"The Last Leaf" is written in the naturalistic style, which emphasized social conditions as the primary influence on human behavior, and strove to depict those environments with a degree of realism. Naturalistic fiction often depicts characters overcome by forces outside themselves. In "The Last Leaf," O. Henry uses personification as a key stylistic element in expressing the relationship between his characters and pneumonia, the most poignant outside threat to their safety. The figure of “Mr. Pneumonia” is invested with tremendous power; he is described at the story’s inception:
In November a cold, unseen stranger, whom the doctors called Pneumonia, stalked about the colony, touching one here and there with his icy finger.
In the urban jungle of New York City, pneumonia functions as the predator: he “stalks” his victims through the city. He is an invisible (“unseen”) threat that holds unchecked power over the characters, lifting them away at the mere touch of his finger. He completely rearranges the lives of those he does not kill outright (consider how Behrman, prior to his diagnosis, and Sudie, all through the narrative, must dedicate an enormous amount of time and energy to helping save Johnsy).
Even descriptions of Greenwich Village itself use personification to help develop a sense of chaos and unease. The streets are said to “have run crazy and broken themselves into small strips called ‘places,’” creating a habitat that is literally unnavigable to its inhabitants. The neighborhood itself has abandoned its reason, broken itself apart; the conditions of Greenwich Village do not support its residents in any way, and they are made to contend with them without institutional support.
Naturalistic works were also known to eschew filigreed or excessively lyrical writing. The style of "The Last Leaf" relies on straightforward use of detail to paint a realistic image of these characters' lives. The text is full of specific and timely references that would have resonated with readers in early twentieth century New York. Johnsy and Sue meet at the “table d’hote of an Eighth Street Delmonico’s”; they bond over their mutual love of “chicory salad and bishop sleeves." The artists of Greenwich Avenue are searching for “north windows and [...] Dutch attics,” moving into the neighborhood with their “pewter mugs” in hand. These details lend the story a sense of believability and immediacy; they pull the reader into the specific world of the characters.
