Hi All,
Here is a little historical perspective which can explain why so many cacti plants have Cereus in their common name, even though they are not members of the genus Cereus as it is now described. Originally, when Linnean plant names were first being used in 1753, all the cacti known were lumped together in the one genus called Cactus. Fairly quickly though, as more plants were discovered, and botanists realised how varied the group of plants actually was, they started to split them up into separate genera.
Cereus was first described in 1754 and it means torch. The name was chosen to reflect the branched candlestick-like shape of the first Cereus described. This is also why they are sometimes called torch cacti. There have been over 900 species names used within the genus Cereus since 1754, but nowadays fewer than 20 are considered to be valid, and all of theses plants are of the tall spiny branching columnar type of cactus found in the desert and semi-desert areas of Central and South America.
When the first Epiphyllum plants were discovered, some botanists placed them in the genus Cereus because their large white nocturnal flowers are quite similar in appearance to the flowers of the big columnar Cereus plants. The old common names like Night Blooming Cereus and so on all date from this time. The name Epiphyllum was not coined until 1812 when it was first used for Epiphyllum phyllanthus. This particular species was, in fact, known to Linneaus himself and he called it originally Cactus phyllanthus in 1753. This was then changed to Epiphyllum phyllanthus by Haworth in 1812. This name translates roughly as "Epiphytic (Tree Dwelling) Leaf Plant with stems like leaves", which is a pretty good description of an Epi in my book! The long time gap between 1754 and 1812 (over 50 years) was quite long enough to cement the use of "Cereus" in many common names and popular horticultural books of the time.
The other thing which seems to confuse people is the presence or absence of spines. Actually Epiphyllums DO have spines, they are just very small on the mature stems, so we tend to think of them as "spineless". As some of you may know, baby epiphyllums have quite noticeable spines when they are young and do, in fact, look rather remarkably like baby Cereus seedlings! However there are many types of epiphytic (note the lower case "e") cacti which are genuinely spiny - Aporocactus, Monvillea, and Nyctocereus are just three examples.
I hope this helps sort out the confusion,
TTFN, KK.
