The mathematical model of plant growth based on the analysis of photosynthesis has been developed. In the analysis, the leaf was treated as a control system, where the photosynthetically active radiation is an input value of the system and the leaf or plant dry mass is an output one. Environmental factors which influence plant growth are treated as disturbances. Part I presents the theory for standard plant growth conditions. Theoretical and experimental results were compared for lettuce cultivated in a greenhouse and phytotron. Part II develops the model for nonstandard conditions.