Background and Aims: Cotton shows a marked plasticity vs. density in terms of branch development and geometry, internode elongation and leaf expansion. This paper proposes interpretations for observed plasticity in terms of light quantity and quality. Methods: 3-D virtual plants were reconstructed from field observations and 3-D digitization and were used to simulate the light regime in cotton stands of different densities. Key Results: All densities showed the same linear relationship between LAI and the sum of light intercepted by the canopy, from seedling emergence up to flowering. Simulated R : FR ratio profiles can very likely explain (1) the longer first internodes on main stem and branches and (2) the azimuthal re-orientation of branches toward the inter-row. Conclusions: Simulation tools were used to analyse plant plasticity in terms of light quantity and quality. The methodology applied here at the stand scale will now be continued at the plant scale to further strengthen the above hypotheses.