Traditional English pottery design.
"Two pigeons flying high,
Chinese vessels sailing by,
Weeping willows hanging o’er
Bridge with three men, if not four,
Chinese Temple, there it stands,
Seems to take up all the land,
Apple trees with apples on
A pretty fence to end my song"
- Mabel C. Bolton
It's a mystery where it came from, but few care, as the design is timeless. The "Willow pattern" pottery was developed in Staffordshire, by one or other of the many potteries of the Five Towns in the late 18th century, about 1780. Thomas Turner, Josiah Spode and Thomas Minton are frequently credited with the development of the theme, based possibly on an old Chinese legend (although many sources say that it is apocryphal). One or two historians make the claim that the story followed the design rather than the other way round.
The story concerns one Tso Ling, a rich Chinese mandarin whose daughter Koong-se, elopes with his secretary (Chang). The father considered Chang unworthy, and the secretary was banished from the household and denied access to Koong-se. Meanwhile she was betrothed to a warrior, against her wishes, and managed to meet with her true love, who had smuggled a message into the garden on a floating shell. They met and escaped, her father in hot pursuit, but the pair are overtaken on a bridge over the river in her father's garden. They do however escape and settle far away, where Chang becomes famous as a writer and poet. His fame contributed to their undoing, as Tso Ling subsequently finds them and has them put to death in their house. The gods take pity on them and both are transformed into immortal birds, leaving them to enjoy their love and freedom forever.
All is Blue
In the same way as the story, the design itself also varies from one pottery to another, but many elements are held in common - the major part of the design (usually in a deep blue) features the river bridge, the fence built around the house, an enormous willow tree, a house in traditional Chinese pagoda style, and the two lovers flying free away from the father. The plate is bordered with many other symbols, ranging from geometric designs, flowers, butterflies and daggers. That the design is English is undoubted, even if based on the Chinese porcelain which was popular at the time. The major differences in the design appears in the borders, being known as the Spode and Mosquito patterns, the Spode having irregular geometric figures, the Mosquit or "Brocade" being more Chinese in style.
Whoever came up with the original design matters little to most people, as it has been popular for over two hundred years, both in the West and the East (Spode designs were exported in great quantity in the nineteenth century). Coals to Newcastle, you may say, and you would be correct. According to Spode's website, Josiah Spode (the most likely candidate for the original conception) developed it from an original Chinese pattern named Mandarin, and other potteries took up the design to cash in on its popularity.
The design is usually carried in a dark blue on a white or pale background (again, a call to much Chinese influence), although many variations are still in circulation, as you can imagine. It is known and loved worldwide, in some areas being referred to as Blue Willow. Furthermore, the essence of the design is used in more modern settings - one plate I saw in Germany depicted a political cartoon, in which people were climbing a wall and being transformed into birds. The house was replaced by a guard tower, and there was a tiny tank in the background. The Berlin Wall is now fallen, and that particular story has ended, happily enough. Just as Chang and Koong-se's story is immortalised in the china cabinets and kitchen dressers of thousands of households.
Thanks to yclept for pointing out information regarding the border designs, and Lometa for her Blue Willow writeup