The mechanical behaviour of single roots being extracted from soil was modelled as a process in which tension is transferred from the upper regions of the root to the soil via shear. Quantitative predictions were made about the extraction forces and the shape of the uprooting curves, and these were tested using leek radicles of different lengths in soil of two different strengths.Results of uprooting tests were qualitatively similar to the predictions. The pullout resistance rose with root length, until the breaking strength of the root was reached, at around 30 mm: longer roots all broke before the tip was stressed. In whole root systems, therefore, failure will occur proximally before the line distal roots are mechanically stressed, so these can have no anchorage function.Resistance to an upward force will be most economically achieved by having many strengthened proximal root axes, as in the adventitious root systems of grasses, sedges and stoloniferous dicots. © 1990 Annals of Botany Company.
