The herbarium of the California Academy of Sciences (CAS) is the largest collection of vascular plants in the western U.S. It is the sixth largest collection in the United States. Together with the Herbarium of the University of California at Berkeley (UC) the San Francisco Bay area is regarded as a National Resource Center for systematic botany. These two major collections have an informal agreement to avoid duplication, thus providing botanists with a rich and varied resource for research.
The CAS collection includes approximately 2 million plant specimens. More than 95% of the specimens are vascular seed plants; the remainder are ferns and a growing collection of bryophytes. There are more than 11,000 types (holotypes, isotypes, syntypes, lectotypes, and neotypes) housed separately from the general collection.
The collection was built largely as the result of research collections by the curators and staff of CAS and the Dudley Herbarium (DS) and augmented by purchases, exchanges, and bequests made over the years. The collection is strongest in vascular plants from North and Latin America (particularly California and western and southern Mexico) and the Galapagos Islands, as well as important collections from other areas of the world. Some vascular plant families and genera that are especially well represented are Acanthaceae, Brassicaceae, Carex, Eriogonum, Hydrophyllaceae, Lupinus, Madinae (Asteraceae), Malvaceae, Melastomataceae, Onagraceae, Penstemon, Poaceae, Polemoniaceae, and Quercus. The herbarium also includes the largest collection of ornamental plants in California.
The main focus for growth presently involves the acquisition of plants from Latin America (particularly Mexico, Central America, and northern South America), Eastern Asia (particularly China), and Western North America (particularly California, Nevada, and Arizona).