Common name. Alyssum. Family Brassicaceae.
Habit and leaf form. Small, hoary-tomentose shrubs, or herbs. Annual. Leaves cauline. Plants with neither basal nor terminal concentrations of leaves. Stem internodes solid. To 0.2–0.3 m high. Leaves small; alternate; spiral; ‘herbaceous’; petiolate to sessile; non-sheathing; without marked odour; simple; epulvinate. Leaf blades entire; oblong to obovate (to oblanceolate), or linear; one-veined, or pinnately veined. Leaves without stipules. Leaf blade margins entire. Leaves without a persistent basal meristem. Leaf anatomy. Hairs present; glandular hairs absent; complex hairs present. Complex hairs stellate. Extra-floral nectaries absent.
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite.
Inflorescence and flower features. Flowers aggregated in ‘inflorescences’; in corymbs, or in racemes. The terminal inflorescence unit racemose. Inflorescences terminal. Flowers pedicellate; ebracteate; ebracteolate; minute to small; regular; 2 merous; cyclic. Floral receptacle with neither androphore nor gynophore. Free hypanthium absent. Hypogynous disk present. Perianth with distinct calyx and corolla; 8; 3 -whorled (K 2+2, C 4). Calyx present; 4; 2 -whorled; polysepalous; erect; decussate; regular; persistent. Corolla present; 4; 1 -whorled; alternating with the calyx; polypetalous; imbricate, or contorted; regular; white, or yellow. Petals clawed. Corolla members bilobed, or entire. Androecial members definite in number. Androecium 6. Androecial members branched (in that the inner whorl of 4 is derived from only 2 primordia); free of the perianth; markedly unequal; free of one another; 2 -whorled (2+4). Androecium exclusively of fertile stamens. Stamens 6; tetradynamous; all more or less similar in shape; hypogynous, on the receptacle. Filaments (at least some of them) appendiculate. Anthers basifixed; non-versatile; dehiscing via longitudinal slits; introrse; unilocular to bilocular; tetrasporangiate; unappendaged. Pollen shed as single grains. Gynoecium 2 carpelled. The pistil 2 celled. Gynoecium syncarpous; eu-syncarpous; superior. Ovary plurilocular; 2 locular. Locules secondarily divided by ‘false septa’. Gynoecium transverse. Ovary sessile. Gynoecium stylate. Styles 1; apical. Stigmas 1–2 (1 2); commissural; 1–2 - lobed; capitate (or emarginate). Placentation parietal. Ovules 2–10 per locule; with ventral raphe; non-arillate; anatropous.
Fruit and seed features. Fruit 4–7 mm long; non-fleshy; dehiscent; a silicula (circular or oval, latiseptate). Capsules valvular. Fruit 2 celled; (4–)8–12(–20) seeded. Seeds (2–)4–6(–12) per locule. Seed rows per locule 2. Seeds scantily endospermic, or non-endospermic; not mucous; compressed; small to medium sized; winged. Embryo well differentiated. Cotyledons 2; accumbent. Embryo bent.
Physiology, biochemistry. Mustard-oils present.
Special features. Fruit body with no clear differentiation into valve and beak regions. Replum present and complete; broad. Fruit bilaterally compressed; compressed parallel with the septum. The inner (lateral) pair of sepals not noticeably saccate. Petals not peculiarly elongated as in Stenopetalum. Nectariferous glands lateral only. Valves of the fruit neither winged nor keeled. Fruit not apically notched.
Etymology. From the Greek for "curing madness or hydrophobia"; hence alysson, the plant madwort, which was supposed to have the power of allaying rage or frenzy and also to be an antidote to rabies; used in the latter sense also by Pliny for Asperugo procumbens.