Sclerophylly: Primarily Protective?

The leaves of plants show an almost infinite variety of form although most leaves are essentially organs utilized for the same function, photosynthesis. Analyses of the diversity of plant leaf form have revealed the presence of one major trend in the variation, i.e. from large, relatively thin and soft leaves to small, thick and tough ones (Givnish 1987; Reich, Walters & Ellsworth 1992; Turner 1994), although there is a great deal of scatter about any regression line. The subject of this paper is the leaves at latter end of the spectrum which I will call sclerophylls, using the term in its literal, descriptive sense of 'hard leaves'. Soft leaves will be called malacophylls. The sclerophyll is the predominant leaf form in vegetation occurring over a wide geographic and climatic range. This has been recognized for at least the last century (Seddon 1974) but the functional significance of sclerophylly remains controversial (Cody & Mooney 1978; Grubb 1986). Hypotheses concerning the significance of sclerophylly fall into three categories. It has been argued that sclerophylls are primarily adapted for water conservation, nutrient conservation or the prevention of damage. I believe that critical examination of the data available indicates that the anatomical features typical of sclerophylls serve a fundamentally protective function. The reasons for this statement will be discussed after reviewing the characteristics of sclerophylls.