Species name: Pereskia bleo
Common name: rose cactus, leaf cactus
Location: Dominican Republic
This species of rose cactus (there are 17 species in the genus, all of which carry the common name "rose cactus") is native to the Caribbean basin, from Panama south to Colombia. It has been spread throughout Central America and the Caribbean Islands as an ornamental species, and it is now incredibly popular in gardens and as landscaping plants around hotels. I saw this plant at the farm we went to, and weren't given much information (if any; I was a straggler at the back of the group because of my compulsive picture-taking) about it. It's unfortunate; the people living there probably would know more about the plant than even Wikipedia does!
Surprisingly enough, this plant is a true cactus. You don't have to tell me it looks nothing like what you would picture to be a "cactus" because I most certainly agree with you. Unfortunately, we would both be wrong no matter how adamant we were about this not being a cactus; DNA sequence data tells us it is in the cactus family. Ah, well. Better luck next time to us! :) The genus can be split into two groups called clades, where each clade is a group of species that are most closely related to each other than they are to any other member of the other clade. Each clade has a very interesting characteristic that can be used to define members of that clade: geographic distribution. All members of clade A, where you find the species Pereskia bleo, are native to the land around the Caribbean Sea, from Mexico south to Venezuela (and also some species native to Hispaniola, the island that is made up of Haiti and the Dominican Republic, as well as Cuba, Jamaica, and the Dutch Antilles). Clade B, on the other hand, is made up of species native to South America near the Amazon basin in Brazil, Uruguay, Argentina, Bolivia, Paraguay, and Peru. The one exception to this rule is in clade B where we find the species P. aculeata which is native to a huge area from Mexico all the way south to Argentina. One thing that all of these species have in common is that they don't tolerate full sun very well; they much prefer to be found in shady areas. If exposed to full sun they will more than likely lose all their leaves and turn into a bush of sticks. This definitely won't kill the plant; the stems are green and they would be able to manufacture sugars using the energy of the sun quite easily (photosynthesis). They won't, however, flower or be anything fancy to look at. Best to plant it where it receives at least shading from a building or tree.
In Singapore, where it was introduced a few centuries ago, it is a very important medicinal plant in their traditional medicine, and the plant is called the "Seven Star Needle." The leaves are either chewed (if young) or brewed into a tea (if older) and consumed. It is believed that the regular consumption of these leaves will prevent cancer, but there is absolutely no scientific evidence to support these claims. I doubt the plant is toxic, as the people who make these claims would likely be dead from chronic poisoning if this were true, but don't start eating the leaves in an attempt to ward off cancer. The only evidence that this might one day be used to fight cancer is in mammalian cell lines; extracts from the leaves do kill cancer cells and prevent their proliferation. A word of caution before you get to excited: cell lines in culture behave very differently than what cells do in the body, so the fact that this has been demonstrated to be true has no weight with the medical community. Once (if) there is a long-term full-scale human trial with leaf extract, then the results might be meaningful to the human medical field.