Description from Flora of China
Gamosepalum Haussknecht; Meniocus Desvaux; Moenchia Roth (1788), not Ehrhart (1783); Odontarrhena C. A. Meyer; Psilonema C. A. Meyer; Ptilotrichum C. A. Meyer; Takhtajaniella V. E. Avetisian; Triplopetalum E. J. Nyárády.
Herbs annual, biennial, perennial, or rarely subshrubs. Trichomes stellate, stalked or sessile, with 2-6 minute basal branches from which originate up to 30, simple or branched rays, sometimes trichomes lepidote, rarely mixed with simple and forked. Stems erect or decumbent, simple or branched. Basal leaves petiolate or sessile, rosulate or not rosulate, simple, entire. Cauline leaves petiolate or sessile, cuneate or attenuate, not auriculate, entire. Racemes few to many flowered, dense or lax, ebracteate, corymbose or in panicles, elongated or not in fruit. Fruiting pedicels ascending, divaricate, or reflexed. Sepals ovate or oblong, base of lateral pair not saccate. Petals yellow, white, or rarely pink; blade suborbicular, obovate, or spatulate, apex obtuse or emarginate, glabrous or pubescent outside. Stamens 6, tetradynamous; filaments wingless or uni- or bilaterally winged, appendaged or not, toothed or toothless; anthers ovate or oblong, apiculate or not at apex. Nectar glands 4, lateral, 1 on each side of lateral stamen; median glands absent. Ovules 1 or 2(or 4-8) per ovary; placentation apical or parietal. Fruit dehiscent silicles, oblong, ovate, obovate, elliptic, obcordate, or rarely globose, strongly latiseptate or rarely inflated, sessile; valves veinless, pubescent or glabrous, smooth; replum rounded; septum complete, membranous, translucent, veinless; style distinct; stigma capitate, entire. Seeds biseriate, winged or wingless, orbicular or ovate, flattened; seed coat smooth or minutely reticulate, mucilaginous or not when wetted; cotyledons accumbent or incumbent.
In his protologue of Ptilotrichum, Meyer (in Ledebour, Fl. Altaic. 3: 64. 1831) distinguished the genus by having white flowers, edentate filaments, and 1-seeded locules. However, these characters are also found in Alyssum, in many species of which one of the two ovules fails to mature into a seed, while in others the filaments are edentate. As for the flower color, it is definitely unreliable in Alyssum sensu lato, just as is the case in numerous other genera of the family, namely Cardamine, Draba, Lepidium, and Rorippa. In fact, the filament base in most plants of P. canescens, the generic type of Ptilotrichum, produces a small, basal tooth. If one accepts Ptilotrichum as a distinct genus, then at least five of the other segregates of Alyssum (Gamosepalum, Meniocus, Odontarrhena, Psilonema, and Takhtajaniella) should also be recognized, an action that the present authors do not support, especially after examining the genus critically on a worldwide basis.
Although Alyssum fedtschenkoanum N. Busch was suspected in FRPS to occur in W China, the present authors have seen no Chinese material in the numerous herbaria they consulted both inside and outside of China. The species is narrowly endemic to Kazakhstan. It is likely that the plant recorded in FRPS represents a minor variant of the widespread and highly variable A. tortuosum.
The ovule number and placentation are important in the identification of the species and can be easily observed in the fruit.
About 170 species: primarily in SW Asia and SE Europe; ten species in China.