed are pretty simple. In order to fossilize, the freshly fallen leaf needs an undisturbed place with little/no oxygen. It could be buried in a landslide or fall to the bottom of a lake.
Some fossils of tree roots have also been found. One of the well known type are the stigmaria, which are the roots of extinct lycopod trees. Some of these trees date back to over 400 million years!
Fossils of wood (called Petrified Wood) on the other hand, are a bit easier to happen, because it takes more time for wood to break down. When this process happens, the wood becomes harder and turns into a substance similar to stone. Petrified wood has been found worldwide, on every continent.