Pseudolilium contortum

The twisted lily bears a slight resemblance to Terran lilies when it flowers, but there the resemblance ends. This unique “planimal” is very tall but soft and herbaceous, with thick rope-like stems that twist and undulate with movement due to the presence of highly evolved contractile fibrils similar to those found in animals and giving the plant primitive neuromotor ability. The leaves are rounded, thick, and somewhat heart-shaped, with striations radiating out from the point where they attach to the petiole, the smaller stem that attaches the leaves to the main stem. (Read more)
The bright radiating lines on the leaves mark the location of primitive sensory cells that converge at the petiole and travel down the length of the plant. When a living organism approaches, the cells bioluminesce brightly, sending a signal to the rest of the plant. This signal triggers a rapid twisting movement of the main stem in the direction of the intruder, who can be slapped by the leaves if he doesn’t quickly move away.

The slapping leaves of the twisted lily are deliberately provoked for fun by young Na’vi and in all seriousness by adults as a fitness and training exercise.