The tracheids in conifer xylem both provide mechanical strength and transport water to the crown. Diameter and wall thickness of conduits influence xylem conductivity and protection against cavitation and implosion. There is a trade-off between these functions. We aimed to develop an optimality model and calculate the optimal value of the ratio between lumen diameter and secondary wall thickness when the compromise between the two functions is reached. The efficiency of conductance function was evaluated by the volumetric speed of water flow. The efficiency of mechanical function was evaluated by the area of wall cross-section. The calculated optimal ratio was compared to actual data for three species growing in Middle Siberia. In the examined rings there were found few tracheids with the optimum ratio but the straight line of the equation for optimal tracheid separated cells in the diagrams of wall thickness vs tracheid or lumen diameter into two clusters corresponding to early- and latewood. We proposed to define the latewood as tracheids in the outer layer of the ring with wall thickness-to-lumen diameter ratio more and the earlywood as tracheids in the inner layer of the ring with values of ratio less than the optimal value (0.25).