Words in context

The following text is adapted from Nathaniel Hawthorne’s 1837 story “Dr. Heidegger’s Experiment.” The main character, a physician, is experimenting with rehydrating a dried flower.

At first [the rose] lay lightly on the surface of the fluid, appearing to imbibe none of its moisture. Soon, however, a singular change began to be visible. The crushed and dried petals stirred and assumed a deepening tinge of crimson, as if the flower were reviving from a deathlike slumber.

As used in the text, what does the phrase “a singular” most nearly mean?