Annuals or perennials [biennials, subshrubs]; not scapose; trichomes sessile, stellate, with 2–6 minute basal branches (branches as many as 3–25), rays branched or not, sometimes trichomes simple [lepidote]. Stems erect, ascending, or decumbent, unbranched or branched. Leaves basal and cauline; petiolate or sessile; basal rosulate or not, petiolate or sessile, blade margins entire; cauline petiolate or sessile, blade (base cuneate or attenuate), margins entire. Racemes (few- to several-flowered, sometimes corymbose or paniculate). Fruiting pedicels ascending, divaricate, or reflexed, slender or stout. Flowers: sepals ovate or oblong, lateral pair not saccate; petals yellow or white [rarely pink], suborbicular, spatulate, oblanceolate, linear-oblanceolate, or, obovate (apex obtuse or emarginate); stamens tetradynamous; filaments not winged, uni- or bilaterally winged, appendaged, or toothed; anthers ovate or oblong; nectar glands (4), 1 on each side of lateral stamen, median glands absent; (placentation apical or parietal). Fruits sessile, ovate-oblong, obovate, or elliptic [obcordate, rarely globose], usually strongly flattened, latiseptate, rarely inflated; valves each not veined (smooth), pubescent or glabrous; replum (visible), rounded; septum complete, (membranous, translucent, veinless); ovules 1 or 2 [or 4–8] per ovary; stigma capitate. Seeds biseriate or aseriate, flattened, winged or not, orbicular or suborbicular to ovoid; seed coat (smooth or minutely reticulate), mucilaginous or not when wetted; cotyledons accumbent or incumbent.

Distribution

North America, se Europe, Asia, n Africa.

Discussion

Species ca. 170 (6 in the flora).

Alyssum has five introduced and one native species in North America. It is taxonomically difficult and is centered in Turkey and adjacent countries. For the determination of most species, both flowers and mature fruits are needed. Alyssum has been split into nine or more segregates; the segregates are based on the presence of staminal appendages, petal color, and number of ovules per ovary. In the absence of thorough molecular studies on Alyssum and its immediate relatives, it is more practical to delimit the genus broadly, as done here.

Selected References

Dudley, T. R. 1964. Studies in Alyssum: Near Eastern representatives and their allies, I. J. Arnold Arbor. 45: 57–100.

Dudley, T. R. 1964b. Synopsis of the genus Alyssum. J. Arnold Arbor. 45: 358–373.

Dudley, T. R. 1965. Studies in Alyssum: Near Eastern representatives and their allies, II. Section Meniocus and section Psilonema. J. Arnold Arbor. 46: 181–217.

Dudley, T. R. 1966. Ornamental madworts (Alyssum) and the correct name of the goldentuft alyssum. Arnoldia (Jamaica Plain) 26: 33–48.

Dudley, T. R. 1968. Alyssum (Cruciferae) introduced in North America. Rhodora 70: 298–300.

Lower Taxa

Alyssum alyssoides, Alyssum desertorum, Alyssum murale, Alyssum obovatum, Alyssum simplex, Alyssum szowitsianum

Key

1	Perennials; ovules 1 or 2 per ovary	> 2
1	Annuals; ovules 2 per ovary	> 3
2	Cauline leaf blades broadly oblanceolate, obovate-spatulate, or obovate; stems 0.7-1.5(-2) dm; ovules (1 or) 2 per ovary; seeds not winged or margined, 1-1.7 mm.	Alyssum obovatum
2	Cauline leaf blades narrowly oblanceolate to linear; stems (2.5-)3-6(-7) dm; ovules 1 per ovary; seeds broadly winged, 3-3.8 mm.	Alyssum murale
3	Fruits usually glabrous, rarely sparsely pubescent (when young).	Alyssum desertorum
3	Fruits pubescent throughout	> 4
4	Sepals persistent; filaments not appendaged, toothed, or winged (slender).	Alyssum alyssoides
4	Sepals caducous; filaments at least some appendaged, toothed, or winged (expanded basally)	> 5
5	Fruits orbicular; fruiting pedicels divaricate.	Alyssum simplex
5	Fruits ovate-oblong; fruiting pedicels ascending to suberect.	Alyssum szowitsianum