Just in case there's not room for 15-20 sentences, I provided links below to the actual story so that you can go over it.
"The Last Leaf" is a story about 2 female roommates and aspiring artists that live in Greenwich village. One of them, Johnsy, gets sick with pnemonia, and the doctor doesn't give her much of a chance. Johnsy's roommate, Sue, keeps watch over her and notices her counting. She asks why, and Johnsy replies that she is counting the "Leaves. On the ivy vine. When the last one falls I must go, too." Soon, there is only one leaf on the vine, and Sue is in despair that it will fall, and Johnsy will die. She expresses her fears to the unconventional and often grumpy old man that lives downstairs, Behrman. He comes up and sits with her for a while. The next morning, Sue opens the blinds to find the leaf still there, and Johnsy recovers. Later it is revealed that it was painted there by Behrman, who then, from exposure to Johnsy and the cold, dies of pnemonia. It's a classic O. Henry "surprise" ending, but a sweet commentary on not judging one by their outward personna.