Fig. 2.

a Measured optical depth $\mathit{\tau }$ as function of the leaf water mass $M_W$. The experimental data (black squares) are linearly fitted (red straight line) as $\mathit{\tau }=C_1+C_2·M_W$. The linear fit has a coefficient of determination $R^2=0.31$; the coefficients are $C_1=3.2\pm 0.1$ and $C_2=2.0\pm 0.1{\text{g}}^{-1}$. b Measured optical depth versus $M_W{A}^{-1}$. The coefficient of determination is greatly improved with respect to the previous model, by the simple measurement of the leaf projective area A. In this case the linear model produces a best fit with $R^2=0.74$. There are two statistically significant parameters: the intercept $C_1=1.0\pm 0.1$ may be ascribed to residual scattering and absorption from the leaf dry mass and vapor, whereas the slope $K=0.33\pm 0.03{\text{cm}}^2{\text{mg}}^{-1}$ is linked to the effective absorption coefficient of water; according to Eqs. (5) and (6) ${\mathit{\alpha }}_{\text{eff}}=330\pm 30{\text{cm}}^{-1}$. If the intercept is set to 0 as proposed in Eq. (6), the absorption coefficient changes to ${\mathit{\alpha }}_{\text{eff}}=444\pm 2{\text{cm}}^{-1}$, and the model fit accuracy is reduced (see the green line in the graph)