Not to be semantically confused with : the Genus
Colutea [plants belonging to the Bladder-senna Genus (in the Pea family Fabaceae) such as Bladder Senna itself].
Not to be semantically confused with : Coronopus species (Swine-Cresses) [plants with similar scientific name] which have similar leaves and vaguely similar stem, but the flowers are not on long stalks nor are they like Tansy (Tanacetum vulgare). Nor with Buck's-horn Plantain (Plantago coronopus) which also has similar leaves and similar scientific name.
Not to be semantically confused with : Cottonweed (Achillea maritima) [which has a similar sounding name and belongs to the same Dandelion & Daisy Family (Asteraceae)]
Unlike most other plants belonging to the Dandelion & Daisy Family (Asteraceae) the stems are succulent and 'telescopic', the leaves forked a little like those of Oxford Ragwort, the flowerheads very similar to those of Tansy but the florets have 4 rather than 5 'petals', the sepals behind the flowerhead very similar to those of Corn Marigold, the florets arranged in Fibonacci spirals like those of some Asteraceae and some Brassicaceae (such as
The disc of florets is not un-like those of the ray-less Tansy (Tanacetum vulgare), but with several important differences. Firstly, the tiny petals number four rather than the usual five. Secondly, those petals do not splay out like those of normal disc florets, but curve inwards.
The plant is salt-tolerant and grows both in damp places and in brackish water, often near the sea but dislikes the shade and any frost. It is a neophyte not native to the UK but to Southern Africa from where it has been introduced to many parts of the world. In the USA it is a problem plant, less so in the UK since it does not yet occupy many hectads. Commonly grown in gardens it is an annual which will re-grow from its own seeds readily.
It can be confused with other Cotula species such as Leptinella (Cotula squalida, but that has Tansy type leaves) and with another 11 species. However the above photos are definitely Buttonweed itself on account of the succulent stems that this specimen possesses, although not every specimen of Buttonweed (as in Cotula coronipifolia) possesses succulent stems. But because this specimen does, it can be no other Cotula species (according to Prof. Clive Stace's key for Cotulas).
Buttonweed is dioecious, with male and female flowers on separate plants, but not all Cotula species are. The sex of the above plant(s) is not known to your Author.
Has it got maybe 4 petals or zero petals? Well, it has no ray florets (aka petals), only tubular florets, and moreover the tubular florets have but 4 lobes rather than the far more prevalent 5.
CONSTITUENTS OF BUTTONWEED
The plant is aromatic and contains about 57 components in the essential oil, the most important compound detected in the leaves being the spiro-sesqi-terpenoid
Agarospirol at 10.4% , 1-Eicosanol (aka Arachidyl Alcohol, CH3(CH2)38OH) at 17% in the stem and Hexacosane (CH3(CH2)24CH3) at 31% in the flowers. The most abundant oils obtained from the roots were Heptacosane (CH3(CH2)25CH3) (28%), 1-Eicosanol (15%) Octacosane ( (CH3(CH2)26CH3)) (5%) and the terpene 4-Amorphene at 5% which has similarities to the terpenoid
Cadinene, all of which are solids at 20°C and probably not volatile so your Author suspects some other more volatile substances are responsible for the aromatic odour.
Agarospirol (aka Ephinesol) was only the second spiro-terpenoid to be found in nature. It is a sesquiterpene alcohol which also occurs in Agarwood, hence its name. It possesses neuroleptic properties affecting the central nervous system. Agarwood was used as a sedative in oriental medicine. It is a spiro-vetivane.
Two new alkaloids have also been found in Buttonweed, one an O-glycosidic compound called Cotuzine but Cotuzine A and Cotuzine B are mentioned as the two in scientific literature (however, your Author can only find the formula for Cotuzine and he does not know if it is really called Cotuzine A or Cotuzine B), which are based on the BenzoAzocine skeleton, and a new polyphenolic substance, Corimen, for which your Author can find no structural formula.