Rosa foliolosa Nuttall ex Torrey and Gray 1840
This species is so rare in cultivation I know of only two Botanic gardens where it can be seen. The Lady Bird Johnson Wildflower Center at Austin where it is part of the native plants of Texas and at the Santa Barbara Botanic Garden in California. Rosa foliolosa has a limited distribution in northern Texas, Oklahoma and barely into Arkansas and Kansas. It is the only native North American species with white flowers hence it’s called the “White Prairie Rose.” What passes for Rosa foliolosa in commerce is actually a tall growing Rosa palustris form or hybrid with intense red flowers, narrow pointed leaflets and most tellingly, curled stipules. Distributed by Hilliers of Westminster, England, its a beautiful rose to which I’ve given the study name “Hilliers foliolosa” to distinguish it from the true species.
I’ve been growing the “Hilliers foliolosa” for many years believing it to be true. But I also grow Rosa palustris whose most distinctive character is curled stipules at the base of each leaflet. I had noticed “Hilliers foliolosa” also has curled stipules and was mystified. Rosa palustris is a species of Eastern North Anerica and far separated from the White Prairie Rose but I assumed somehow they were related.
Serendipitiously the mystery was finally solved in 2015 by a misidentified species growing in the Santa Barbara Botanic Garden. In 2014 a friend sent me photographs of an unidentified but unique species with white flowers and short, densely colonizing growth habit planted in the Santa Barbara Botanic Garden. I contacted the staff and asked if they had any identity. They replied it was Rosa spithamea and the source was Lester Rowntree’s garden in Carmel Highlands! I was astonished by this identity because other than prickly hips and low growth habit there was no other resemblance between the two roses.
In August of 2014 I visited the SBBG to see the rose and was allowed to take root divisions. It was definitely something new unlike any species I’d seen or read about before. The arroyo where it grew at SBBG was pretty dry in late August but the rose was thriving quite well, even blooming. I inventoried all its characters, low growing with few prickles and glossy elliptic to obovate leaves, very short flower pedicels, prickly hips and most spectacularly pure white flowers. I researched and pondered. Then in January, 2015 Vern Yadon, Director Emeritus of the Pacific Grove Museum of Natural History alerted me to the publication of Volume 9, Rosa in the Flora of North America. I obtained a copy and saw the entry for “White Prairie Rose”, Rosa foliolosa Nuttall. The description matched exactly the SBBG species! How I missed not knowing we had a white flowered North American rose species but there it was. Ironically, at the HelpMeFind website, the photos at the entry for Rosa foliolosa are of the red flowered “Hilliers foliolosa” but if one bothered to read the “Reference” entries, it's plainly made clear they’re white!
Scroll through the photos to the Cass post of a Garden and Forest illustration of Rosa foliolosa with curled stipules showing this error in naming goes back at least to 1890.

Note: An expanded version of these comments was published in the January, 2016 issue of Gold Coast Roses, Edited by Jeri Jennings and titled: “The Rose Prize of Santa Barbara Botanic Garden and Rosa Adventures Along the Central Coast” by Don Gers

The Southwestern Naturalist, Vol. 3, No. 1/4 (1958), pp. 151-152
A Monograph of the Genus Rosa in North America. II. R. foliolosa
Walter H. Lewis

Although the mitotic leaf chromosomes from only one plant were studied in detail, several plates from this specimen showed the following karyotype (Figure 3):

1 pair long median chromosomes (aa)
1 pair long subterminal chromosomes (cc)
2 pairs medium median chromosomes (dd, dd)
1 pair medium submedian chromosomes (ee)
1 pair medium subterminal chromosomes (ff)
1 pair short median chromosomes (gg).

This morphology is similar to that for one form of R. Woodsii Lindl. except that--as observed here--one pair of long chromosomes has a median centromere in R. foliolosa and the corresponding pair in R. Woodsii has a submedian centromere. The chromosome morphology of R. foliolosa is also similar to a second diploid species, R. palustris Marsh. The latter has a long submedian pair (bb) and two pairs of medium subterminal chromosomes (ee, ee), while R. foliolosa has a long median pair (aa) and only one pair of medium subterminal chromosomes (ee).

p. 153
From the data presented in Table 1 and the descriptive summary of the characters of R. foliolosa, it is apparent (for this sample studied) that the individuals exhibit an extensive degree of homomorphism. There is no doubt, however, that intraspecific variation does exist. Petal colour from white to pink, peculiar modifications from galls, as well as others mentioned in the species description, are known examples. Regardless, the populations of this taxon are more consistent for both morphological and cytological characters than are those of most rose species. Its limited range and its geographical isolation from diploid Cinnamomeae species (except a partial distribution overlap with R. Woodsii Lindl.) have undoubtedly contributed to this unique phenomenon in the North American Rosa.