Light is one of the most important environmental factors influencing the cambium activity and consequently the formation of the secondary xylem. Its deficiency more or less inhibits the lignification of cell walls and also the formation of conductive tissues represented by the vessel system. From these results, if compared with those of bent stems, the conclusion may be drawn that some growth substances, namely the indoleacetic acid are the most important metabolites taking part in the formation of the quality of secondary xylem determined by the action of light. The importance of the total leaf area is assumed, acting as a perceptor of external light impulses and balancing to a certain extent the metabolic supply of the cambium by its own inertia.