The horse-chestnut tree symbolizes the relationship between Jane and Mr. Rochester. Jane goes for a walk in the orchard, down to the horse-chestnut tree. She is momentarily at peace:
Here one could wander unseen. While such honey-dew fell, such silence reigned, such gloaming gathered, I felt as if I could haunt such shade for ever . . . .
Mr. Rochester enters the orchard and they walk toward the horse-chestnut tree. He is telling her that he is going to marry Miss Ingram and she is going to Ireland. They discuss her leaving and never seeing each other again, and Mr. Rochester says,
here is the chestnut tree: here is the bench at its old roots. Come, we will sit there in peace to-night, though we should never more be destined to sit there together.
Jane weeps and says she doesn’t want to leave him, and then he tells her not to, and then he kisses her. A wind comes through and “trembled through the boughs of the chestnut: it wandered away-away-to an indefinite distance-it died.”
Mr. Rochester is wrestling with morality and convincing himself what he is doing is okay. Jane writes,
“And what ailed the chestnut tree? It writhed and groaned while the wind roared in the laurel walk, and came sweeping over us.”
That night the horse-chestnut tree is struck by lightning “and half of it split away.”
In Chapter 25 Jane goes down to the orchard:
I faced the wreck of the chestnut-tree; it stood up black and riven: the trunk, split down the centre, gasped ghastly. The cloven halves were not broken from each other, for the firm base and strong roots kept them unsundered below . . . .
This is symbolic of the troubles they are about to face which will separate them, yet they remained connected.
At the end of the book, Mr. Rochester says, “I am no better than the old lightning-struck chestnut-tree in Thornfield orchard,” yet Jane says, “You are no ruin, sir-no lightning-struck tree: you are green and vigorous.”