I enjoyed painting the bottom view of the dragon fruit (DF) yesterday. My watercolor sketch didn’t quite portray the type of spiral symmetry one sees quite clearly in the flower of a sunflower or a pineapple. In all honesty, the symmetry in the DF isn’t that clear. Perhaps I got a defective one.
When I bought the DF, I also purchased the pineapple and something called a ‘Star Fruit’. I never saw or heard of a star fruit before, but it looked interesting. When I posed it for sketching, I was surprised to see that it was a five-sided star. Having been immersed in the hexagonal shapes of the pineapple and the hexagon-looking shapes of the scales on the DF, I must have been thinking, “Star fruit? Oh yes, the Star of David, a six-sided star.”
Here is the sketch:
Before I talk about where a pentagonal shape fits into the whole plant symmetry issue, I’d like to say a few words about the sketch itself.
The colors are fairly accurate. I used thalo yellow green and lemon yellow for the planes of the fruit in the light. The edges of the fruit were nearly uniformly brown, so I used a mixture of yellow ocher and Van Dyke brown, with stronger (perhaps too strong) application on the lower front edge of the fruit. For the green in shadow, I cut the thalo yellow green with black. I used latex mask for the reflective spots, which seemed to work except for the ones in the shadows. I need to tone them down so they don’t look like white spikes.
The first stop on my search for plant symmetry yielded two terms: zygomorphic and actinomorphic. Zygomorphic refers to a flower with one plane of symmetry. In other words, bilaterally symmetric (like the anatomy of a human). Actinomorphic refers to a flower which is radially symmetric, or having several planes of symmetry. I illustrated this with the crude sketch below:
Aha! The star fruit is bilaterally symmetric (if it is proper to apply flower parlance to the fruit). Actually I’m not at all sure this is correct. I have no idea about the mechanics of star fruit emergence. Is the spiral symmetry of a fruit (resulting from the stacking of the hexagons in the pineapple, for example) a consequence of the actinomorphic structure of its flower? I need to do more research to understand this.
Yesterday I speculated that there must be some kind of genetic basis for plant symmetry. While this is way over my head, from what I understood of an article by Wenhen Zhang, et al, floral zygomorphy evolved independently more than 38 times, but they investigated a gene called CYCLOIDEA2 that is responsible for symmetry in a diversity of plants belonging to a certain evolutionary branch.(Wenheng Zhang, Elena M. Kramer, and Charles C. Davis Floral symmetry genes and the origin and maintenance of zygomorphy in a plant-pollinator mutualism PNAS April 6, 2010 vol. 107 no. 14 6388-6393)
The rest is too complex for me, but I’m satisfied knowing that there is a gene somewhere at the bottom of all this. I would like to know more about symmetry of fruit and its relation to the type of plant from which it springs.