Abrupt and gradual speciation are not mutually exclusive. Fossil evidence and biosystematic results are sources of data, not methods to deduce phylogeny. The plant kingdom evolved from a photosynthetic species, that if we had with us today, we would classify as a green alga. There is no perfect solution. Species are equivalent by designation only.
Species are convenient categories to which a name can be attached.
The species category is a flexible, indefinable unit of practical convenience. An alphabetical list of plant names with bibliographical references to the place of first publication. Not all of the names are validly published, most are. It can be searched using any aspect of the plant name, author or title of the publication. No infraspecific taxa prior to 1980.