Sclerophylly: evolutionary advantage or mere epiphenomenon?

The possible functional significance of sclerophylly (hard and coriaceous leaves) is discussed on the basis of different interpretations reported in old and recent literature. These, have been schematically divided into four groups of hypotheses, i.e. the water conservation hypothesis, the resistance to negative turgour pressures hypothesis, the nutritional hypothesis and the protective hypothesis. In particular, a section is devoted to discussing the first and second hypotheses in terms of relationships of sclerophylly to drought resistance strategies. Data reported in the literature lead to the conclusion that the sclerophyllous habitus per se seems not to he related to drought resistance of plants as previously thought. Nor do clear-cut relationships appear to exist between leaf sclerification and living leaf cells with thicker walls, leading to a high modulus of elasticity (epsilon), the latter known to be of importance for generating low leaf water potentials, i.e. higher driving forces for water uptake. Because leaf sclerification has proved to be related to soil nutrient deficiency, this leaf habitus can be considered as a mere epiphenomenon of nutrient stress. On the other hand, the selective advantage of sclerophylly as protection against herbivore attacks, though not demonstrated in different environments, seems to be of great interest and deserves further studies.