The fruit of the cannon ball tree is so distinctive that, after seeing it for the first time, I could immediately recognize the tree by its fruit. I have no such ability with most of the other trees and plants I encounter in Singapore.
The cannon ball tree, officially known as *Couroupita guianensis*, is not native to Southeast Asia. It is a deciduous tropical tree originally from the lowland rainforests of Central and South America. Because of its fragrant flowers and unusual, large fruits, it has been introduced and cultivated in many tropical parts of the world.
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| A cannon ball tree with flowers and fruit |
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| The left tree has many flowers but no cannon balls, the right tree, the right tree has many cannon balls, scarce flowers. Singapore Botanic Garden |
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| A broken fallen Cannon Ball at Pasir Ris |
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| A normal Cannon Ball Tree |
The mystery begins to unravel
A little online research suggested that there are at least two possible explanations for the difference we observed.
The first is simply different flowering and fruiting cycles.
Tropical trees do not necessarily follow the same seasonal rhythms as temperate trees. Individual trees can have different internal cycles. One tree may be producing flowers while another is developing fruit. Even trees growing side by side, experiencing essentially the same weather and soil conditions, may not be synchronized.
This could explain some of the differences we saw.
But it doesn't completely explain the two cannon ball trees at the Singapore Botanic Gardens. They are right next to each other, yet one repeatedly produces abundant flowers while the other produces abundant fruit.
That raises another possibility: individual differences in reproductive biology.
Perhaps the flowering tree has some characteristic that makes it produce flowers but rarely develop fruit. A genetic difference, reproductive incompatibility, or another biological factor could potentially be involved.
But this is only a hypothesis. Without examining the trees or conducting botanical studies, we cannot know whether genetics—or something else—is responsible.
And that uncertainty made the mystery even more interesting.
What I learned about cannon ball trees
My curiosity about the mystery led me into the fascinating reproductive biology of the cannon ball tree.
Each flower contains both male and female reproductive organs. The flowers have numerous stamens that produce pollen, as well as a central ovary and stigma capable of receiving pollen.
Yet having both reproductive organs in the same flower does not necessarily mean that a tree simply fertilizes itself. Cannon ball trees rely heavily on large bees for pollination. Their flowers even produce different types of pollen: some fertile and used for reproduction, and some apparently serving as nutritious food for visiting insects.
Another fascinating fact is the length of the fruit-development process.
Once pollinated, a cannon ball fruit can take roughly **12 to 18 months** to mature and eventually fall from the tree, and under some conditions it can take even longer.
Suddenly, those cannon balls hanging from the branches looked quite different to me. They were not simply fruits sitting on a tree—they represented a reproductive process that had been unfolding for well over a year.
Curiosity creates mystery
Looking back, I realize that I created this mystery simply by noticing something that most people would probably walk past.
I saw two trees that looked different and asked: Why?
That simple question took me from an ordinary walk in Singapore to tropical botany, pollination, reproductive biology, flowering cycles, and the remarkable life history of a tree from South America.
I still don't know exactly why those two cannon ball trees behave so differently.
And perhaps that's part of the fun. A simple observation turned into an exciting puzzle.
Only when I became curious enough to look deeper did I discover how much wonder was hiding behind something I had walked past many times before.
Curiosity creates mystery.
And sometimes, mystery is the beginning of discovery.
Notes
This post is a polished version by chatGPT (with minor edit) of my original full draft
Here is chatGPT's comment on the original draft:
Absolutely. I think the strongest aspect of this essay is the **curiosity-driven structure**: you notice something unusual, it becomes a mystery, you investigate it, and the investigation leads to a broader appreciation of plant biology. I would make that progression more prominent and reduce some of the technical detail that interrupts the story.
One important point: I would also **soften the Google AI explanation** about the “genetically barren” tree. The essay currently presents that as an explanation, but the evidence you describe doesn't establish that this particular tree is triploid or genetically defective. It is better presented as a hypothesis rather than a conclusion.
I particularly like the ending **“Curiosity creates mystery. And sometimes, mystery is the beginning of discovery.”** It connects nicely with the broader theme that appears in several of your nature essays: you don't just photograph what you see—you **notice something unusual, ask why, and then investigate it**.
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Appendix
1. original full draft
2. chat with Gemini
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Original Draft of the post
We saw the cannon ball trees for the first time at Fort Canning in July 2021.
The tree's fruit, the cannon ball, is so unique and relatable, I am able identify it by name after the first exposure. I have no such ability with all other flora in Singapore.
We see cannon ball trees at many places, Fort Canning, Pasir Ris, West Coast Park, ... It is so common, I did not pay much attention to their flowers and fruit.
Nevertheless I did notice a peculiar phenomenon about the tress early on, after sighting of the trees at a second location, West Coast Park, later in November 2021. What was unique here at west coast park was that, there are many cannon ball trees, and quite a few of them have many cannon balls, compared to none or only one cannon ball from cannon ball trees at Fort Canning area, with many fallen flowers.
I observed the same phenomenon at Singapore Botanic Garden, where two cannon ball trees are right next to each other. One has many cannon balls, some flowers, the other is covered by flowers but no cannon balls.
The mystery deepened for a moment. However we forgot about it afterwards.
Last week, when we walked by the two cannon ball trees again, saw the same two cannon ball trees, one has only flowers, one has a full tree of cannon balls, just like what we observed numerous time before. I wondered aloud to Lily. She challenged me to find out the reason.
I did a bit online research. The following is what I found out.
The Mystery Explained by Google AI
There are two possible (simplified) reasons for what we observed (see Appendix for expanded discussions).
1) Mismatched Timelines - this does not apply to the two SBG next to each cannon ball trees
Unlike temperate trees that bloom simultaneously due to rigid winter-to-spring temperature changes, tropical rainforest trees often follow individual internal clocks. Even when planted side-by-side in identical soil and weather, individual cannonball trees can be completely out of sync with each other. When one finishes fruiting and drops its crop, it will swap phases—quieting down its fruit production and exploding into bloom like its neighbor.
2) SBG Cannon Ball trees: The Left Tree is "Genetically Barren" (Triploid or Defective), The Right Tree is normal
In botanical collections like the Singapore Botanic Gardens, individual trees are sometimes accidental clones or possess minor genetic mutations. The Left Tree: It likely has a chromosomal defect (such as triploidy) or a mutation in its reproductive organs that renders its fertile pollen or ovules completely non-functional. Because it never sets fruit, it avoids the massive metabolic drain of growing heavy cannonballs. It hoards 100% of its nutrients every year exclusively for flower production, turning it into a perpetual blooming machine. The Right Tree: It has perfectly healthy, normal genetics. It easily accepts pollen carried by bees from the blooming left tree, allowing it to stay constantly laden with heavy, growing fruit.
Because of the answers from Google AI, I had more questions about the plant biology related to reproduction, which led to the following secton.
Cannon Ball trees and plant biology
1. Cannon ball trees are bisexual ,meaning each individual flower contains both male and female reproductive organs.
- Male Organs: The flowers feature hundreds of pollen-producing stamens arranged in a unique, hood-like structure.
- Female Organs: The centre of the flower contains a functional fertile ovary and stigma to receive pollen.
- Pollination Strategy: Despite having both sexes in one flower, the tree relies heavily on large bees for cross-pollination. Interestingly, the flower generates two types of pollen: fertile pollen to reproduce and sterile, nutritious pollen to feed the visiting insects. [1, 2]
- It takes 12 to 18 months for a cannonball flower to develop into a mature fruit and naturally drop off from the tree. In some environments, it can take up to 24 months.
2. The vast majority of flowering plants (angiosperms) are bisexual. Approximately 85% to 90% of all flowering plant species worldwide produce perfect, hermaphroditic flowers containing both functional male and female reproductive organs
3. Trees distribute their sexes slightly differently than smaller herbaceous plants. Globally, around 75% of all tree species are bisexual. Trees rely slightly more on wind-pollinated, single-sex configurations (like monoecy found in pines, birches, and walnuts) because their height grants them better access to the wind.
Curiosity Creates Mystery
I created this cannon ball tree mystery because I noticed the obvious differences in some cannon ball trees and wondered about it.
Only when I was interested enough to dig deeper into the reasons, I find wonder.
A simple observation turned into an exciting puzzle!
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2. Chat with Gemini
Prompt 1 - i saw two cannon ball trees which are next to each other, one has many flowers but no cannon ball, the other has many cannon balls, few flowers. what caused this phenomenon, see the photo attached
Google AI reply
1. High Resource Consumption of Fruits
- The Tree on the Right: It has heavily invested its stored nutrients, sugars, and energy into sustaining the crop of large fruits you see hanging from its trunk. Because its energy reserves are maxed out by fruiting, it has minimal resources left to produce new flowers.
- The Tree on the Left: It currently has no fruits draining its energy. As a result, it can channel all its accumulated sugars into a spectacular, high-energy mass-flowering phase. [4]
2. Asynchronous Phenology (Mismatched Timelines)
3. Low Pollination Success Rate
Prompt 2 - The two trees are in Singapore Botanic Garden, I see the same phenomenon, every year, never see the left tree has cannon ball on it except one time I saw one cannon ball; the right tree always have many cannon balls on it, not too many blooming flowers. It does not seem to be Mismatched Timelines.
Google AI reply
1. Late-Acting Self-Incompatibility (LSI)
- Even though the flowers are bisexual, a tree cannot fertilize itself. If a bee transfers pollen from the left tree back onto its own flowers, the tree recognizes its own DNA inside the ovary and actively aborts the fruit in its first month. [1]
- The single cannonball you saw on the left tree was likely a rare fluke where a pollinator managed to bring compatible pollen from a third tree located much further away.
2. Clonal or Mismatched Genetics
- The Right Tree: It likely has a slightly different genetic makeup that allows it to accept pollen from the left tree (or other cannonball trees in the area), converting its blooms into fruit. [2]
- The Left Tree: It is stuck in a genetic "dead end." It cannot accept its own pollen, and it cannot accept its neighbor's pollen. Because it fails to set fruit, it doesn't deplete its energy reserves, allowing it to spend 100% of its resources on massive, continuous flower displays every year. [1]
3. Asymmetric Pollinator Behavior
- When a bee moves from the left tree over to the right tree, it carries a heavy load of the left tree's pollen, easily fertilizing the right tree.
- However, because the right tree has very few flowers, bees rarely fly from the right tree back to the left one. Even if they do, the genetic incompatibility blocks the left tree from forming fruits anyway. [1]
