Symbolism, Imagery, Allegory
When Anne goes to sleep, the nature outside her window's almost always mentioned. Anne can see plenty of farm through her window, so even when she's inside, she always has a sense of the vast natural world that surrounds her home.
Know what else Anne can see through her window? Diana's bedroom window. (Yes: this is one of the reason that femslash writers love the Diana/Anne friendship.) The windows are their connections to each other when they aren't together. They even have a signal system where they pass cardboard over their candles to send each other messages—sort of like Morse code.
The final image in the book is also of what Anne sees in her window:
The wind purred softly in the cherry boughs, and the mint breaths came up to her. The stars twinkled over the pointed firs in the hollow and Diana's light gleamed through the old gap. (38.62)
Everything Anne sees is familiar, even friendly. Wind "purrs" like a cat, and the mint breaths travel to her. Lucky Anne—she gets to commune with nature from the warmth of her room.