This work was carried out with central leaflets of the first trifoliolate leaves of common bean plants ( Phaseolus vulgaria L.), developed under different conditions of light, under the sun (average daily intensity of radiation = 620,000Wm 2 ) and under shade (intensity mean daily radiation = 9,078Wm 2). The plants, in a total of 20 in each treatment, were grown in pots containing soil whose chemical composition and amount of water were identical in both treatments. The measurements were carried out 10 days after the emergence of the first trifoliolate leaf. The quantitative effect of the sun and the shade was verified by evaluating the following parameters: leaf area, stomatal density and stomatal index of each epidermis, mesophyll thickness and central bundle vascular bundles. Comparing the extreme values obtained between leaves under sun and shade conditions, it was verified that the amplitude of variation, in all parameters, is higher in plants developed under sunlight. Also the measurements obtained in each of these parameters were always higher in sunny conditions, except the leaf area, whose average value was higher in shade conditions. The statistical analysis of the means obtained in each treatment revealed a very significant difference between leaves developed under the ground and under the shade.
