The Hagen-Pouiselle principle (which relates water flux rate to the fourth power of the radius of a pipe) has cross-disciplinary applications (in engineering and human and plant physiology) and is thus useful for teaching general laboratories in which we want the students to see connections between scientific disciplines. Students in a general botany measured frequency distributions of xylem diameters in bean stems. They then calculated total xylem conductance based upon these models: the sum; the sum of squares; the sum of cubes; and the sum of fourth powers, of measured diameters. They compared conductance calculations with actual transpiration rates, determined earlier by the instructor, using linear regression. The expected result was that transpiration would be most closely related to the sum of the fourth powers. However, all of the models were equally good at predicting transpiration, except the sum of diameters. This contradictory result led to an interesting consideration of the differences between a simple physical law and the complexity of biological processes.

Key words: xylem conductance transpiration teaching