Abstract
Summary: With certain precautions the “disappearance” of guard cell chloroplasts may serve as a quantitative measure of ultraviolet light damage to leaves.Irradiated French bean leaves are still appreciably photoreactivable after 30–60 min in darkness, but eventually they cease to be. This critical time is the same as with irradiated viruses photoreactivated in their hosts.Guard cells seem much less susceptible than other epidermal cells to damage by ultraviolet light because ultraviolet light doses 15 times as great as those that cause collapse of ordinary epidermal cells do not abolish reversible osmotic closure of stomata.