5. Crocanthemum Spach, Ann. Sci. Nat., Bot., sér. 2. 6: 370. 1836.
Frostweed, rushrose, rockrose [Greek krokos, saffron, and anthemon, flower, alluding to petal color] Frostweed, rushrose, rockrose [Greek krokos, saffron, and anthemon, flower, alluding to petal color]
Bruce A. Sorrie
Helianthemum Miller subg. Lecheoides (Dunal) Reiche
Herbs, perennial, or subshrubs, 0.4–8 dm. <Stems not dimorphic, hairy, usually some hairs (chasmogamous), 1–20 per capsule (cleistogamous). x = 10.
Species 21 (15 in the flora): North America, Mexico, West Indies (Dominican Republic), Central America, South America.
Crocanthemum was created in 1836 to accommodate the New World species of Helianthemum, but most authors have not recognized the distinction. Studies by J. Arrington (2004) and B. Guzmán and P. Vargas (2009) showed that traditional Helianthemum is not monophyletic. Helianthemum is now a genus of some 80 species restricted to the Old World, and Crocanthemum is a New World genus most closely related to Hudsonia. Differences in leaf arrangement, pollen type, style architecture, and funicle and embryo shape all serve to distinguish the two genera. In particular, the style base in New World species is broad (broader than the apex) and confluent with the outline of the ovary, whereas in Old World species the style is tapered from apex to the narrower base. The style in New World species is short and straight, but long and curved in Old World species. In New World species, leaves are alternate and estipulate; in Old World species leaves are mostly opposite and stipulate. Regarding the apparent close relationship between Crocanthemum and Hudsonia, Guzmán and Vargas noted that these two genera have unique pollen types, and that both differ from Old World Helianthemum. More complete sampling of New World taxa may be necessary to resolve the issue of whether Crocanthemum is distinct from Hudsonia.
A prominent feature of Crocanthemum is production of dimorphic flowers in all but the Californian species. Showy, chasmogamous flowers are produced early in the growing season, followed one to three months later by apetalous, cleistogamous flowers (simultaneous in C. georgianum and C. glomeratum). Flowering dates cited are for chasmogamous flowers.
All species of Crocanthemum are fire-tolerant, readily resprouting from the woody caudex.
SELECTED REFERENCE Daoud, H. S. and R. L. Wilbur. 1965. A revision of the North American species of Helianthemum (Cistaceae). Rhodora 67: 63–82; 201–216; 255–312.