For years scientists and laymen alike have casually noted the impact of solar ultraviolet radiation upon the nonhuman component of the biosphere. Stratospheric ozone functions effectively as an ultraviolet screen by filtering out solar radiation in the 220-320 nm waveband as it penetrates through the atmosphere, thus allowing only small amounts of the longer wavelengths of radiation in this waveband to leak through to the surface of the earth. Although this ultraviolet radiation comprises only a small fraction (less than 1%) of the total solar spectrum, it can have a major impact on biological systems due to its actinic nature. Many organic molecules, most notably DNA and proteins, absorb UV-B radiation which can initiate photochemical reactions. The defense mechanisms that serve to protect both plants and animals from current levels of UV-B radiation are quite varied.