LIGHT-HARVESTING AMONG PHOTOSYNTHETIC ORGANISMS

1. Examination of light absorption by photosynthetic tissues of 156 sp ecimens from 128 species of chlorophyll a containing photosynthetic or ganisms, ranging from single-celled cyanobacteria to trees, confirmed the existence of a universal law involving the asymptotic increase in light absorption with increasing chlorophyll a density. 2. The chlorop hyll a concentration of photosynthetic tissues decreased as the tissue s become thicker, thereby avoiding high areal chlorophyll a density an d inefficient light absorption. Light absorption per unit photosynthet ic tissue weight was strongly, linearly related to the chlorophyll a c oncentration, explaining the lower growth rates and higher light requi rements of thick, compared to thin photosynthetic organisms and the do minance of thin photosynthetic organisms in shaded environments. 3. Th ese results confirm the importance of the thickness of photosynthetic tissues as a major factor in light harvesting and photoadaptation, and help explain the strong dependence of plant growth on the thickness o f the photosynthetic tissues.