This remarkable cactus goes by the common name of Queen of the night or Orchid cactus. They have large flowers, roughly 10 to 15 centimetres across that bloom only after sundown and wilt by dawn.
The cactus belongs to the genus Epiphyllum, a name meaning "upon the leaf" because the flowers are sent out from the edge of the plant's "leaves" (They are actually stems). The most common species are Epiphyllum hookeri, Epiphyllum strictum and Epiphyllum stenopetalum. The first name was given by Adrian H Haworth in homage to Sir William Hooker, the first botanist to record the plant, albeit incorrectly, as Cactus phyllanthus.
Native to areas south of Mexico, including Guatemala, Costa Rica, Guyana and Venezuela, the cactus has found renown around the world as an ornamental plant. Thus the original white and yellow petalled flowers now have many colourful variants due to hybridization with related genera, such as Selenicereus, Hylocereus, Heliocereus and Nopalxochia.
Apart from the obvious ornamental and nocturnal charms, the flowers are said to be a treatment for Bradycardia (slow heart beat) and in Japan the petals are coated in tempura batter, then deep fried.
If you ever come across one of these plants, grab a small cutting of the stem and plant it at home, as they are very easy to propagate from a cutting. I did it and I can kill parsley!
Cuttings and further information can be had at http://www.epiuniverse.com/