The objective of this method paper was to examine a computational algorithm that may reveal how vessel length might depend on vessel diameter within any given stem or species. The computational method requires the assumption that vessels remain approximately constant in diameter over their entire length. When this method is applied to three species or hybrids in the genus Populus, vessel length is sometimes a linear function of vessel diameter and sometimes an exponential function of vessel diameter within a stem, based on R(2) values. Our results give within-species variation of vessel length versus diameter, and we compare this to between-species variation of mean diameter versus mean length.