The significance of this letter lies in its rendering of the especially close relationship that exists between Nettie and Celie. Nettie's trying to persuade their father into letting Celie attend school is strong evidence of her deep, maternal compassion for her sister and her concern for her sister's future.
Celie is about twenty years old when Mr. _______ returns to take "another look" at her; he appraises her from horseback, much as he would reckon the worth of a farm animal, and then he decides to take her — after Fonso reassures him that a cow is included in the bargain.
Despite the depressing thoughts about what Celie's future might hold, there is one light note: Nettie and Celie have a delightful exchange about the world not being round. To Celie, the world seems literally
flat — and metaphorically flat, made up of unending misery and monotony. But Celie doesn't tell Nettie this; it is important to Celie that Nettie continue to think that she isn't dumb.