Habit and leaf form. Shrubs. Plants succulent (leaves succulent); unarmed. Leaves cauline (ass.). Plants with neither basal nor terminal concentrations of leaves. Stems not ‘jointed’. Stem internodes solid (ass.). Leaves minute to large; alternate; spiral, or distichous; fleshy; petiolate to sessile; simple; epulvinate. Leaf blades entire; solid; semi-terete (usually); linear, or obovate; attenuate at the base, or cuneate at the base. Leaves without stipules. Leaf blade margins entire. Leaves without a persistent basal meristem. Leaf anatomy. Hydathodes present, or absent. Hairs absent (some axillary pubescence). Extra-floral nectaries absent (ass.).
Reproductive type, pollination. Fertile flowers hermaphrodite. Unisexual flowers absent. Plants hermaphrodite.
Inflorescence and flower features. Flowers solitary; axillary; sessile; ebracteate; ebracteolate (ass.); minute, or small; regular; cyclic. Hypogynous disk present, or absent. Perianth sepaline; 3–5; 1 -whorled; joined (imbricate); fleshy, or non-fleshy; persistent; accrescent, or non-accrescent. Calyx present; not replaced by accrescent bracteoles; 3–5; gamosepalous (ass.); blunt-lobed; imbricate; tubular (and sometimes unguiculate); fleshy, or non-fleshy; persistent (in the fruit); accrescent. The fruiting calyx not berrylike; wingless, spineless and without tubercles. Corolla absent. Androecial members definite in number. Androecium 3–5. Androecial members free of the perianth, or adnate (to the base of the perianth); all equal (ass.); free of one another (ass.); 1 -whorled. Androecium exclusively of fertile stamens. Stamens 3–5; all more or less similar in shape (ass.); isomerous with the perianth; alternisepalous. Anthers bent inwards in bud; dehiscing via longitudinal slits; tetrasporangiate. Gynoecium (2–)5 carpelled. The pistil 1 celled. Gynoecium syncarpous; synovarious to synstylovarious; superior. Ovary unilocular; 1 locular; sessile. Gynoecium stylate. Styles 2–3; partially joined. Placentation basal. Ovules in the single cavity 1; pendulous, or ascending; non-arillate; campylotropous.
Fruit and seed features. Fruit non-fleshy (pericarp thin, membranous, brittle above; fruiting perianth may be succulent); indehiscent; capsular-indehiscent, or a nut; enclosed in the fleshy perianth, or without fleshy investment; 1 celled. Gynoecia of adjoining flowers combining to form a multiple fruit, or not forming a multiple fruit. Fruit 1 seeded. Seeds more or less non-endospermic. Perisperm present, or absent. Cotyledons 2. Embryo curved.
Etymology. After Caleb Threlkeld (1676–1728), English botanist, published an account of the Irish flora.