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Number of cambium cells and tracheids observed in different maturation stages (radial enlargement, thickening, maturing) of P. sylvestris and P. nigra trees grouped in three defoliation classes (D, severe defoliation; I, intermediate defoliation; N, scarce or no defoliation). In the lowermost graph the bars correspond to the number of latewood tracheids. Means are given with standard errors (n = 5 trees for each defoliation class).
