The pod plant was a plant species that served as a repository for thousands of microscopic spores, which were discharged from its flower upon finding a Human body to inhabit. The infected a person was then protected from berthold rays, but also giving them perfect physical health (even regrowing missing organs) and peace of mind.
The pod plants were from unknown origins that had drifted through deep space until they came to the planet Omicron Ceti III, a planet constantly bombarded in berthold rays, which the plants thrived on.
They were discovered on the planet by botanist Leila Kalomi in 2264. The plant's spores infected Kalomi, Elias Sandoval, and the other members of the Omicron colony. Where they continued to live for many years, in an environment otherwise deadly to Humans.
In 2267, the USS Enterprise arrived at Omicron Ceti III, when the colonists tried to infect the crew, which was exacerbated by an infected Doctor Leonard McCoy, who had numerous pod plants beamed up to the ship, where the spores quickly spread through the ventilation system.
Only Captain James T. Kirk remained unaffected, and he was able to deduce that his apparent immunity was due to the intense emotions he was feeling. These emotions destroyed the spores, and Kirk was able to incite anger in his science officer, Commander Spock, to free him from the spores as well.
Together, Kirk and Spock built a subsonic transmitter that they used to induce anger into the entire colony at once over the Enterprise crew's communicators, wiping out the spores. (TOS: "This Side of Paradise")