How is plants growing in cracks biological weathering?

Weathering is the process of breaking away at a substance, such as stone. By growing inbetween a crack of a rock, the plant will begin to exert pressure on the walls of the crack as it grows larger. In doing so, the rock's crack will gradually become larger and larger until, lo and behold, the crack might break completely down. Of course, this outcome may take decades upon decades to occur (depending on the type of plant, size of the crack, and fragility of the rock).

In short, the plant's growth between a crack widens the crack and wears down upon the rock.