In Letter 13, Celie uses metaphor to describe herself as a "tree"—one that has been hardened in the face of Mr. ________'s physical abuse.
He beat me like he beat the children. Cept he don’t never hardly beat them. He say, Celie, git the belt. The children be outside the room peeking through the cracks. It all I can do not to cry. I make myself wood. I say to myself, Celie, you a tree. That’s how come I know trees fear man.
Through metaphor, Walker suggests that the violent impulse driving domineering men to abuse women is the same impulse driving those men to "dominate" the natural world, stripping it of resources until nothing is left. Celie makes herself into a tree only to realize that "trees [also] fear man." The perceived strength and impenetrability of wood does nothing to discourage men from cutting down entire forests.
Celie's strategy is to endure the abuse with a stiff upper lip, but this does nothing to help her situation or discourage Albert from harming her. When Celie meets Shug Avery, who stands up to Albert on her behalf, she learns that it will not suffice to simply endure abuse stoically and wish for its absence. Through Celie's evolving perspective on Albert's domestic abuse, Walker asserts that one cannot remain immobile in the face of gendered violence, like a tree—one has to fight back.