T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Caltha palustris (Marsh Marigold)

Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
Order: Ranunculales
Family: Ranunculaceae
Genus: Caltha
Species: C. palustris
Binomial name: Caltha palustris
Synonyms: Trollius paluster
Common names: Marsh Marigold, Kingcup

Caltha palustris is a perennial herbaceous plant of the family Ranunculaceae (Buttercups), native to marshes, fens, ditches and wet woodland in temperate regions of the Northern Hemisphere. In the United Kingdom, it is probably one of the most ancient native plants, surviving the glaciations and flourishing after the last retreat of the ice, in a landscape inundated with glacial melt-waters. They grow in shallow water margins or damp soil, forming clumps (> 80 centimetres high) of shiny green heart-shaped leaves. The leaves are rounded to kidney-shaped with two lobes, 3–20 centimetres across, with a bluntly serrated margin and of a thick, waxy texture. The stems are hollow.
The flowers are a bright yellow, 2–5 cm in diameter, with 4-9 (mostly 5) petal-like sepals and many yellow stamens; they appear in early spring to late summer. The flowers secreted nectar from small depressions, one on each side of each carpel. After fertilisation by insects and bees, the carpels form into green sac-like follicles >1 cm long, each opening to release several seeds.

Caltha palustris is a highly polymorphic species, showing continuous and independent variation in many features. The photos below are of a form with double flowers. The common names for this form are 'Double Marsh Marigold' and Double Kingcup 'Flore Pleno'.

As is the case with many members of the family Ranunculaceae, all parts of this plant are poisonous and can be an irritant. Skin rashes and dermatitis have been reported from excessive handling of the plant. It is known to have kill cows. 

Double flower form.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/