the hallucinogenic plant he eats

There is a scene near the end where the man-ape eats a plant and it makes him hallucinate, mainly about the past events in the film. This is also what makes him realize the ax he has been carrying around is a weapon. I think the scene was reference to Terence McKenna's stoned ape theory. However, his theory was more about the effects of mushrooms. What plant did our ape friend eat?