"Till it bore an apple bright,..."
See in text (A Poison Tree)
The apple serves two primarily symbolic functions. The apple is the result of the speaker’s long-harbored, unexpressed wrath. The image offers an appropriate touch of redness, underscoring the tone of rage. The apple also serves as an allusion to the fruit of the knowledge of good and evil in the book of Genesis. Satan offers Adam and Eve the fruit; their to decision to eat it marks the “original sin.” Thus the apple is the hinge between innocence and experience, to use Blake’s terminology.