Why does Marguerite think that Ira Sargent has grown sullen? What does she do about it?

Marguerite believes that Ira Sargent has grown sullen because he is in love with Abby Welles, but she is being courted by Ethan Jordan, who just brought her an expensive china teacup set from Portsmouth. Marguerite spots the red ear of corn and points it out to Ira so that he might be the lucky one said to be the next to marry.