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In another question, Are flying plants possible?, a point was raised that because of the square-cube law, a large, hollow, spherical plant could fly by having the air inside be a little bit hotter than the air outside. I'm wondering how a plant could heat the air inside of it up. Any ideas?

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Jetscooters
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  • You might want to specify exactly what part of the plant will take flight. Presumably the only reason for this is to spread seeds. – iAdjunct Mar 04 '15 at 13:20
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    You should also note that it's not so much the fact that the air is HOTTER than makes it float, it's that the hot air is less dense that it floats. If your plant has an enclosed pod, even heating it up will not make it fly unless its volume increases or it releases some of the air. This hints at another approach it can take other than heating: if it creates an air-tight pocket, then causes it to grow (i.e. increase its volume) without letting air in, it would have the same effect as a hot air balloon. – iAdjunct Mar 04 '15 at 13:29
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    Just a note, some plants run hot enough to melt snow. – user3082 Mar 04 '15 at 14:32
  • Fascinating. Look up thermogenesis or thermogenic plants. Thanks @user3082. – Brian Lacy Mar 04 '15 at 20:27

6 Answers6

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Compost

A symbiotic relationship between bacteria and the plant could produce a significant amount of heat. If the plant, as it grew, shed plant material into an inner cavity then it could be providing its own biomass for composting. By controlling the airflow in through stomata, the plant could control the amount of oxygen and therefore the speed at which the composting takes place. As the biomass is composted, heat would be produced inside the plant's expanding gasbag.

Sunlight

The large gasbag would naturally absorb sunlight as well, providing additional lift during the day. In the night the plant could allow more oxygen in to increase the composting action and provide more lift without the sun.

Thermogenesis

Of course, some plants have the ability to produce their own heat. This is an expensive (in terms of energy) process, so it's possible a plant could use this sparingly to take flight, but not stay aloft all the time.

The most successful plant would likely use a little of all three methods.

Samuel
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  • Compost is just as energy intensive as thermogenesis. It's aerobic respiration from plant energy stores either way, just done by plant vs bacteria. – Kevin Kostlan Jan 10 '20 at 02:08
  • Sunlight is the best option. It is cheapest as the plant wouldn't need to carry an internal energy source/warmth production method. The plant could have a darker shade color on the top side to ensure it heats up more than the surroundings. If this takes place in a rainforest, once the balloon reaches above the treetops, it will raise quicker because it gets more sunlight, and thus heats up more. I imagine the 'balloon' having a thin strand connecting it to the ground, which will snap once the balloon generates enough lift. Ensuring it will only start its journey in the best of circumstances. – vinzzz001 Nov 23 '23 at 13:58
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The obvious answer would be by trapping sunlight, in the same way that greenhouses tend to be hotter than the air around them.

Biological processes could also be used, but that would cost the plant energy so would most likely be avoided.

Tim B
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    That would require the plant to be transparent to high-energy IR but not low-energy IR, which is the basis of how greenhouses work. – iAdjunct Mar 04 '15 at 13:19
  • Or just.. black and light absorbent, with near perfect isolation. – Pica Nov 24 '23 at 08:04
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While I agree with Tim in that the most likely would be warmth from the sun as a green house, an alternate would be bacteria that produce heat, maybe a bacteria that breaks down the water into Hydrogen and Oxygen so the plant can float with it. It would also help the plant from crashing into the ground at night, (though that might be a 'special harvest' for those living on the ground)

The plants would also need some kind of 'steering', to follow (drift with) the sun north and south as the seasons change.

bowlturner
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Well, there are a number of ways that a plant can raise its temperature. The obvious are:

  • Absorbing sunlight
  • Energy released from respiration burning fuel

However, to get the fuel, you need to photosynthesise. Again, capturing sunlight. Luckily, photosynthesis isn't endothermic (it just uses sunlight energy), so it won't steal your thermal energy out of the system.

As mentioned in a comment [@iAdjunct] you'll need an enclosed space with elastic properties (read: a balloon) that can be expanded as the air heats up so that density reduces inside. Energy needs to be effectively transmitted directly into this inner space to heat the air.

Best bet would be some high-respiration specialist plant growth inside a rubber-balloon releasing energy into your trapped inside air, with the green photo-synthesising cells on the outside creating fuel. Luckily enough, as the plant rises its sunlight exposure will increase allowing for greater fuel creation. You'd have a nice little feedback cycle.

Your plant would still need contact with the ground at some point, or else have very good absorbency from its environment. Water is essential, and there will be a need for nutrients and such for repair and growth.

FelixMarcus
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Perhaps you don't need the plant to heat up the air at all? (bear with me here)

The plant in question (perhaps in winter, with the deluge of water to help build the bubble) begins forming the walls around the "bubble-to-be", sort of like a U shape. This could be grown as a new, vertically-growing attachment in the middle of the plant, while the light-gathering leaves spread out from the base. For the bubble-to-be, everything's ready, except the top hasn't closed up yet due to a small hole right at the top, and small holes at the bottom for rain to escape.

In the lead up to summer, the plant gets its seeds ready (attached to the bubble-to-be).

In summer, during a particularly hot day, the plant finishes closing up the hole at the top of the U (as well as the bottom holes) so that it's now an O, i.e. a fully enclosed bubble, which traps the hot summer air inside.

Summer passes. As the weather cools down, heading into autumn, the plant's connection to this bubble dies, and eventually breaks off (once in winter, the mother-plant can begin working on a new bubble-to-be). The bubble still contains the summer air, which has since cooled down but is otherwise less dense than the cool wintery air, and as such, begins to slowly float away. Slowly and over time, the seeds break off from the bubble, falling to the ground in an attempt to start more of these plants. At the same time, this helps the bubble stay afloat as its weight slowly diminishes with the release of the seeds.

Come next summer, the bubble has very possibly degraded away and fallen back to earth. Even if it hasn't, the hot air of the coming season is now as thin as the air inside the bubble, so gravity takes over. Either way, by the end of the cool season, the bubble will come back down.

Depending on what you want, I get that this may not be an ideal solution for you if you want these bubble-plants floating around year-long.

Also, it's highly likely that these bubbles would die shortly after being released from the mother-plant, their purpose solely to float around and drop seeds. The fact that they're dead would help the bubble float, as the dead plant tissue can have a reduced weight from not needing to contain water to sustain a living organism. All of this would counter an idea that these plants are floating around while alive, though you may be able to come up with a way to explain that these bubbles (should) remain alive.

Feel free to correct me on anything scientific I've missed, or otherwise play with this idea. Furthermore, while this provides a great way for the plant to spread its seed around (theoretically at least), the evolutionary steps required for a plant to get to this point elude me.

RPBCL
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  • You're right about my want to have the plants float around year round, and this is a great answer. I hadn't thought of the bubble being dead. – Jetscooters May 15 '15 at 02:35
  • Well, then I guess my answer really doesn't help you. Sorry about that haha :) ... Hopefully it can help someone else with a similar idea. – RPBCL May 15 '15 at 02:37
  • Also, sorry about replying to a two month old post, but I really like the idea of bubble-plants, and thought I could contribute nonetheless with my first post. – RPBCL May 15 '15 at 02:39
  • I'll adapt some parts of this into the story. Welcome to Stack Exchange! – Jetscooters May 15 '15 at 02:40
  • Heh, really? Awesome. Glad it gave you something to think about :) And thank you. – RPBCL May 15 '15 at 02:41
  • I think the issue with using lower-pressure, atmosphere-temperature air inside the planet like this is that it requires the "bubble" to be rigid and strong enough to withstand this pressure difference. And we don't know of natural materials that are capable of being this rigid, while at the same time low-mass enough to float. – zeta May 15 '15 at 05:18
  • what you propose is a kind of natural vacuum ballon, this wont work. – Jorge Aldo May 15 '15 at 05:28
  • @sumelic What about on a smaller scale? I didn't mention it in the comment, but while the OP seems to be asking about large bubbles, in my mind I couldn't help but imagine the "bubble" as no larger than, say, 10, maybe up to 15cm diameter. Furthermore, I imagined it as semi-rigid as opposed to "blimp"-style, i.e. the vines on the inside help to hold out the outer-covering. On a smaller scale, and perhaps without a huge pressure difference, just enough to get the bubble to float, would this be more plausible? – RPBCL May 18 '15 at 00:24
  • @JorgeAldo see above comment reply to sumelic, does this make it any more plausible? – RPBCL May 18 '15 at 00:26
  • nope, vacuum is very hard to contain and leads to too much heavy walls that end up increasing the overall weight to volume ratio of the system, preventing it to fly, or, if the walls are not reinforced enought to prevent collape, they simple buckle. – Jorge Aldo May 18 '15 at 00:56
  • @JorgeAldo It doesn't need to be a vacuum though, just a difference in pressure, i.e. the pressure difference between hot summer air and cold winter air. Surely a pressure difference between these two is far easier to work with compared to the pressure difference between the air and a vacuum? – RPBCL May 18 '15 at 02:58
  • Pressure difference is not directly the cause of buoyance. Buoyance is the result of differences of density between something and a fluid. – Jorge Aldo May 18 '15 at 03:52
  • @JorgeAldo Aye, so in the same way that a bubble "containing" (in a manner of speaking) a vacuum would float if the total weight of the bubble were less than the weight of the air that would otherwise occupy that space, can the same not be said of a bubble which contains air which is less dense than that of the air surrounding it? (Also, for the record, not trying to start a fight here :) Genuinely just trying to understand what the problem is so that I can figure out if the solution needs tweaking, or is just completely wrong...) – RPBCL May 18 '15 at 04:21
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Use methane instead of heated air.

Methane is readily produced by all sorts of bacteria, and it's significantly lighter than air at room temperature (0.716 g/L vs. 1.2 g/L). You would need to heat the air to 200°C (390°F) to get the same density.

You can of course use heat to add to the buoyancy, but with methane, you don't need to maintain the temperature to keep the plant in air if it can float in the first place.

Jani Miettinen
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