This annual mostly occurs on xeric to subxeric calcareous sites in Mo. and Kans., where it is widely scattered, plus parts of adjacent states, where it is generally rare to endangered. There are disjunct eastern clusters of records from glades of c. Tenn., barrens of w. Ky., and rocky pastures or roadsides of nc. Ky. (Hill 1982; K). Typical associates include Panicum flexile, Croton monanthogynus, Acalypha rhomboidea, Opuntia cespitosa, Cuphea petiolata, Sedum pulchellum, Isanthus brachiatus etc. In Ky. most records date from before 1970 and the species seems to have been generally reduced due to succession, spread of fescue or various artificial developments. Although usually found on rocky sites in Ky., it is known from alluvial sites further west. A few observations in Ky. suggest that rabbits (or perhaps other herbivores) can often browse plants, and that an insect may often consume seeds. Malvastrum is a widespread, rather weedy genus of wet tropical regions. M. hispidum is anomalous in several characters. It may deserve treatment as the monotypic genus, Sidopsis, with the name S. hispida (Pursh) Rydb. (Hill 1982; p. 169).