In the palace court room, King Duncan receives the news of the execution of Cawdor and delivers formal thanks to Macbeth and Banquo for their part in the battle. Then, to the private astonishment of Macbeth, Duncan announces that his successor as king, whenever that may be, will be his son Malcolm.
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.
Note the way in which Shakespeare plays with images such as these. Often he builds up a cluster of related images (as here, "plant," "growing," "grow," and "harvest") precisely in order to establish a sense of irony. In the next speeches, for example, the king first invests all those who deserve his thanks with "signs of nobleness, like stars." Only a few lines later, Macbeth, frustrated and angry at the news of Malcolm's investiture as Prince of Cumberland, breathes to himself the words "Stars! Hide your fires! Let not light see my black and deep desires" (50-51).
Here, the juxtaposition of images of starlight and the cancellation of starlight emphasizes the great opposition between the king and Macbeth and between good and evil, an opposition that is ironically reinforced by the king's final lines to Banquo, once more praising Macbeth. The phrase "peerless kinsman" gives added poignancy: The historical Macbeth was the cousin of Duncan, and his crime will not simply be regicide, but the willful destruction of the head of a family.
became him (8) suited him
construction (12) intention
which is not us'd for you (44) which you are not used to
harbinger (45) forerunner