According to the Book of Engaro, there is a crystal capable of holding the psychic essence of a male and female orhatea. Once joined in the Seed the two are—in effect—immortal. As long as one is able to create a body for the other, they can transfer a retained essence to that body.
In practice, the Seed is a crystalized storage unit. Grown in the shell of the Amah it records the life experiences—engrams—of the Amah and her male. Once stored these engrams are available for instant retrieval. This allows the pledged pair to study a wide range of subjects without having to retain that knowledge through traditional mnemonic devices.