A method was devised to calculate the factor of safety (based on any criterion of failure) for any population of similar biological structures that must cope with mechanical loads. This method is based on a formula using empirically determined values for the maximum load a structure can sustain before it mechanically fails (load capability), the load the structure normally sustains (the actual load), and the means and standard deviations of these two kinds of loads. The method is illustrated for Psilotum nudum axes drawn from different levels of branching. Failure in bending or twisting was used as the criterion for mechanical failure. The actual loads, load capabilities, and the difference between the two loads measured in bending and twisting for representative axes in each branch level were normally distributed. Based on the formula, the factor of safety in bending and twisting was found to decrease rather than increase in a basipetal direction: distal axes had the highest factors of safety, while the most proximal axes in trusses had the lowest factors of safety. Nevertheless, all axes where mechanically reliable load-bearing members.

Key words: biomechanics, mechanical safety, stems