by Grant Bayley,  Auckland

Hylocereus and Selenicereus, like Epiphyllum hookeri and E. oxypetalum all have these features in common:

1. All have flowers that last 12 hours or less – night flowering
2. Generally self incompatible for pollination
3. Pollen is mushy wet-ish with a short life
4. Ripe seed enveloped in a spongy clear sac like tadpoles eggs
5. Seeds are small and bean-like
6. Strong tendency to be a climber, creeper or vine
7. There is no juvenile growth stage as the seedling develops
8. Each cotyledon size can grow to 15mm long.

This last feature (the cotyledons or seed leaves), is critical in identifying plants and it seems it hasn’t been considered with classification of this group of cacti. As you know, plants are classified as monocotyledons or dicotyledons – all cacti are dicotyledonous plants. So consider the distinctions between cotyledons as an aid for classification - their size, shape, growth and then look at other factors. The nature of the seed leaves probably reflects evolutionary relationships, and is a fundamental feature of plants. It should therefore be useful for sorting out plant relationships.

Based on the current information, Anderson’s book "The Cactus Family" (2001), a number of plants are grouped under Epiphyllum, apparently more on growth form than anything else. But another group of plants, now grouped together, even though they look so different, are now classified under the Disocactus genus, namely Nopaloxchia, Heliocereus, Aporocactus, Disocactus, etc (ranging from the very spiny to spine free, from the cylindrical to flat stems, and a wide range of flower sizes and shapes) - Besides their ability to interbreed and have flowers open for around three days, they also have identical cotyledons. Also Epiphyllum crenatum and E. cooperi (see note below) fit comfortably here as they hybridize with Disocactus species and hybrids. These plants all have a juvenile stage – some seem never to move from this stage.

If we use the cotyledon criteria for Epiphyllum spp we have two distinct groups.

One group is identical to Disocacti and this clearly encompasses E.crenatum, E. cooperi.

The second group, Hylocereus and Selenicereus which clearly would include E. oxypetalum, E. hookeri..

How and why Disocactus spp were grouped in Anderson’s book is not clear. It was not by their cotyledons, but because they hybridized. The cotyledons are clearly the consistent and distinct feature. A hybrid between E. crenatum and a Selenicereus was believed to have created E. cooperii, but that could not have occurred as these species are totally incompatible.

So to move forward, I ask have E. oxypetalum or E. hookeri been crossed with Hylocereus or Selenicereus? These should be highly probable for producing hybrids, given their similarities.

To confirm my hypothesis that Hylocereus and Selenicereus, and some Epiphyllum species could be classified under the same genus, please keep an eye out for seedlings growing from a cross of an Epiphyllum (E. oypetalum or E hookeri etc) and a Hylocereus or/and Selenicereus species – it would be a hybrid between these night flowering cacti of the rainforest!.

Note