Scientists find proof plants are capable of complex arithmetic

Careful now. Computers are a similarly mindless arrangement of atoms. The only difference is that humans designed computers, and natural selection designed plants. Under your reasoning, computers are incapable of arithmetic. At that point, how are brains any different?

Perhaps that's right, but such a "pure" definition of computation is hardly useful.

More examples of plants mechanistically doing math: http://www.youtube.com/watch?v=ahXIMUkSXX0