Resistance added by hair layer
The resistance added by leaf hairs, r h, can be estimated from the difference in boundary layer resistance between bald and hairy patches, as described in Supporting Information Methods S4. Because estimating r h requires a value for an unknown parameter – the ratio (β ), between adaxial and abaxial leaf surfaces, of the component of boundary layer resistance that is not caused by hairs – we inferred r h at a range of values of β (Fig S4) and presented results below assuming a nominal value of β = 1. We compared the resulting values ofr h with theoretical predictions that assumed leaf hairs entrain a layer of still air equal to the depth (d h) of the hair layer; the diffusive resistance through such a layer would be r h =d hRT /D haP , where R is the gas constant (8.31446 Pa m3mol-1 K-1), T is temperature (298.15 K), D ha is the binary diffusivity for heat in air (2.36⋅10-5 m2s-1), and P is the atmospheric pressure (105 Pa). We measured d h by microscopy in fresh leaf sections for three leaves per species, as the distance between the exterior edge of the trichome layer and the leaf cuticle at the base of the trichome (Table 2). We measured trichome length in SOLY (genotype Wo ) from images in Gasparini et al. (2021); the results were consistent with another study on the same genotype (Zhang et al., 2020).