Nature is so ingrained in her, it is an inherent part of her, so much that instincts are those which lead her life and make decisions for her.
Explanation:
Sylvia is a girl stuck between two worlds. On one hand, she's a girl born in the city but on the other, she never felt actually alive till she left out to the county with her grandmother. “Whatever reserves were lost to her, woodlands and summer-time, master! Bring your gifts and blessings and tell your puzzles to this desolate country child and she was deeply impressed by nature.
I'd say the answer is that she "operates on instinct". Nature is so ingrained in her, it is an inherent part of her, so much that instincts are those which lead her life and make decisions for her.
