Hypotheses are created regarding the control of size scale and geometry of softwood tracheids within a genotype, and tested using two independent Norway Spruce (Picea abies) materials. Variation in physical dimensions of tracheids is controlled predominantly by cambium maturity. The accumulated amount of production appears to be a somewhat better descriptor of cambium maturity than cambium age. Thus, the mean tracheid dimensions within an annual ring can be predicted on the basis of the distance from the pith of the tree, at least as well as using the number of the annual ring. Cross-sectional tracheid geometry appears to be controlled by the growth rate of the tree.