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I'm developing a tree that has a peculiar fruit. Instead of having water in said fruit, there is nothing but sugar. The tree pumps sugar into a hollow orb which slowly "bakes," caramelizing into an amber shell of stained-glass sugar. This caramel shell fruit contains tiny seeds. What's more, to protect this fruit from dissolving or getting soft or tacky, the tree coats it with a super thin layer of waterproof, edible, tasteless wax. Sometimes, though, when the fruit falls and isn't eaten/smashed, it explodes violently, throwing shards of sugar-glass through the air, as it impacts with the ground (or after lying there for a while.

Info

The reason for the explosion is because the same heat that the tree generates for caramelizing the sugar into a single shell is sometimes expelled in waves through the tree roots because the tree needs to cool down. The now cool "fruit" suddenly gets a blast of intense, fiery heat, causing it to explode (hence, "sugar-bomb").

Question

How would the tree

  • catalyze the heat and expel it (Edit: to clarify, expel it through its roots)
  • the tree generate that level of heat? (I already made the tree have black leaves for maximum heat/sun absorption: anything else the tree could do?)

In accordance with this meta post, please do not use magic as an answer. Try not to use "it's handwavium/impossible/magic" answers. I've set my premise (caramelized sugar fruit, ignoring water/uselessness).


Thank you to all in the Sandbox who helped me develop this question.

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    $\begingroup$ As everyone has said, plant/wood material can't get that hot so you need another approach! :O $\endgroup$ – Fattie Jun 20 '18 at 15:19
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    $\begingroup$ Note to Downvoters (and VTC Casters): Please leave a detailed comment outlining why you are downvoting(or VTCing) this question and leave a suggestion showing how you think this issue can be fixed. Thank you for being considerate. $\endgroup$ – FoxElemental Jun 20 '18 at 17:21
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    $\begingroup$ Instead of having water in said fruit, there is nothing but sugar Water is fundamental to organic chemistry as we know it. I don't know of any biochemistry that could do what you want without water to allow cells and the organism as a whole to function. $\endgroup$ – StephenG Jun 20 '18 at 17:44
  • $\begingroup$ What exactly does the plant get out of caramelizing its sugars? That is quite a difficult evolutionary task for it to serve no purpose. $\endgroup$ – John Jun 21 '18 at 14:24
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    $\begingroup$ What liquids and fluids are involved is fundamental to the question of how this reaction could happen (if at all). If you handwave these things you may as well remove the "science-based" tag because you're not remotely science-based anymore. $\endgroup$ – StephenG Jun 22 '18 at 1:19
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Imagine if the tree had a symbiotic relationship with yeast. A developing fruit would have a rind with a semipermeable membrane that performs reverse osmosis on fermenting sugar water within the rind. The alcohol content in the outer chamber of the rind would rise as the fruit developed eventually becoming highly flammable. Eventually, an outer transparent skin on the rind would develop fluid blisters that would act like magnifying glasses focusing sunlight onto the highly flammable alcohol layer. When the developing fruit finally bursts into flame, the outer rind would burn off forming a caramelized shell around the sugar center.

After the mature fruit falls yeast in the fruit's stem would kickstart fermentation in the core of the fruit (separate from the flesh of the fruit...which is of course pure sugar). Alcohol would accumulate in the core in a similar fashion to the rind, but the heat generated by the fermentation process would accumulate in the highly insulated fruit and could eventually result in a nice ethanol explosion.

Now while the tree itself has evolved to be fairly flame resistant to catching on fire, the periodic exposure to brief bursts of extreme heat from its fruit means the tree needs a way to shunt excess heat in its leaves and branches for brief periods of time to avoid bursting into an alcohol-fueled inferno when its fruit ripens. To facilitate this the tree has heat sensitive cells that cause the capillaries in the roots to contract pushing cool pure water from underground up into the leaves and causing the fermented sap in the leaves to be forced back down the tree to the outer skin of the roots and into surface nodules in the root system. These root nodules further cool the tree via evaporative cooling as the alcohol in the sap evaporates through the skin of the nodules.

While the rapid cooling mechanism employed by the root system does work to quickly cool the tree the alcohol vapors released by the root nodules means that occasionally a spark from a ripening fruit will drop into the vapor cloud surrounding the roots catching the vapors on fire. This has been known to light the stems of fallen fruit in the vicinity of the roots which works like a wick causing the alcohol reservoir in the fruit's core to light on fire and explode.

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  • $\begingroup$ I added a bit on how you can get brief bursts of heat from the roots that help cool the tree down :) $\endgroup$ – user3389672 Jun 21 '18 at 0:21
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Caramelization of fructose requires A minimum temperature of 110°C and a perfectly uniform distribution of the heat, so the fruit would bake itself the moment the process started. Even worse, if it started baking from the outside, for thermal energy would be lost before it started cooking the sugar lump inside. You'd have a fruit-flambé with a barely caramelized lump inside. And the tree should be made of asbestos: Imagine all those hanging fruits taking fire one after the other, yikes!

Not to mention that this GMO tree would have the less efficient way to spread its seeds: the idea of the fruit falling, rotting rapidly or being devoured is that one way or the other the tree reproduces efficiently. Once the seeds are trapped in the fructose crystals, it would take days of rain before they can go to Earth -o well, not to mention they'd be nicely cooked in the caramelization process. And for sure, they couldn't be eaten by birds: caramel is tough and comes in shards, it would be like eating glass.

EDIT: The answer is: This tree cannot generate the necessary heat, unless it is not a tree at all, made of non-organic, fireproof material.

Because a tree doesn't just generate enough heat as to burn itself. Even using the most advanced genetical engineering, the resulting organism would be something that only looks like a tree: it should have conduits of highly reactive incendiary chemicals that, in contact with the sugar, start melting it and cook it. Then the tree should 'know' that the optimal temperature is reached and at that point the fruit expels the excess heat until its core is a mass of solid caramel. A core that, by the way, should be made of a thick layer of hardwood or, again, the fruit would cook during caramelization. but if we have a layer so tough around the core, it will not 'explode' by falling or by any other means, unless it is made of explosive material.

And speaking of heat dissipation, at this point we should think of another kind of fruit entirely: It should look like a pinecone grenade with microvents between its 'leaves', so to dissipate the heat during the caramelization of the core. But the problem of the hard-protected core would remain

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    $\begingroup$ hi @FoxElemental - on this site, for science-based, quite often the correct and actual answer is "it is not possible", with an explanation why. $\endgroup$ – Fattie Jun 20 '18 at 13:42
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    $\begingroup$ @Fattie I meant that at the original time of the answer, it didn't go into depth as to how the tree would work and didn't make any attempt to answer the question. Now, though, Valerio edited the post to answer the question, and I'm retracting my downvote. $\endgroup$ – FoxElemental Jun 20 '18 at 13:48
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    $\begingroup$ "This answer was flagged as low-quality because of its length and content." - I'm going to click skip, just saying. $\endgroup$ – Mołot Jun 20 '18 at 14:17
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    $\begingroup$ It is possible for seeds to survive fire (onlinelibrary.wiley.com/doi/pdf/10.1111/btp.12276), not to mention many seeds lie dormant for years, meaning disolution by rain for many days shouldn't be an issue for reproduction. Sandbox trees have exploding fruit (en.wikipedia.org/wiki/Hura_crepitans), but I don't see if the fruit explodes before or after it's dropped, or what causes the explosion. $\endgroup$ – Devyzr Jun 20 '18 at 22:09
  • $\begingroup$ @Devyzr I believe it's stored up mechanical energy from the seed pod drying out, and it happens when the seed pod falls and hits the ground, but I wasn't able to find any footage of it happening naturally. Touch-Me-Nots are even more violent, exploding at the slightest touch, often setting off a chain reaction. $\endgroup$ – Draco18s Jun 21 '18 at 1:43
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Ideas:

1: Your sugar bombs as proposed have a hard shell of caramel. That entails heat at the edge of what biological systems can produce which makes it tricky. You could instead make a shell of crystalline sugar: rock candy. Sugar will crystalize out at room temperature as the solvent (water) evaporates. Crystals can form on fruit that is damaged by bugs but there is no reason your fruit could not do this by itself. That gives you the hard shell without the heat.

2: Your sugar bombs heat up via spontaneous combustion. You can find reports of stored sugar and molasses spontaneously combusting. As regards carbohydrates usually one reads about hay piles - bacterial fermentation starts things and then the material gets to a heat in which the reaction is self sustaining. So too your sugar bombs - they do not receive heat but rather generate internal heat by oxidation / fermentation of the sugar within the fruit.

3: They want to blow up. Having something in a hard shell getting hotter and hotter is a fine recipe for an explosion. If nothing eats the fruit (to later poop out the seeds), the fruit does not want to be eaten by ants which do nothing for seed dispersal. Having it explode violently is a fine way to spread seeds at least out of the immediate vicinity of the parent tree.

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  • $\begingroup$ Rock sugar shell coupled with incendiary flesh? Mmm, tasty! $\endgroup$ – Joe Bloggs Jun 21 '18 at 6:47
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Overall, I think your frag-grenade tree concept isn’t impossible, but it needs a different mechanism for heat.

Heat

Sugar starts to caramelise at perhaps 110°C, but to reach a hard consistency, you need to go higher - a quick google for recipes gives 160°C for “hard crack” caramel. In practice, due to how heat is lost from the pod, some parts will need to be significantly above this temperature.

Obviously this is pretty bad for any seeds inside. Worse still, Wood starts to char at 120°C, and it’s actually it’s above the flashpoint for decayed Wood (150°C), and nearing the flash point for fresh Wood (190-260°C, depending on the Wood).

So you can’t use heat to caramelise it, or your tree is very likely to spontaneously combust.

Unless you make this part of the tree’s lifecycle, that it occasionally bears fruit and burns down, leaving just the roots to regrow from? You’ve still got to deal with the seed surviving the temperature though.

Heat waves in roots

Isn’t going to happen. Due to the insulation of the ground, your roots would need to be extremely hot for a “heat wave” to reach the surface. It’s likely this would cause the soil to burn (soil can and does burn, e.g. due to fires in mines).

Even if your tree is made of asbestos you need an alternative mechanism here.

I’d suggest that your caramel pods have some kind of internal stress, or perhaps a pressure build up inside, which can cause the explosion you’re looking for. This would be a lot more plausible. If you want them to sometimes explode, we can rely on caramelization being a tricky beast at the best of times. Many factors affect how crystals grow and their size, so it’s entirely possible for some pods to be weaker, some to vent safely and slowly, etc.

Catalytic caramelization

So if not heat, try a catalyst? A quick Google shows catalysts can be used to assist caramelization, so it’s not ridiculous that your tree could produce a catalyst which allowed caramelization at a survivable temperature.

So you can create your caramel without igniting your tree.

Propagation

Whilst some animals might not want to eat caramel, others would, and the sharp corners would be the first bit to go once they’re exposed to damp air. And ants would absolutely love it!

It would also protect the seeds inside until the caramel had dissolved, which may be preferable, since not all plants want their seeds to germinate immediately.

So no issues there.

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I'm surprised it hasn't been suggested, but there are plenty of chemical reactions that are violently exothermic.
What you could do is have your tree synthesise two different compounds in lots of small pockets evenly distributed throughout the fruit, and when it's ripe, the walls between the pockets break down, the two compounds combine and the fruit explodes.
Take a look at the internal structure of an Orange for example, where it's clearly defined into segments inside, each partitioned from the others and containing a lot of juice.

This has the advantage of also detonating when the fruit hits the ground because of the shock of impact breaking the dividers.

Alternately, if simply producing heat during the ripening process is your goal, making the fruit produce both chemicals and immediately allowing them to mix will produce a constant heat rather than explode.

I'm no chemist, so I couldn't tell you what specific compounds would produce that kind of temperature, but I'm fairly sure it's not out of the realm of possibility!

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Re The sugar bomb tree. High temperatures could be generated very quickly using an exothermic chemical reaction between two compounds. This is how Bombardier Beetles (family Carabidae) generates hot caustic repellant that builds up in a reaction chamber which is then expelled at high pressure. I kid you not! Ive been hit by them - couldnt resist touching one.

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  • $\begingroup$ Welcome to Worldbuilding, Peter, and thanks for the answer! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods. Have fun! $\endgroup$ – FoxElemental Aug 10 '18 at 0:23

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