On the relationship between the composition, morphology and function of leaves

1. It is common practice to express measurements of leaf composition and morphology using dry mass as a basis. Relationships established using these measurements are difficult to interpret because the leaves may have different liquid contents or fractional air space. 2, In a previous theoretical investigation, we showed that the liquid content of leaves was important and the following hypotheses were proposed: (1) the mass of nitrogen per unit mass of liquid is relatively constant within leaves and (2) the surface area to volume ratio of leaves is proportional to leaf liquid content. That investigation also proposed that fractional air spaces were important, because they confer plasticity in construction, with apparently minimal cost in terms of CO2 uptake. In this paper we use a set of comprehensive measurements to address the above hypotheses and to assess whether the fractional air space does vary in a consistent manner. 3, We found that the specific gravity of the non-gaseous fraction in the leaves we measured was within the range approximate to 0.9-3.0 and increased with the fractional air space in a regular manner. Consequently, the specific gravity varied over a much smaller range (approximate to 0.9-1.4). The specific gravity at high liquid contents approached unity but at low liquid contents it was variable. 4, The mass of nitrogen (N) per unit mass of liquid was found to be relatively constant (0.01 +/- 0.003, n = 76). 5. The surface area to volume ratio was positively correlated with the liquid content (R-2 = 0.87, n = 27). 6, The mass of carbon (C) per unit dry mass was relatively constant (0.49 +/- 0.04, n = 76). Because N was a constant fraction of the liquid mass but C was a constant fraction of the dry mass, the N:C ratio was positively correlated with the liquid content (R-2 = 0.76, n = 27). 7. A comparison with leaf measurements from several external databases confirm that the relationships described above are widely applicable. 8. It is concluded that the composition and morphology of leaves are linked and that leaf function is a consequence of that linkage. Both the liquids and air spaces are integral to leaf function and need to be considered in analysis of experimental results.