SUMMARYThe fracture toughness of sun-leaves of 42 tropical tree species growing in Singapore was measured along a 'least tough' path using an instrumented cutting technique. Punch-and-die (penetrometer) tests were also conducted. Intercostal material was analyzed for crude fibre and crude protein (total nitrogen concentration x 6-25) and the ratio of the two, the index of sclerophylly, calculated for each species. Leaf anatomy was examined and specific leaf area was calculated. Thirty-seven of the species were from three definable plant communities, namely mangroves, trema belukar (secondary forest on undegraded sites) and adinandra belukar (secondary forest on highly degraded soils).There was a highly significant positive correlation between fracture toughness and index of sclerophylly for the 42 species (R^ = 0 43, P < OOOl) brought about largely by a high correlation between toughness and crude fibre content. Fracture toughness was not correlated with lamina tissue dorsi-ventral thickness but high toughness was clearly associated with the presence of sclerenchyma bundle sheaths and sheath extensions. Analysis of the fracture path for 26 leaves of ten mangrove species showed a high correlation between the area fraction occupied by veins and fracture toughness if one species, Avicennia rumphiana, was excluded. The regression equation for the other nine species gave estimates of the fracture toughness of vein tissue and lamina matrix of 6053 and 327 J m~r espectively. Punch-and-die test results only correlated well with the cutting tests when values were divided by lamina thickness.The ratio of protein to fibre in a leaf (the reciprocal of the index of sclerophylly) has been established as a good predictor of leaf palatability and digestibility to generalist chewing folivores. The high correlation between fracture toughness and the index of sclerophylly for a large sample suggests that leaf toughness could be the factor used as a proximate cue for determining food quality in herbivore foraging strategies.

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Leaf colour, size and toughness were investigated in five plant species important in the diet of Macaca fascicularis in Singapore. Leaf colour and size were examined as potential visual cues for food selection, whereas toughness mirrored fibre content, the inverse of food quality. As leaves matured, they changed colour and toughened. Leaf lightness and yellowness were strongly negatively correlated with toughness, but variation in both the red-green axis of the CIE Lab colour space and leaf size were not. Leaves selected as food by the macaques were distinguished by being very light, yellow to slightly green. Some leaves were dappled with red. The literature suggests that these leaves are relatively rich in protein without being tough and therefore would be sought after by primates. We argue that leaf colour is an important indicator of the nutritive value of leaves. Trichromatic vision is an important advantage in finding those palatable leaves that are dappled red. These would appear dark to dichromatic primates and be deceptive by making leaves look older (lower in quality) than they actually are. This would decrease the perceived window of feeding opportunity for such primates who would be at a disadvantage in trying to find these leaves. It is possible that trichromatic vision in catarrhine primates may have originally evolved for the detection of red coloration in the leaves of shade-tolerant tropical plants, enabling the better exploitation of a food resource.

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.

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