If a symbol is an action or thing that points beyond it's concrete, literal meaning, what might the turtle symbolize ?

The turtle can be seen as a metaphor for both the Joads and the migrants in general: the turtle is tough, tenacious, and unstoppable.

Steinbeck writes, "All over the grass at the roadside a land turtle crawled, turning aside for nothing, dragging his high domed shell over the grass." As the turtle tenaciously carries forward with his life, so too do migrants as they protect both their own lives and the lives of their families.

Further, in another section of the inter chapter, it is revealed that the turtle is transporting a seed in his shell. The turtle not only carries life, but also transplants that life to a new place: “The wild oat fell out, and three of its spearheads stuck in the ground”. This situation is analogous to the way the men end up taking their pregnant wives away where they raise new life in new lands.