Madrean region

Crossosomataceae, Garryaceae, Lennoaceae, Limnanthaceae and Stegnospermataceae have their principal development here; for Onagraceae, Polemoniaceae and Hydrophyllaceae it is the major center of diversity.

Species endemism is also moderate (about 25%), but is much more considerable in such genera as Astragalus, Eriogonum, Penstemon, Cymopterus, Lomatium, Cryptantha, Chrysothamnus, Erigeron, Phacelia, Castilleja, and Gilia.

The vegetation in the central Great Basin shrub steppe part of the province is dominated by Artemisia species and Chenopodiaceae genera.

More than 50 genera (e.g. Adenostoma, Bergerocactus, Carpenteria, Cneoridium, Dendromecon, Fremontodendron, Jepsonia, Lyonothamnus, Neostapfia, Odontostomum, Ornithostaphylos, Pickeringia, Romneya) are endemic or near-endemic as well.

Arctostaphylos, Brodiaeinae, Calochortus, Caulanthus, Streptanthus, Ceanothus, Cryptantha, Downingia, Dudleya, Eritrichieae, Eriogonoideae, Gileae, Hydrophyllaceae, Limnanthaceae, Lotus, Madiinae, Mimulus, Onagreae, Epilobieae, Orcuttieae, Eschscholzioideae, Platystemonoideae, Astragalus and Cupressus have a principal center of diversity within the province.