Light micrograph of pine stem, 2011. Light micrograph, two-colour fluorescence. by Lauren Piedmont, a technician at at Harvard Medical School in Boston, MA.

In this cross section of a pine branch, the irregular green line defines the craggy edge of the pine's bark. A single row of larger, dark vascular cambium cells shows the dividing line between the Chartreuse-coloured wood and the orange tinged bark. The British scientist Sir George Stokes described and named fluorescence in 1852, observing that mineral fluorite emitted red light when illuminate by ultraviolet excitation, and that the fluorescence was alway at a longer wavelength than the excitation light. In the later nineteenth century, experiments with specimens showed that they fluoresce when irradiated with ultraviolet light. But it was not until the 1930s that the potential for using fluorochrome in biological investigations was fully understood.