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.

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
 * Chrona