Plants respond to their environment by adjusting physiological and phenotypic traits, which has been called plasticity. In this thesis it has been investigated how variation in plastic responses affect plant growth while competing for light. The focus was on the shade avoidance responses that plants express in response to changes in the light environment. Detailed plant experiments illustrated that different parts of the plant differ in their sensitivity to changes in the light environment, and this also induces different responses. 3D model simulations and game theoretical analysis illustrated that subtle differences in shade avoidance responses have strong effect on plant competitiveness, and that selection for variation in these shade avoidance responses depend on the environment itself. Eventually, the thesis shows how combining plant experiments, 3D modelling and evolutionary theories can give insights on how natural selection could have affected variation in plastic responses of plants.