A detailed quantitative analysis of the three-dimensional organization of the mesophyll was performed, and mesophyll diffusion resistance to CO 2 in the leaves of Chamaerion angustifolium formed under different irradiance was calculated using an original method of stereometric cellular packing. For each type of leaves (sun and shade), we determined structural components of gas exchange: the volume of mesophyll per unit leaf area (V mes), the volume of the intercellular space in the mesophyll (V is), the area of the total mesophyll surface (S), the area of the free mesophyll surface facing the intercellular spaces (Smes), and the ratios of the total and the free mesophyll surfaces to its volume (S/V and Smes/V). As compared with sun leaves, in the shade leaves of Ch. angustifolium, S and Vmes decreased twofold, tissue density was reduced twofold, and the share of the intercellular space in the mesophyll rose from 49 to 72%. In shade, the diffusion resistance of the mesophyll increased by 1.8 times because of changes in the leaf structure. At the same time, the ratio Smes/V was found to increase by 1.4 times, which facilitated the diffusion of CO2. In the shade leaves of Ch. angustifolium, the diffusion resistance of the intercellular air spaces was reduced twofold as a result of an increase in their share in the leaf mesophyll and simplification of their geometry. Thus, the method of three-dimensional reconstruction of sun and shade leaves of Ch. angustifolium showed a comprehensive rearrangement of the mesophyll spatial organization in shade and revealed the structural mechanisms of changes in the resistance to CO2 diffusion within the leaf. © MAIK "Nauka/Interperiodica", 2006.