How has Behrman proven himself a source of new life to Johnsy in "The Last Leaf"?

Behrman has at long last painted his "masterpiece," a painting that gives rebirth to Johnsy's spirit and will to live.

A poignant tale, O. Henry's "The Last Leaf" relates the story of two young aspiring artists who have come from Maine and California in May to the quaint Greenwich Village where they establish themselves in a studio. But, when the New York winter besets the Village, Johnsy, who is from warm, sunny California, is not acclimated to such weather. Consequently, she is stricken with pneumonia.

Despite the efforts of the physician and the tender care of her dear friend Sue, Johnsy continues to weaken and the frail young woman begins to despair of getting well. In fact, she tells Sue that she has been watching the leaves fall off the ivy outside her window: "When the last one falls I must go, too. I've known that for three days. Didn't the doctor tell you?"

Sue will not hear such despairing talk:

"Don't be a goosey. Why, the doctor told me this morning that your chances for getting well real soon were...ten to one!"

In truth, Sue is really worried, so she goes to Mr. Behrman, who lives on the ground floor and asks him what they can do to make Johnsy more positive. He accompanies Sue upstairs, and they find Johnsy sleeping. But, when they look out at the ivy vine, they stare at each other holding their breaths; then, Sue pulls down the window shade. Later on, after posing as a model for Sue, Behrman departs.

The next morning when Johnsy awakens, she asks Sue to roll up the shade. Reluctantly, Sue does so; however, despite the cold rain and wind of the previous night, one leaf yet clings to the window. Johnsy has a conversion from her despairing thoughts of dying:

"I've been a bad girl, Sudie," said Johnsy. "Something has made that last leaf stay there to show me how wicked I was.  It is a sin to want to die. You may bring me a little broth now, and some milk with a little port in it, ...."

When the doctor comes in the afternoon, he sees promise that Johnsy will get well. The next day Johnsy's condition is, indeed, much improved.

Unfortunately, however, Mr. Behrman died of pneumonia because on the night of the cold rain, he dragged a ladder outside, climbed it, and painted an ivy leaf onto the window. He then fell ill, and lay all night in his wet clothes. By the time the janitor found him, it was too late. 

Although Mr. Behrman's death was unfortunate, he had painted a masterpiece because, like all great art, it inspired the viewer: Johnsy. As a result, Johnsy's spirit felt a rebirth with the sight of it and she regained the will to live.