Do veins and stomata patterns in angiosperms obey general rules?

Plants have evolved a huge variety of leaf forms, vein structures, stomata dimensions and behavior. Here we investigate whether general principles might be evoked to explain the distribution of leaf veins and stomata across species. We analyzed 20 angiosperms grown in different environments (alpine, middle latitude, tropics). We took 5-15 mm2 samples from the proximal, middle and distal part of a mature leaf of each species. Stomata imprints were collected using acrylic varnish on transparent tape; veins were highlighted by chemical treatment using NaOH. With a semi-automatic GIS-based procedure we identified the smallest leaf area completely enclosed by veins as a loop and we measured area, contour, stomata number, pore lengths for each loop. About 60-700 loops were measured for each species. Stomata density was species-specific (ranging 50-350 pore/mm2) and seemed not to change with position on the leaf nor with the order of surrounding veins. Notably, in each loop the number of stomata scaled almost isometrically with loop area (exponents ranging 0.9-1.0) (i.e. Ns~A1) and loop contour (0.9-1.2) (i.e. Ns~L1) in all measured species; moreover the scaling coefficients appeared related to pore length. This would suggest that a simple scale-free pattern has evolved regardless of species or environment.