They are called dragon fruit because the red scaly leaves that cover the fruit look like the scales of a dragon.
Our white dragon fruit plant is the smaller of the two but when we got it there was a beautiful flower on it which has subsequently been knocked off.
The red dragon fruit plant is bigger and I'm not sure how we're going to train it along that fence yet.
We also broke off a couple of segments from each of the plants and have planted them in the ground to see if they will grow.
When I traveled through Vietnam I ate a lot of the white fleshed dragon fruit. I loved how easy it was to peal and consume and its kiwi fruit like texture and taste. When you can find them in the supermarket they frequently cost around $10/pound which is fairly pricy. I'm hoping that this will develop into a fence of fruit bearing cacti.
I've always associated the dragon fruit with Asia but apparently it's of Mexico, Central and South American origin and was probably taken to the Far East by the Europeans.