Cover: Variation in leaf chlorophyll content represented by senescing leaves of muscadine grape (Vitis rotundifolia Michx.). Responsive to a variety of environmental stressors, chlorophyll content can provide a relative indication of plant vigor. Leaf optical properties in key wavebands correlate strongly with chlorophyll concentration, and thus may prove valuable in evaluating environmental quality. See Carter and Spiering, "Optical Properties of Intact Leaves for Estimating Chlorophyll Concentration," p. 1424-1432. Photo by Gregory A. Carter, NASA, John C. Stennis Space Center.