Leaf aging, once associated with the disintegration of leaf metabolic functions, is increasingly viewed as a controlled disassembly, freeing leaf constituents for investment in other parts of the plant. This dissertation examines leaf aging in a drought-deciduous chaparral shrub, Lepechinia calycina (Benth.) Epling, with emphasis on the implications of leaf aging for the photosynthetic carbon gain of whole plants. The thesis is organized into four chapters, each of which is abstracted briefly here. Chapter 1 describes the design and construction of a fully portable, steady-state gas exchange system for measuring the carbon dioxide exchange and transpiration of single, attached leaves. The leaf cuvette provides temperature, humidity, and carbon dioxide concentration control. Portability allows the study of leaf aging in natural habitats while high accuracy permits the identification of subtle leaf-age and seasonal effects. Chapter 2 describes seasonal and leaf-age effects on the structural, photosynthetic, and water-use characteristics of L. calycina. Though photosynthetic capacity decreased as leaves aged or as the season progressed, most of the changes could be attributed to linear effects of increasing leaf specific weight, decreasing leaf-nitrogen concentration, and variation in stomatal conductance. The quantum yield of photosynthesis, a measure of photosynthetic efficiency, and the stomatal response to vapor-concentration gradient, a measure of sensitivity to environmental factors, were unchanged by leaf-age or seasonal effects. The ensemble of leaf-age effects is consistent with the hypothesis that aging represents resource redistribution rather than an uncontrolled deterioration. Chapter 3 demonstrates the qualitative similarities between leaf aging and shade acclimation and establishes that leaf aging tended to increase net photosynthesis at low light intensities. With leaf aging, increased photosynthesis at low light was due to reduced respiration. Leaf nitrogen content appeared to control respiration and maximum photosynthesis in parallel. Nitrogen contents consistent with high photosynthesis at low light intensities were inconsistent with high photosynthesis at light saturation. Simulations indicated that, except on very bright days, the total carbon gain of an actual leaf-age series should be greater than that of a hypothetical series without physiological aging. Chapter 4 considers the adaptive significance of leaf aging in terms of photosynthetic carbon gained relative to the costs of leaf construction, maintenance, and supply. The model developed here identifies the distribution, among leaf ages, of photosynthetic capacities and stomatal conductances that maximizes whole-plant carbon gain, assuming that the total supply of nitrogen and water is fixed. The optimum resource distribution corresponds closely to that observed in nature.