In 1944, M.L. Bowerman published a flora of Mount Diablo, an isolated mountain in central Contra Costa County, California, that includes several endemic or near-endemic species. This exemplary work, done under the tutelage of W.L. Jepson, set the standard for regional floras, incorporating extensive discussions of ecological factors, plant communities, and phytogeographic analyses. In 1995, the Jepson Herbarium took on the challenge of updating the flora, which had long been out of print. This undertaking has provided an excellent case study in the dynamic nature of floristic knowledge, well beyond the expectations of any participant. In addition to nomenclatural updates of previously included taxa (40%), 23% of the currently included taxa are additions, an unexpectedly high percentage for such a well-done flora. Of the additions, over half represent native taxa, but the percentage of non-native taxa has nevertheless increased from 14% to 21% of the total flora. One, Aira caryophyllea var. cupaniana , is apparently a first record for North America. On the other hand, several taxa were deleted or have apparently disappeared, including one presumed to be globally extinct (Eriogonum truncatum). The absolutely essential nature of Bowerman’s original vouchers was particularly underscored, with numerous changes resulting from taxonomic “splits”, circumscriptional shifts, reinterpretations, and outright misidentifications. Potentially undescribed taxa are also involved. These changes can be interpreted as taxonomic and floristic hypotheses, which have been tested and modified in the face of new evidence. Keys have also been reworked to maximize their usefulness (e.g., avoiding reliance on underground parts, which the average user will not have permission to examine in a state park).

Key words: Aira caryophyllea var. cupaniana, California, flora, floristic hypotheses, Mount Diablo, vouchers