An analysis of photosynthesis for a leaf treated as a control system was carried out. Some similarities between selected functions of the leaf and certain mechanical and hydraulic processes were discussed, and on this basis a leaf analog and its mathematical model were elaborated. Properties of the model, which can be treated as a mathematical model for the growth of a young leaf or a young plant in comfortable conditions, were worked out with the help of automatic control theory. The theoretical results obtained were compared with experimental results for lettuce cultivated in a greenhouse.