Further Evidence to Support a Nutritional Interpretation of Sclerophylly

Abstract

A statistical analysis is made of published data on the leaf composition of eighty-nine species, covering both sclerophyllous and mesophytic types of leaves, from North and Central America, Africa, and India. Expressing degree of sclerophylly by the ratio crude fibre dry weight × 100/crude protein dry weight, the analysis shows: (1) that in terms of organic matter an increase in sclerophylly is associated with a decrease in percentage protein content and with an increase in percentage fibre content, and (2) that in terms of mineral matter a decrease in phosphorus content below about 0.3 per cent. results in a proportional increase in degree of sclerophylly. These two results are considered complementary. Since phosphorus is essential for protein synthesis, the observed protein deficiency of sclerophyllous leaves can be correlated with their low phosphorus content. It is also reasonable to expect the observed reciprocal relationship between protein content and fibre content on the basis of a common pool of intermediate metabolites. The above conclusions support the author's hypothesis, suggested in a previous paper, that a sclerophyllous leaf is the expression of a metabolism found in plants that can tolerate low levels of phosphate. Strong circumstantial evidence is also provided by the fact that phosphate deficiency is characteristic of the soils, either wet or dry, which carry sclerophyllous vegetation. The hypothesis thus provides a ready answer to the apparent paradox of why the same specialized type of leaf should occur in both wet and dry habitats.