Plant fossils can be difficult to identify since small details can distinguish two different species of plant. Finding plant fossils in the first place is difficult since plants do not have body parts that tend to remain preserved, unlike the skeletal remains of humans and other animals. Still, examining plant fossils is crucial to understanding how plant species evolved.
Plant fossils can be found in the form of plant grasses etched into limestone when the grasses pressed against the limestone. Fossils can be found in microscopic pollen granules, and as petrified wood that has been preserved in silica when the water washes the silica into the pores of the wood. Plants that fall into the lake and float to the bottom sometimes form carbon films and impressions that can be taken as fossils. Some of the oldest plant fossils, which were ferns, are 300 million years old.
Identifying the fossil of the plant sometimes does not require any sophisticated equipment other than a hand lens. For example, if a conifer fossil is found, the cells will be elongated and will tend to be smoother. Angiosperm plant cells will be made up of vessels and fibers.
The body structure and functions of the plant must be identified in the fossil remains in order to determine the species. By studying the plant structure, the examiner can identify the phylum of the plant, and can then narrow the plant down to the class, order, family, genus and finally the exact species. By narrowing down the different classifications of the plant, not only does the plant not need to be compared to species of plants that it is not related to---which saves time---but the identifier can at least get closer to the type of plant if the final species cannot be identified. For example, the plant might be identified as a fern but the archeologist might not be able to determine the exact species of fern.
Laboratory equipment can identify a plant species by examining the DNA of a plant cell. This is accomplished by placing the DNA in a buffer solution and sending electrical impulses through the solution so that the nucleotides become excited and move through the pores of the buffer solution. Those nucleotides that move more quickly through the pores in the buffer solution are smaller. Comparing the sizes of the nucleotides to those of an existing plant species can narrow down the species of the plant fossil.
Another way to identify plant fossils is by using an x-ray to examine chemical differentiation in fossilized plant cell walls. Radiation is sent through the plant walls on to a film on the other side of the plant. The films then create images that give archaeologists a sense of the chemical changes that occurred in the fossilized plant.