Yellow primroses in Hidden Valley Regional Park, Nevada

Beautiful yellow primroses were flowering this May at the base of the slopes—just above the washes—in Hidden Valley Regional Park, east of Reno in Nevada.The flowers come in clusters, each flower with four petals. The stamens are yellowish white and somewhat shorter than the pistil, which is often quite long with a bulbous stigma (knob) at the tip (see the flowers at the upper left side of the cluster in the picture). Some basal leaves with red spots can be seen in the backgound, close to the sandy and dry soil. 

In Laird Blackwell's Wildflower Guide [1], I found a picture and description of the yellow club-fruited evening-primrose (Camissonia claviformis) of the evening-primrose family (Onagraceae), which closely matches the Hidden Valley flowers. But other sources are associating names such as browneyes or brown-eyed evening-primrose with the species Camissonia claviformis. Further, they include plants with white flowers [2,3]. Despite differences in common names and flower color, the bulb-tipped, female reproductive organe is always there and sometimes becomes the main focus [4].  

The yellow-flower-clustered primrose, shown above, is not the only evening-primrose growing in the Hidden Valley: hiking a few steps upslope, the tufted evening-primrose (Oenothera caespitosa) with large pinkish white flowers can be found: in May, Hidden Valley is primrose paradise!

References and more 
[1] Laird R. Blackwell: Tahoe Wildflowers • A Month-by-Month Guide to Wildflowers in the Tahoe Basin and Surrounding Areas. A Falcon Guide, Morris Book Publishing, LLC, 2007; page 35.
[2] CalPhotos: calphotos.berkeley.edu/cgi/img_query?query_src=photos_index&where-taxon=Camissonia+claviformis.
[3] CalFlora: Camissonia claviformis (Torrey & Fremont) Raven [www.calflora.org/cgi-bin/species_query.cgi?where-calrecnum=1410].
[4] Camissonia claviformis (Brown-eyed Primrose) [www.stanford.edu/~rawlings/kengif/camissonia.html].