Description from Flora of China
Malva rhombifolia (Linnaeus) E. H. L. Krause; Sida alba Cavanilles (1785), not Linnaeus (1763); S. insularis Hatusima; S. rhombifolia subsp. insularis (Hatusima) Hatusima; S. rhombifolia var. rhomboidea (Roxburgh ex Fleming) Masters; S. rhomboidea Roxburgh ex Fleming.
Subshrubs erect or prostrate, many branched, to ca. 1 m tall. Branchlets stellate. Stipules spinelike, 3-5 mm; petiole 2-5(-8) mm, stellate puberulent; leaf blade rhombic to oblong-lanceolate or obovate, rarely linear-lanceolate, 1-4.5 × 0.6-2 cm, abaxially gray-white stellate pilose, adaxially sparsely stellate pilose to subglabrous, base broadly cuneate, margin dentate, apex obtuse to acute. Flowers solitary, axillary. Pedicel 1-2.5 cm, densely stellate tomentose, articulate above middle. Calyx cup-shaped, 4-5 mm, abaxially stellate pubescent, lobes triangular, apices acute. Corolla ca. 1 cm in diam.; petals yellow, obovate, ca. 8 mm, base attenuate, apex rounded. Filament tube 4-5 mm, glabrous. Style branches 8-10. Fruit semiglobose to broadly turbinate, 6-7 mm in diam.; mericarps 7-10, 2.5-3 mm excluding awn, shallowly grooved to near base, eventually dehiscent, side walls usually thin, not veined, stellate puberulent, apex usually (1 or)2-awned, awns to 1.5 mm. Seeds reniform, ca. 2 mm, blackish. Fl. autumn-winter.
The entire plant is used medicinally.
Sida rhombifolia is the type species of the genus Sida. The taxonomy of this species/species complex is controversial and is discussed by Verdcourt (Kew Bull. 59: 233-239. 2005). The lectotype of S. rhombifolia almost certainly came from Jamaica, not India as suggested by some authors, and is a match with neither African nor Asian material. Verdcourt recognized six varieties for East African material, differing most obviously in mericarp morphology: most notably the degree of dehiscence, which varies from completely indehiscent to dehiscing by an apical slit to breaking into two valves, the degree of sculpturing, and the presence or not of awns. Chinese material appears to have a comparable range of variation, though many collections lack fully mature mericarps, and more detailed studies could lead to the recognition of more, comparable taxa.
Hu (Fl. China, Malvaceae [Fam. 153], 20-21. 1955) recognized two varieties, var. rhombifolia and var. corynocarpa (Wallich ex Masters) S. Y. Hu (Fl. China, Malvaceae [Fam. 153], 20. 1955), differing only in minor quantitative characters. The validity of var. corynocarpa is open to question, as Masters (Fl. Brit. India 1: 324. 1874) only mentioned the supposed basionym "Sida corynocarpa" in a note under S. rhombifolia var. retusa and therefore cannot be said to have accepted it as a species. Plants with prostrate stems from Taiwan, Japan (Ryukyu Islands), and the Philippines have been placed in S. rhombifolia subsp. insularis. Their status needs more detailed investigation. Borssum Waalkes included S. alnifolia Linnaeus within S. rhombifolia as var. retusa.
Hu 12784, from Hong Kong, has 10 indehiscent mericarps per flower, each with a single awn, suggesting a relationship to Sida rhombifolia var. maderensis (Lowe) Lowe (S. maderensis Lowe; S. unicornis Marais). That taxon has rather more strongly sculptured mericarps than Hu 12784, which might represent a distinct taxon.
Scrub, open slopes, streamsides. Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hubei, Sichuan, Taiwan, Yunnan [Bhutan, Cambodia, India, Laos, Nepal, Thailand, Vietnam; pantropical].