It takes Mr. ____ most of the spring to decide whether or not to marry Celie. In the meantime, Celie and Nettie study together, although only Nettie is still in school. They read in particular about Christopher Columbus, and the Native Americans whom Columbus brought back to Europe as slaves.
This passage introduces, through the presence of Christopher Columbus, the idea of American slavery which is one of the novel's undercurrents. Nettie and Celie are only two or three generations removed from black slavery in the American south.
Celie's father took Celie out of school the first time she got pregnant, saying that Nettie was the smarter of the two sisters anyway. When Miss Beasley, their teacher, comes over to beg with Celie's father to let Celie back in school, Miss Beasley sees Celie, dressed in a tight dress and pregnant, and leaves, disgusted in her former student.
Miss Beasley wishes to help Celie, but it appears that this help is limited by Miss Beasley's sense of propriety. Celie's teacher assumes that Celie has made a "choice" to get pregnant—and Miss Beasley therefore judges Celie based on this "choice," deciding that it is not worthwhile to argue she should be placed back in school.
Mr. ____ returns toward the beginning of summer and looks anxious and tired. He says he needs a woman to take care of his children, and asks to see Celie again. Celie's father repeats that Celie would be a good stepmother. Mr. ____ asks whether Celie comes with a cow, and her father replies that the cow is hers: a dowry, or wedding present.
Pa's logic has been proved correct. Mr. ____ cannot take care of his four children alone (it is revealed later he has four, not three). Celie's ability to quietly do her duty, at any cost, is, perversely, exactly what Pa exploits, and what he praises when encouraging a marriage between Mr. ____ and Celie.