Fortitude
Adds +4 to Fortitude saving throws vs. poison from Plant creatures
This does not affect saving throws for poison derived from plants (e.g., hemlock or oleander), it only helps with poison delivered by plant monsters.
Does not affect AC vs. plant creatures
Adds +2 to Constitution (+1 to max HP per character level and +1 to Fortitude saving throws)
Adds +1 to Constitution (depending on initial Constitution score, either 0/+1 to max HP per character level and 0/+1 to Fortitude saving throws)
Adds +1 to Fortitude saving throws but user may not equip any other ioun stone
Adds +2 to all saving throws. If the user of this ioun stone has also equipped a Skull of Cavadar, the bonus is +3 to all saving throws instead of +2.
Adds +1 level to character. If another level bonus is in effect, adds +3 to saving throws instead.