I like the first principles thinking, but what if they do have a secret place?
What if it goes to the roots and then into fungi and bacteria in the soil or something?
What if it goes to the roots and then into fungi and bacteria in the soil or something?
By the same token, plants convert carbon into fruit, which animals eat and then metabolize back into CO2. At the end of the day, all of a plant's captured carbon will return to the ecosystem one way or another. Most of the time, anyway.
Then, at some point, remove and weigh the plant to see if it contains all of the accumulated mass.
If I recall correctly, other than the last step, that was how someone first noticed that plants must be gaining mass from the air.