The fracture toughness of the leaf of the dicotyledon Calophyllum inophyllum L. (Guttiferae)

Abstract

The fracture toughness (specific work of fracture) of the leaf of the dicotyledonous angiosperm, Calophyllum inophyllum L. (Guttiferae), which has a lamina with a reticulate venation comprising secondary and tertiary veins only, was investigated by cutting, notched tensile and punch-and-die tests. Toughness was found to depend on the presence of veins in the fracture path, with both tensile and cutting tests in agreement that fracture at right angles to secondary veins was 2.5-3.0 times as tough as fracture parallel to them. Values from tensile tests were smaller than those from cutting tests. The cutting method had the advantage of specifying the direction of fracture and of severing structures serially, allowing easy recognition of tough structures from force-displacement records. This showed that the fracture toughness of the lamina could be modelled by a ‘rule of mixtures’ whereby the veins (vascular bundles supported by sclerenchyma strands above and below), with a combined toughness of about 6000 J m -2 , are contrasted to other tissues - the mesophyll and the epidermis and its cuticle — which together probably have an average toughness of only 220-300 J m -2 . Bounds on the toughness of mature leaves are probably set close to these limits. However, we could not test all these tissues separately to confirm this. The model predicted the fracture toughness expected in a cylindrical punch-and-die test, which is the commonly used test in ecological studies. However, observed values from this test were twice those expected, which is mostly attributable to yielding of a large volume of mesophyll but also in part due to the debonding of secondary veins from the rest of the lamina. The results suggest that relevant classifications of leaves (sclerophyll, pachyphyll, etc.) should pay close attention to the venation of the leaf and the structure of these veins. It is proposed that the thickening of the walls of smaller veins to form a venous network is a defence against invertebrate herbivores.