AbstractPhotosynthesis, respiration, net assimilation, chlorophyll b concentration, and levels of potassium, calcium, magnesium and iron were measured in 2 cm shoot segments ofPleurozium schreberiin order to examine patterns of physiological decline with ageing in a continuously growing species. Photosynthesis and chlorophyll b levels declined rapidly after the apical shoot segment and were undetectable after a distance of 8 cm from the shoot apex. Respiration rate also fell rapidly at first but then declined more gradually and some respiration was detected in all shoot segment samples. Potassium and magnesium levels declined approximately linearly in relation to segment number. Calcium levels, expressed on a concentration per unit weight basis, became progressively higher in older segments but the calcium content per segment may actually decline with age. The disparity may be due to a decline in the proportion of cytoplasm compared with cell wall material in older segments.No clear division could be made between living and dead shoot segments. Different physiological processes decline at different rates as ageing occurs and the importance of the age structure of shoot samples is discussed in relation to design of physiological experiments.