The term fossil, from the Latin fossilis, means "something dug up." Fossils are the remains of ancient organisms - both plants and animals - preserved in a form close to their original shape.
Fossil plants are commonly preserved in three ways: compressions or impressions, casts or molds, and petrifactions.Compressions and impressions: These occur frequently with leaves. The remains of twigs and leaves fell into water, became soaked, settled to the bottom and were covered by sediments. Overlying sediments accumulated, and their weight forced out the air and water, leaving only a thin layer of carbon that preserved the shape and structure of the plant.
Casts and molds: A mold forms in the rock when a cavity is left by decay of a stem, leaf, root or seed. A cast results when this cavity is filled, often by mud or sand. No organic material or cellular structure remains in a cast, but the shape is preserved.
Petrifactions: These are mineralized plant remains that result when wood or other structure is preserved by a deposition of mineral material such as silica, pyrite or calcium carbonate along the cell walls. The foreign material permeates and encloses the organic remains and, thus, often retains all the structural details of the original plant material.
These types of fossils are important botanically because they often reveal the internal structure of the plant. They can also be very colorful, due to various oxides of iron and manganese. In some particularly spectacular samples, the wood has been replaced by precious minerals, such as opal.
Others: In some parts of the word, plant fragments have been preserved in calcareous nodules called coal balls and in amber or fossilized resin. And ash that has fallen from volcanoes has entombed some plant samples.