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. Thus no varieties, no subspecies named before 1980 are listed. One of the primary goals of the taxonomist is to keep classifications stable.
Taxonomists think carefully before making changes, e.g., uniting two genera into one.
We weigh the desire for stability against the need for change. The use of subspecies, variety and form has changed over the years. Thus it is impossible to use these terms in a consistent fashion. To these workers subspecies and variety are synonyms in a biological sense. Thus, there can be subspecies without varieties.