What actually is a Terrarium?
Terra- Meaning Earth
Arium- A suffix denoting a place
This is is the basics of what a terrarium is, we are creating a landscape protected from the outside elements, in which the plants and minerals used can interact and grow as if they are in their natural ecosystems.
The glass protects plants from dust, pollution and any dramatic changes in temperature or pressure; allowing enough light in for the plants to photosynthesise as normal. A sealed terrarium should mimic exactly what is happening in our world, for example when it rains, plants and mosses will take in as much moisture as they need, as the plants Photosynthesise they then push this moisture back out through the heat and oxygen that is created. This moisture evaporates and is caught in the clouds to then fall again as rain, in a Terrarium though the moisture rises to cause condensation on the inside of the glass, this condensation then trickles down the glass and is used by the plants for watering. When these plants are in a closed vessel they then become self-sustaining, as the glass and cork stops any of the humidity created from escaping.
The humidity inside a sealed Terrarium should be about 70–90%, the plants used in these terrariums all thrive in the high humidity - think tropical jungle plants like Fittonia and Maranta but also palms, ferns and mosses.
Another type of terrarium is the open terrarium. As this vessel will not be sealed, the same amount of self-sustainability is not exercised, so you will still need to water the plants as usual. Quite often you will see Cacti and Succulents in an open terrarium as they need very little water and thrive in a dry and arid climate, they will still be partially protected from outside elements by the glass but not to the same extent as a closed Terrarium.