I had the question of, how do the plants get C02 once they use it all up? Answer: they don't, or the terrarium is not sealed. > A closed terrarium is a closed

I had the question of, how do the plants get C02 once they use it all up? Answer: they don't, or the terrarium is not sealed. > A closed terrarium is a closed ecosystem just on a much smaller scale [than earth]. The supply of carbon dioxide in the terrarium air is very small because of the limited volume of air. Therefore, unless the terrarium contains animals to provide carbon dioxide or microbes break down large amounts of organic matter in the soil and release carbon dioxide, the plants in a sealed terrarium cannot gain significant dry weight. They would basically refix the same carbon dioxide over and over. The carbon dioxide the plants release during cellular respiration would be refixed via photosynthesis, then respired, refixed, respired, refixed, etc. > Most terrariums are not completely sealed so carbon dioxide can enter and plants can gain dry matter via photosynthesis. The inventor of the terrarium, Nathaniel Bagshaw Ward (1791-1868), supposedly had a "sealed terrarium" that survived four years but it is questionable if it was completely sealed because of the substantial plant growth he observed. http://www.madsci.org/posts/archives/1999-08/935417556.Bt.r.html http://www.madsci.org/posts/archives/1999-08/935417556.Bt.r.... Side-note: it's not often that a webpage from 1999 is the first search result when googling (I searched "terrariums where does the c02 come from" on DuckDuckGo).