The dramatic function of this short scene is twofold. First, it gives an opportunity to observe the relationship between Macbeth and Duncan; second, it provides Macbeth with further fuel for his ambitious claim on the kingdom.
Malcolm's report of the execution of the disloyal Thane of Cawdor emphasizes the dignity with which even a traitor can go to his death, but Duncan's reply is even more ironic. "There's no art to find the mind's construction in the face" has a proverbial flavor to it — never judge a book by its cover — but it's also a sad admission that even Duncan was unable to predict the treachery of Cawdor. Such is the human side of kingship. Exactly at the moment that Duncan speaks the line, Shakespeare seals the irony by having Macbeth enter the court room.
Formal speeches are exchanged, both Macbeth and Banquo giving humble and loyal replies to their king. The imagery at this point in the scene largely refers to growth and fertility. The king clearly sees Macbeth as a potential successor: "I have begun to plant thee, and will labour / To make thee full of growing" (28–29). The metaphor is continued by Banquo, who promises the king that, if he too is allowed to grow in the king's favor, he will dedicate "the harvest" to Duncan. At this point, the scene recalls Banquo's earlier line when he asked the Witches if they could "look into the seeds of time / And say which one will grow, and which will not" (I:3,58–59). The irony of giving the earlier "seeds" line and now the "harvest" line to Banquo is that these expressions symbolize the seed, or children, of Banquo himself, who are to inherit the kingdom, according to the Witches' third prophecy.
