It's notable that Sue and Mr. Behrman are described as peering “fearfully” out of the window at the withering vine. That they then look at each other without speaking is a further sign of how disturbed they are by the vine's decaying condition.

For both Sue and Mr. Behrman know all too well what's at stake. Johnsy, in her dangerously ill condition, almost knocking at death's door due to a particularly nasty bout of pneumonia, has said that she will hang on to life for as long as there is a leaf on the vine outside her window. But once the last leaf has fallen from the vine, then she will give up the ghost and pass away.

It's absolutely essential, then, that there should be at least one leaf left on the vine. Sue and Behrman are acutely aware of the fact that it is only a matter of time before the vine has been stripped of all its leaves. As they don't want that to happen, for obvious reasons, and as they can't hold back the forces of nature, they have no choice but to use the power of art to trick Johnsy into believing that there really is still a last leaf outside her window.