File:Fossil wood without tree rings (49019752401).jpg

(public display, Sternberg Museum of Natural History, Hays, Kansas, USA)

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.

Seen here is a cut slice of "petrified wood". While buried in sediments, the wood had its microscopic porosity filled with minerals as groundwater percolated through. This fossilization process is technically known as permineralization ("petrified" is a horrible term - never use it in a scientific context). Permineralization can also occur with fossil bones. The most common permineralization mineral is quartz (~pure silica - SiO2), which is the most common mineral in Earth's crust.

From exhibit signage:

A growing tree is like a tape recorder that tracks climate season by season, year after year. During harsh winters or very dry seasons, a tree stops growing and a ring is formed. Trees like this fossil that do not show rings grow in climates with mild temperatures year round.