Learn about the plants united by one common theme–we lack the necessary information to assign them to one of the other ranks or to reject them.

Using the RPI's advanced search allows you to unlock even more capabilities! List/Rank, Location, Biology, and more.

Over one-third of California’s flora is considered rare. Learn expert tips to search the RPI to prepare a list of rare plant species for a project.

It sounds mysterious and magical, like an ancient tome you would check out of the library. Where does it come from?