7. Tanacetum
L. (tansy)
Plants perennial
herbs (annual elsewhere), sometimes with rhizomes, weakly to more commonly
strongly aromatic, glabrous or sparsely to densely hairy. Stems erect or
ascending, branched or unbranched, finely ridged. Leaves alternate and basal
(basal leaves sometimes withered by flowering time), sessile or short- to
long-petiolate, sometimes with slightly broadened, more or less clasping bases.
Leaf blades unlobed and bluntly toothed or deeply 1–2 times pinnately
compound (sometimes only deeply many-lobed) with the leaflets deeply lobed and
the ultimate segments sharply toothed and/or lobed, sharply pointed at the tip,
hairy and/or glandular to nearly glabrous, 1- to 5-veined. Inflorescences
panicles at the stem tips, these flat-topped or dome-shaped, bractless or with
a few reduced leaves toward the base. Heads radiate or appearing discoid.
Involucre cup-shaped to broadly cup-shaped, the bracts more or less in
2–5 overlapping series, the outer ones somewhat shorter,
oblong-lanceolate to narrowly elliptic, rounded to bluntly or sharply pointed
at the tip, sparsely to moderately hairy, tan to brown, sometimes with a green
or brown midvein or subapical area, the midrib sometimes keeled, the margins
and often also the tip thin and papery, somewhat irregular. Receptacle flat to
somewhat convex at flowering, not conspicuously elongating at fruiting, solid,
naked. Ray florets (when present) 10–20 or rarely more, pistillate,
persistent, becoming reflexed after flowering, white (sometimes yellow when
marginal florets are pistillate, not markedly enlarged, and only slightly
zygomorphic), sometimes pinkish-tinged. Disc florets perfect (the marginal ones
sometimes pistillate), numerous, the corolla yellow, minutely glandular, the 5
lobes without resin canals, persistent, the tube often somewhat flattened
toward the tip, becoming swollen at fruiting. Pappus absent or more commonly a
very short collar or crown. Fruits oblong-obovoid to slightly wedge-shaped in
profile, sharply 5-angled or nearly circular in cross-section, not flattened,
truncate at the base, the tip often slightly obliquely truncate, strongly
5-ribbed to slightly 10-ribbed, the ribs slender, smooth, the surface otherwise
sparsely glandular, tan to brown. About 160 species, North America, Europe,
Asia, Africa, introduced widely.
The taxonomic
circumscription of Tanacetum remains controversial, especially the
inclusion of species formerly classified into the genus Chrysanthemum.
The genus is quite heterogeneous morphologically. For example, as presently
circumscribed, the genus contains a large number of species with white to pink
ray florets that formerly were segregated in the genus Pyrethrum (Zinn)
Rchb. f., whereas Tanacetum in the strict sense has been characterized
variously as having discoid heads or radiate heads with yellow rays (Bremer and
Humphries, 1993).
Several,
including all of the escaped Missouri taxa, are cultivated as garden
ornamentals. The genus also contains a number of species used medicinally.
Heads of the eastern European ornamental species T. cineariifolium
(Trev.) Sch. Bip. are the primary natural source of the group of monoterpenes
called pyrethrins. These compounds are the active ingredient in some tick
repellents and also have a long history of other uses in insecticides.
Synthetic derivatives of pyrethrins are known as pyrethroids.