O. Henry is the pen name of William Sydney Porter (1862 – 1910). The stories he wrote are known for their clever twist endings.
In this story, Johnsy has fallen ill and is dying of pneumonia. She watches the leaves fall from a vine outside the window of her room, and decides that when the last leaf drops, she too will die. While Sue tries to tell her to stop thinking like that, Johnsy is determined to die when the last leaf falls.