Like their much larger vertebrate cousins that feature so prominently in natural history museums everywhere, plants also can become fossilized and offer us a window into the past. Plant fossils are found in one of six broad categories. However, some fossils can fall into multiple categories, or outside of any single category, according to the University of California Museum of Paleontology.

...
Plant fossils can be either two- or three-dimensional.

Compression Fossils

These plant fossils are two-dimensional imprints of plants that have been transformed during the fossilization process and smashed into a flat impression of the plant they once were. These fossils often retain some organic matter. Common examples of compression fossils can be found in coal and peat, which contain many types of accumulated fossil plants.

Impression Fossils

Impression fossils are similar to compression fossils in that they are both two-dimensional, but these fossils are not remnants of the plant itself and do not contain organic material. Impression fossils essentially leave an imprint of the plant material in some fine-grained or soft sediment, such as clay or silt. Once the plant matter decays, the impression remains to be fossilized.

Cast and Mold Fossils

Mold and cast fossils are three-dimensional fossils that sometimes retain some organic material. These fossils are formed when sediment fills in an empty space in the plant (casts) or by surrounding the plant itself before the plant decays (molds). These fossils tend to record outside features of the plant but do not reveal cellular information.

Permineralization Fossils

Permineralization is a process through which the plant material becomes impregnated or permeated by mineral rich liquid before the plant matter decays. The minerals seep into the plant and then harden, forming the three-dimensional fossil. Because the liquid can seep into all parts of the plant, these fossils often yield highly detailed information about the plant's internal structure. Petrified wood is a common example of a permineralization fossil.

Compaction Fossils

Like compression, compaction fossils are reduced volume versions of the plants, though they are three-dimensional and are not generally mineralized. These fossils retain organic material and are often found in peat, lignite coals and soft sediments.

Molecular Fossils

Molecular fossils are the chemical remains of the plant material. Molecular fossils are those that reveal the chemical make-up of the plant and can even contain fossilized DNA and RNA. Mass spectrometry, spectrophotometry and other advanced chromatographic techniques are often employed when studying molecular fossil materials.