Caltha palustris
Caltha palustris commonly known as Kingcup or Marsh Marigold (also known as Calthus palustris syn. Trollius paluster Krause) is a member of the family Ranunculaceae, and is native to temperate regions of the Northern Hemisphere (Europe including Iceland and Arctic Russia, temperate and Arctic Asia, and North America).  It grows in wet, boggy places, such as marshes, fens, ditches and wet woods. It becomes most luxuriant in partial shade, but is rare on peat. In the UK, it is probably one of the most ancient British native plants, surviving the glaciations and flourishing after the last retreat of the ice, in a landscape inundated with glacial meltwaters.
It is a herbaceous perennial plant growing to 80 cm tall. The leaves are rounded to kidney-shaped, 3-20 cm across, with a bluntly serrated margin and a thick, waxy texture. Stems are hollow. The flowers are yellow, 2-5 cm diameter, with 4-9 (mostly 5) petaloid sepals and many yellow stamens; they are borne in early spring to late summer. The flowers are visited by a great variety of insects for pollen and for the nectar secreted from small depressions, one on each side of each carpel. Carpels form into green sac-like follicles to 1cm long, each opening to release several seeds.
Caltha palustris is a highly polymorphic species, showing continuous and independent variation in many features. Forms in the UK may be divided into two subspecies: Caltha palustris subsp. palustris, and Caltha palustris subsp. minor.
It is sometimes considered a weed in clayey garden soils, where every piece of its root will survive and spread. In warm free-draining soils, it simply dies away.
As is the case with many members of the Ranunculaceae, all parts of the plant can be irritant or poisonous. Skin rashes and dermatitis have been reported from excessive handling of the plant.

# Other names and etymology
In the UK, Caltha palustris is known by a variety of common names, varying by geographical region.  These include Marsh Marigold and Kingcup (the two most frequently-used common names), Mayflower, May Blobs, Mollyblobs, Pollyblobs, Horse Blob, Water Blobs, Water Bubbles, Gollins and the Publican. The common name of marigold refers to its use in Churches in medieval times at Easter time as a tribute to the Virgin Mary, as in Mary gold.  The specific name palustris, from Latin "of a marsh", indicates its common habitat.
Richard Mabey, in his magisterial Flora Britannica, describes Caltha palustris thus:
"Marsh-marigolds are in decline as agricultural land continues to be drained, but they are still the most three-dimensional of plants, their fleshy leaves and shiny petals impervious to wind and snow, and standing in sharp relief against the tousled brown of frostbitten grasses. Most of the plant's surviving local names - water-blobs, molly-blobs, water-bubbles - reflect this solidity, especially the splendid, rotund 'the publican' from Lancashire."
In North America Caltha palustris is also known as Cowslip. This should not be confused with Primula veris, the original plant to go by that name.
Caltha palustris is a plant commonly mentioned in literature, including Shakespeare:
Kingcup Cottage by Racey Helps is a children's book which features the plant.

# Subspecies, varieties and cultivars
The 2006-2007 edition of the RHS (Royal Horticultural Society) Plant Finder, a British publication which lists over 70,000 plants available in nurseries in the UK, lists the following:
- Caltha palustris (awarded the RHS's Award of Garden Merit)
- Caltha palustris var. alba
- Caltha palustris var. barthei
- Caltha palustris 'Flore Pleno' (a double-flowered cultivar, awarded the RHS's Award of Garden Merit)
- Caltha palustris var. himalensis
- Caltha palustris 'Marilyn'
- Caltha palustris 'Multiplex' (double flowered)
- Caltha palustris var. palustris
- Caltha palustris var. palustris 'Plena' (double flowered)
- Caltha palustris subsp. polypetala
- Caltha palustris var. radicans
- Caltha palustris 'Semiplena' (double flowered)
- Caltha palustris Trotter's form
- Caltha palustris 'Yellow Giant'