Eosian grass was a sentient grass which could stimulate regeneration. It could be found on Gallifrey, where it was protected by the Time Lords, and Jaxus.
The Master used Eosian grass to make a metempsychosis compound to restore himself and become immortal without the need to regenerate. The Fourth Doctor and Cahlo were attacked by the grasses on their way to find those that the Master had kidnapped to produce the compound, but they let them go when the Doctor apologised for stepping on them. (AUDIO: Metamorphosis [+]Loading...["Metamorphosis (audio story)"])