In Macbeth, what is the significance and purpose of this image: "If you can look into the seeds of time, and say which grain will grow and which will not"?
In Act I, Scene iii of Macbeth, Banquo questions the witches, asking them to speak a prophecy to him:
To me you speak not.(60)
If you can look into the seeds of time,
And say which grain will grow and which will not,
Speak then to me, who neither beg nor fear
Your favors nor your hate.
Banquo speaks poetically about the seeds of time and how the witches are prophesying about which grain will grow and which will not. This reference to nature is Banquo's way of comparing the seeds to people. Banquo is indicating that the witches know who will grow and prosper and who will die. Since the witches have such discerning powers, Banquo inquires about his own future.
When Banquo asks the witches "if they can look into the seeds of time," Banquo is referring to the prophecy the witches have just given Macbeth. The witches have just prophesied that Macbeth will be king in future time. If this is to be so, then the witches know which grain will grow and which grain will die. In other words, King Duncan will have to die for Macbeth to be king.
Banquo's metaphor of seeds and grain is a metaphor that compares seeds and grain to present kings and future kings.
This passage from Act I, Scene 3 acts as foreshadowing and, also, as an insight into how the character of Banquo responds to the preternatural element of the Elizabethan drama.
In this scene Banquo and Macbeth cross the heath from the battlefield as they are on their way to the court of King Duncan at Forres. The third witch hears them approaching and calls out to the other two, thus introducing the element of the spiritual. When he sees them, Banquo asks if they are mortal beings, observing that they do not appear to be “inhabitants o’ th’ earth” (1.3.39).
Further, as he addresses the witches, Banquo displays his skepticism of their fortune-telling. Having heard what they have predicted for Macbeth, Banquo also observes how stunned, yet intrigued, Macbeth is. But he challenges the witches, saying that if they truly can "...look into the seeds of time / And say which grain will grow and which will not," then they should tell him what his future holds too (1.3.60-61).