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?
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.
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.
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.
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.
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.
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.