The mechanical stresses required to buckle root tips were measured directly for seven plant species. For two of these, both seminal and primary lateral roots were measured. For four of the plant species investigated, the easier and more rapid method used to measure the buckling stresses of excised root tips gave results which agree closely with those obtained for the growing roots of intact plants. Values of buckling stress were also calculated from previously determined elastic properties of roots. The calculated and measured values of buckling stress are of the same magnitude only, and comparison for ten root types fails to show any consistent relationship between these two methods. From the results from three plant species it has been possible to define empirical functions to account for the observed changes in root buckling stress with air gap length and water stress. Buckling stresses are not significantly affected by the concentration of nitrate in the growth medium.

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The elasticity of root tips was investigated in experiments of the static clamped‐beam type. The response was non‐linear. The results were interpreted in terms of a generalized elastic modulus M, and two parameters, α and β, which account for the non‐linearity. Seminal axes and primary lateral roots of 16 crop species grown in the laboratory show significant inter‐species differences but insignificant inter‐variety differences in the elastic properties. A smaller set of results for field‐grown roots with two tillage treatments shows that, in nearly all cases, the elastic modulus is smaller and the diameter is larger than with laboratory‐grown roots. Environmental factors such as water potential, and nutrition have significant influences on the diameters and elastic behaviour of roots.