Abstract.Net photosynthesis, photosynthetic electron transport, and leaf area density of photosynthetic units have been studied in developing, mature, and old leaves of seedlings ofBetula pendula. The photosynthetic quantum yield under light‐limiting conditions and the leaf area related rale of light‐saturated net photosynthesis were lower in developing than in mature and old leaves. Developing leaves also had more oxygen inhibition of photosynthesis, a lower pool size of plastoquinone in the electron transport chain, a lower chlorophyll content and a lower leaf area density of photosynthetic units than mature and old leaves. The photosynthetic properties of The oldest leaves resulted partly from acclimation to shade and partly from a different ontogeny to that of younger lea