A COMPARATIVE-STUDY OF THE FRACTURE PROPERTIES OF 5 GRASSES

1. The mechanical properties of plants constrain herbivory. Fracture properties of five species of grass were measured using tests which fractured leaves, pseudostems and whole tillers in tension (mode 1) and in out of plane shear (mode III). Force (F), energy to fracture (U) and energy per unit area (U/A) were calculated from force-displacement curves. 2. Fracture properties varied widely, depending on species, morphological unit, sclerenchyma or fibre content, and whether the grass was fractured in tension or in shear. 3. Older leaves contain more sclerenchyma than younger leaves, resulting in higher F, U and U/A. 4. Pseudostem is tougher and stronger than leaf whether fractured in shear or in tension, owing to its complex structure and capacity for plastic deformation, larger cross sectional area, and higher fibre content. 5. Entire tillers tested under tension required much less energy to fracture than leaf or pseudostem. 6. Fracture occurs at a zone of weakness at the intercalary meristem and it is argued that this is an evolutionary advantage to grass species which are commonly grazed. The fibre content of grasses increases the difficulty both of harvest and digestion by herbivores.