In the classification above, only the major divisions and classes of living plants are listed, and a number of entirely extinct divisions are omitted. The classification outlined is somewhat conservative but is one that best conforms to available data and has gained wide acceptance.
Biological classifications were initially mechanical or “artificial”; that is to say, they had no basis in evolution. This was followed by a period of “natural system” construction, whereby plants were grouped together on the basis of their overall similarities or differences, using as many characteristics as possible. Contemporary systems of biological classification are phylogenetic, which means that various plants are arranged together because they are thought to be related by descent from a common ancestor. As additional molecular evidence has become available, classifications have changed to accommodate the new information.
At the turn of the 19th century, the plant kingdom was frequently divided into two major groups, the cryptogamia (algae, fungi, bryophytes, and ferns) and the phanerogamia (gymnosperms and angiosperms). Subsequently, it was common practice among systematic botanists to group all vascular plants together under a single division, Tracheophyta.
More modern taxonomies, such as those of the Angiosperm Phylogeny Group (APG), do not formally recognize groupings at the division level but use more informal groups known as clades, a view that interprets the individual major groups to be less closely related to one another than was previously believed. Difficult and complex questions still exist in the definition and circumscription of certain groups. The phylogenetic relationships, if any, of the bryophytic plants with primitive vascular plants remain unclear.