Cotula is a genus of flowering plant in the sunflower family.[4][2] It includes plants known generally as water buttons or buttonweeds.
The species within this genus can vary extensively in their habit, leaf division, involucre, receptacle and achenes. This makes it difficult to define them by comparing their morphology. The genus can only be defined by looking at the corollas of their flowers. Most are disciform (lacking ray florets). These corollas may be tubular, reduced or even absent. Another characteristic is their solitary heads growing on a peduncle.
Cotula is the largest genus found in the Southern Hemisphere of the tribe Anthemideae. This genus was first mentioned by Carl Linnaeus, who described four species in his first edition (1753) of Species Plantarum. In 1867 the genus was subdivided by George Bentham[5] into three sections. Since his account, only a few changes have been made but the number of species has remained more or less stable. The sections possess different basic chromosome numbers :
section Cotula : largest section with about 40 species; mostly in South Africa, a few in North Africa and Australia the cosmopolitan species C. coronopifolia and the widespread species C. turbinata; this section also includes the former genera Cenia and Otochlamys; basic chromosome numbers x = 8 and x = 10.
section Strongylosperma (Less.) Benth.: a total of eight species, found in warmer parts of Africa and Asia (often lumped as C. anthemoides), Central and South America (C. mexicana) and Australia (five species, including C. australis); basic chromosome number : x = 18
