129. Tripleurospermum Schultz-Bipontinus, Tanaceteen. 31. 1844.
Mayweed [Greek tri-, three-, pleuro-, ribbed, and sperma, seed, alluding to strongly 3-ribbed cypselae]
Annuals, biennials, or perennials, 5–80 cm (taprooted; scentless or nearly so). Stems 1–5+, usually erect or ascending, sometimes procumbent, branched or simple, glabrous or sparsely hairy (hairs basifixed). Leaves (not marcescent) basal (usually withering by flowering) and cauline; petiolate or sessile; blades oblong, 1–3-pinnately lobed, ultimate margins crenate or serrate, faces glabrous or sparsely hairy. Heads radiate [discoid or disciform], borne singly or in corymbiform arrays. Involucres hemispheric to patelliform, 8–12+ mm diam. Phyllaries persistent, 28–60+ in 2–5 series, distinct, broadly ovate, unequal to subequal, margins and apices (pale to dark brown or black) narrowly to widely scarious (abaxial faces glabrous). Receptacles convex to conic (± solid), epaleate. Ray florets  10–34+, pistillate, fertile; corollas white [seldom pinkish], laminae mostly oblong. Disc florets 300–500, bisexual, fertile; corollas yellow [greenish], tubes cylindric (sessile-glandular), throats narrowly campanulate (cylindric or compressed), lobes 5, deltate (with resin sacs). Cypselae trigonous, ± compressed (apices truncate), ribs 3–5 (0–2 abaxial, 2 lateral, 1 adaxial, usually whitish, relatively thick, smooth), faces often rugose or tuberculate abaxially and between ribs and glabrous (pericarps sometimes with myxogenic cells and usually 2–3, sometimes 1–5, abaxial-apical resin sacs; embryo sac development tetrasporic); pappi 0 [coroniform or each an adaxial auricle]. x = 9.
Species ca. 40 (2 in the flora): North America, Eurasia, North Africa; introduced in New Zealand; one species widespread as a weed.
Tripleurospermum often has been included within Matricaria. It is distinguished from the latter by 3-ribbed cypselae with 2 abaxial-apical resin sacs versus 5-ribbed cypselae without apical sacs, and by production of 7-glycoside flavonols versus 3-glycosides (K. Bremer and C. J. Humphries 1993). C. Oberprieler (2001) showed that Tripleurospermum is closely related to Anthemis in the strict sense and not to Matricaria. This is supported by the tetrasporic embryo sac shared by Tripleurospermum and Anthemis (versus monosporic in Matricaria and other anthemid genera), similarity of karyotype, and the presence in both genera of matricaria ester in the polyacetylene pathway, otherwise unknown in the tribe.
Although W. L. Applequist (2002) favored a single species, Tripleurospermum maritimum, for the three taxa (as subspecies) present in North America, based in large parts on the findings of A. Vaarama (1953) on crossing ability among these taxa, on the existence of hybrid populations, and on the (mistaken) supposition that they form a polyploid series, some recent authors (e.g., Q. O. N. Kay 1976b, 1994; A. R. Clapham et al. 1987; E. G. Voss 1972–1996, vol. 3) have maintained two species based on morphologic characters, notably perennial versus annual habit, spacing of ribs (closer versus wider), elongate versus ± circular resin sacs on cypselae, relative infrequency of hybrid populations, and relative sterility of some hybrid individuals, as well as both species harboring diploid and tetraploid populations (Clapham et al.; Kay 1994). Success in interbreeding among plants does not always indicate conspecific status. In North America, authors such as M. L. Fernald (1950) and H. A. Gleason and A. Cronquist (1991) treated (in Matricaria) T. inodorum and T. maritimum subsp. maritimum as a single species with varieties or as a single entity, resulting in confusion about the distinctness of the two taxa in their ranges of introduction, and also difficulty in recognizing the presence of T. inodorum in areas where T. maritimum subsp. maritimum is present in coastal northeastern United States (e.g., D. W. Magee and H. E. Ahles 1999). N. N. Tzvelev (2002b) segregated the subspecies of T. maritimum as species; under his scheme, three species are present in North America: T. inodorum, T. maritimum in the strict sense, and T. hookeri (= T. maritimum subsp. phaeocephalum). Pending further taxonomic work on this complex, I have here retained the taxonomy (except generic assignment) adopted by Kay (1976b), with two species, one including two subspecies.