Fig. 3. Two-dimensional colour plots of the surface profiles of the test surfaces used in experiment 1. The plots were obtained using an optical surface profiler. The surface topographies can be inferred from the colour profiles and the Ra values (i.e. the means of the profile ordinates). Values of Ra (arithmetic means ± S.D.; N=3) are as follows: (A) glass microscope slide (Ra=0.034±0.002 µm), (B) photographic paper (Ra=1.09±0.2 µm), (C) dry leaf of Phragmites communis (Ra=7.95±0.4 µm) and (D) filter paper (Ra=8.96±0.6 µm). The dimensions (length and width) of the measured surface areas can be read off the x- and y-axes.