Fractals caused by expansion of an edge. In plants the
rate of growth at the edge of a leaf or flower petal is
faster than it is in the center: hence there is not enough
room for the edge to stay flat so it crinkles. The same
thing happens at the edge of torn plastic where the plastic
stretches more the thinner it gets and it gets thinnest at
the edge of a tear.

The crenellations at the edge of torn plastic have
the same form when looked at under a microscope as they
have in this picture.