Plants

Plants are the reason humans have been able to survive on Gunsmoke.

Description
"Plant... the thing that produces water, UV rays, oxygen... depending on how much electricity is fed to one, it can even defy the laws of physics and produce organic systems. It is lost technology's biggest black box.  In appearance, they look like messengers of God.  If one of them dies, so dies every living thing depending on it." Trigun, Volume 1.

Most Plants are kept in large light bulb like containers.

Abilities
The abilities of Plants appears to vary between each one. It may be that Plants can be engineered to specialize in a particular area.

All Plants appear to have some form of telepathy or sense between themselves. This caused the Plants powering a weapon the military was going to use on Knives to stop functioning, because they did not want to hurt another Plant.

Regeneration is one of the primary abilities of a Plant. Knives used this ability extensively after the July Incident.

It is said in one point of the manga that Plants have two main powers. One that "takes in" and another that "forces out". The power that "takes in" may relate to how Knives and Vash's main Plant power that act's like a black hole, while the "forcing out" relates to regular Plants and their ability to use electricity to make food and water.

Independent Plants
There are Plants, like Vash, who have formed their own identity and taken on a human form. How or why this happens has not been explained. It appears that all independent plants have blond hair. They can also communicate with regular Plants. Though in human form, they age much slower than humans do.

Independent Plants

 * Vash The Stampede
 * Millions Knives
 * Domina
 * Tesla
 * Chronica