characteristics of the upper leaf epidermis. A) In 'Stacked' images of upper epidermal cells of the Arabidopsis thaliana line FNR/EGFP-7-4, all plastids in a given cell are visible (cell boundaries given as grey line). For better display of stromules, the image has been gray scaled and inverted. Therefore, epidermal plastids are visible in dark grey (arrow) and the larger plastids from the palisade parenchyma appear light grey (arrowhead). Size bar corresponds to 10 μm. B) Histogram of cell size in the upper epidermis, illustrating the huge variety of cell sizes and the predominance of small cells in this tissue. Values given on x-axes are upper limits of size classes. The visible surface area, defined by the lateral cell walls, was used as a measure of cell size, (see A). C) Scatter plot and linear regression line of plastid number vs. cell size showing the strong linear correlation between both variables (r2 0.8016; p value < 0.0001). This underlines that cells of the epidermis can be very different in cell size and plastid number. D) Plot and linear regression line of stromule frequency vs. cell size class suggesting no significant correlation between the two parameters. Because of unequal class size, ' > 44376' has not been considered in the regression test. E) Box plot of stromule frequency found in 109 independent samples of untreated upper leaf epidermis. Specific parameters: maximum = 0.34, 90% percentile = 0.26, 75% percentile = 0.21, median 0.18, 25% percentile = 0.13, 10% percentile = 0.08, minimum 0.02.