The leaf of New Zealand flax, Phormium tenax, has been tested in tension to determine some of its fracture properties. The mechanical and fracture properties of the leaf are almost totally dictated by its high content of stiff, strong fibres, which are orientated parallel along the leaf. The shock waves generated when the fibres in the leaf break in tension have been found to interact with the fracture process, because fracture events always occur in phase with the waves. The forces and displacements generated by the waves are large enough to be significant during fracture. This may be why grazers such as sheep and geese snatch at grass rather than pull evenly as cows do: they are generating a shock wave which will aid fracture. The results also have implications for the fracture of unidirectional composites in general.