What does it mean by blake about the third quatrain in the poem "A Poison Tree" ? "And it grew both day and night, till it bore and apple bright, and my foe beheld it shine, and he know that it was mine."
The Poison Tree is a poem about what happens when we keep anger to ourself. In the first stanza, the narrative persona says that his anger with his friend ends when he tells his friend he is mad at him, but because he didn't tell his enemy he was mad, his anger grew.
The persona's anger has "born fruit" which appears to his enemy as something valuable. The enemy steals the "fruit" of the persona's anger, ingests it, and dies of it. There are many layers to the metaphor. The persona has grown the fruit through trickery ("sunned it with smiles/ and with soft deceitful wiles) and has given it the appearance of something desirable. His enemy knows it is not his fruit, but it appears tempting (it shines) and he knows it belongs to his enemy, so he feels justified in stealing it. This means that the enemy has somehow stolen the fruits of the persona's labours, but what he has taken, though it seems good, is actually something that will destroy him.
This echoes the temptation of Adam and Eve in the Garden of Eden--the serpent tricks them into thinking they are getting away with something but in fact, what they have done has condemned them.