File:Fossil wood (probably Upper Triassic; probably eastern Arizona, USA) (15360188388).jpg

Fossil wood, probably from the Upper Triassic Chinle Formation of eastern Arizona, USA. (24.6 cm across)

Plants are multicellular, photosynthetic eucaryotes. The oldest known land plant body fossils are Silurian in age. Fossil root traces of land plants are known back in the Ordovician. The Devonian was the key time interval during which land plants flourished and Earth experienced its first “greening” of the land. The earliest land plants were small and simple and probably remained close to bodies of water. By the Late Devonian, land plants had evolved large, tree-sized bodies and the first-ever forests appeared.

The fossil shown above is "petrified wood", which is a horrible term for what is technically called permineralization. Biogenic materials such as wood or bone have a fair amount of small-scale porosity. After burial, the porosity of wood or bone can get partially or completely filled up with minerals as groundwater or diagenetic fluids percolate through. The end result is a harder, denser material that retains the original three-dimensionality (or close to it). The wood or bone has become “petrified”. Well, no - it’s become permineralized. Not surprisingly, the most common permineralization mineral is quartz (SiO2). Sometimes, fossil wood and bone have been permineralized with radioactive minerals such as black uraninite (UO2) or yellowish carnotite (K2(UO2)2(VO4)2·3H2O). Recently, fossil bones permineralized with cinnabar have been identified (García-Alix et al., 2013, Lethaia 46: 1-6).