RICE, STANLEY A. Department of Biological Sciences, Southeastern Oklahoma State University, Durant OK 74701-0609. - __Teaching principles of fluid dynamics using xylem and transpiration measurements.__

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