The radial and cross sections of wood samples from individual trees of known age of African Acacia species were examined for growth rings. These were apparent in most species as narrow bands of marginal parenchyma filled with long crystal chains. The crystals were subsequently identified as calcium oxalate through the use of a scanning proton microprobe. Several other chemical elements were concentrated around this zone. The number of parenchyma bands formed annually corresponded to the number of peaks in the annual rainfall distribution. These results suggest that the presence of marginal parenchyma bands and crystalliferous chains define growth phases in African Acacia species, and can be used for age determination.
Gourlay, I.D.; Grime, G.W. Calcium oxalate crystals in African Acacia species and their analysis by scanning proton microprobe (SPM). Iawa Journal (1994) 15 (2) 137-148.