Several green, photosynthetic plants in orchids and Ericaceae were recently found to recover carbon from the mycorrhizal fungi associated with their roots, a dual nutritional capability called mixotrophy. The physiological and cellular processes allowing carbon gain from the fungus are not well understood. We believe that this phenomenon is overlooked and propose several land plant families and ecosystems that should be investigated for possible mixotrophy. We speculate that mixotrophy allowed, in some lineages, the evolution of heterotrophic plants, that is, non-photosynthetic plants that obtain their carbon from organic compounds. Moreover, the amount of carbon gained from the fungus varies from one site to another in mixotrophs. Drawing a parallel with mixotrophy in planktonic algae, we propose some hypotheses that could account for this.