The influence of illumination level during leaf development on the mesophyll cell surface area per unit leaf area (Ames/A), CO2 resistances, and the photosynthetic rate was determined for leaves of Plectranthus parviflorus Henckel. The relative importance of A"'eS A versus C02 resistances in accounting for observed changes in photosynthesis was quantitatively evaluated using equations based on analogies to electrical circuits.When the illumination during development was raised from 900 to 42,000 lux, the leaves more than tripled in thickness as the mesophyll cells increased in size and frequency, which caused Asses/A to go from 11 to 50. Ignoring respiration and photorespiration, the net rate of photosynthesis per unit leaf area (JC02) is Jco2 = co2/Rco2 = cco2/(1.56 R,, + Rc02) (1) where cCO2 is the CO2 concentration outside the leaf, and RCO2 is a total resistance for CO2 fixation expressed per unit leaf area (3,4,8,13,15,16,20). As equation 1 indicates, RCO2 has a gaseous phase component in common with the water vapor pathway, the factor 1.56 accounting for the ratio of the diffusion coefficient of water vapor to that of CO2 in air at 20 C (16). In the liquid phases, the resistance to CO2 movement (RC2) is composed of contributions from cell walls, plasmalemmas, cytoplasm, chloroplast membranes, and the resistance associated with the carboxylation reaction (3,4,8,14,15,20). This internal resistance, expressed per unit leaf area, is related to the resistance per unit area of mesophyll cells (R"j,) as follows:The internal leaf morphology of many plant species varies from that characteristic of shade leaves at low light levels to that of sun leaves when development occurs at illuminations approaching full sunlight (1,7,(10)(11)(12)(21)(22)(23)(24). Not only do sun leaves tend to have more highly developed palisade and spongy mesophyll regions than shade leaves, but also they have higher photosynthetic rates at light saturation (1, 2, 6, 10). The higher assimilation rates have been suggested to result from changes in activity of enzymes involved in photosynthesis (1,2,9,10,14,18) and to variations in the number of chlorophylls per photosynthetic unit (1, 2, 9, 17). However, the higher rates of photosynthesis could also be a consequence of the changes in internal leaf morphology caused by illumination, a matter which apparently has not been systematically investigated. This MATERIALS AND METHODSCuttings from a single parent plant of Plectranthus parviflorus Henckel, a member of the Labiatae commonly known as "Creeping Charlie," were grown at 20 C and 55%c relative humidity for 5 weeks in sterilized soil. The indicated illumination was provided for 12 hr each day using warm-white fluorescent tubes and neutral density screens (15,000 lux corresponded to 29 neinsteins cm-2 sec-I between 400 and 700 nm). Third node leaves approximately 15 cm2 in area were used for measurements.The Ames A ratio was determined using drawings prepared with the aid of a camera lucida using an over-all magnification of 300 X. S...

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