You can discern three kinds of cactus:
a spherical purple kind roughly two feet in diameter, a tubular kind that resembles a mass of writhing brains and covers the ground for twenty square yards and a tall tree-like kind with 1-6 branches.
Between your group, that the first one (spherical) is used by elves to poison their weapons and that the tubular one is also called a 'rope cactus'. The tree-like one is edible and provides some liquid, but you remember that they are somehow not save to approach...