Tall woody flowering cactus

Four days after I first noticed the flowers on this tall cactus that I pass every day, I managed to take a decent photo. A day to remember to put my camera in my backpack. The next day I saw that the flowers were in bad light, and I tried to figure out the best time to take the photos. The third day I realized that I had figured wrong. The fourth day was when I got the light reasonably fine. Just goes to show that there’s a big difference between looking and seeing.

Early exposure to a cactus-mad uncle did not help me to identify this one. About all I can tell is that it is not of the genus Rhipsalis, the only Old World genus. It is about 6 to 7 meters tall, and very woody at the base. I think the plant is less than thirty years old, but not much less. This is the first year that I’ve noticed it flowering, so I wouldn’t know whether it flowers once a year, or more often or less. If you look around the base of the tree, you can see green spiny masses beginning to grow well away from the main stem. These are the plantlets sent out by the parent plant as it reproduces vegetatively. I suppose the gardeners are trying to make up their minds about whether they should let this cactus run loose on this patch, or keep it to an individual tree. I suppose that depends partly on how long these trees last.

Part of the reason why I never noticed it flowering in earlier years is that the flowers and fruits are confined to the top of the plant. At eye level and lower the stems are a mixture of succulent green and woody tissue. Cacti are plants in which the stems are photosynthetic. In the photo above you can see that the lower stems are slowly turning woody. They show the typical areole, the small knob from which spines (and also flowers and new stems) emerge. What’s interesting in this plant is that the areole looks like a dry scab, and is not the hairy organ that you see in many other cacti. The habit of the plant, that is to say that it grows as a tree, and the appearance of the areole allows me to rule out many genera of cacti in the identification. The fact that the plant thrives in wet and humid Mumbai should provide useful clues to a better educated person.

I had to wait for the noonday sun to light up the flowers right at the top of the plant. Interestingly they don’t grow right out of the areole, but are borne on a stem that grows out from it. The small yellow flowers were too high up for me to look at them carefully; I don’t even now how many petals they have. It seemed to me that this had a radial symmetry, although I could not count the number of petals. Most cactus flowers are perfect, a botanical term which means that it contains both pistil and stamen. A wonderful description of the flowers of cacti is given in a web page from U Texas, Austin: “Imagine partially inflating a cylindrical toy balloon, then painting on dots to represent the leaf, sepal, petal, stamen and carpel primordia. Once all dots have been painted on with carpel dots at the closed end of the balloon, imagine placing your finger on the end of the balloon then blowing it up further: the balloon will become a hollow cylinder with the stamen dots on the inside next to your finger, the petal and sepal dots at the end and the leaf dots on the outside.”

It was only after I zoomed in with my camera did I realize that the rounded knobs that I’d initially taken for buds were actually the developed fruits of the tree. You can see the complex branching of the flower-bearing stems, and the three-lobed fruits, presumably developing from fused ovaries. That also explained why, when I zoomed in to the flowers, they looked like they had three clusters of petals, with spaces between them. Each cluster seemed to have three petals (or tepals, which is a more correct word for cacti). This is consistent with counts of petals in several other families of plants in the same order, the Caryophyllales.

My morning’s search for the name of this cactus species led me into these strange pathways. At the end of it I’m left in this very unsatisfactory state: the tree is a cactus (family Cactaceae) from the Americas. Can anyone help me with a better identification?