Quantitative results of an earlier paper by Wuenscher (1970) are corrected, and the implications for the adaptive significance of mullein leaf hairs discussed. Wuenscher's paper seemed to show that the leaf hairs thickened the leaf boundary layer by a distance equal to the thickness of the hair layer, causing boundary layer resistance to increase by a factor of 90. The corrections indicate that the hairs increase boundary layer resistance by a factor of about 2.
The results imply that measured differences in total transpiration resistance were due to higher stomatal (or cuticular) resistance of normal hairy leaves compared with shaved leaves. Such higher epidermal resistances would tend to reduce photosynthesis. The hypothesis is made that plant species having dense leaf hairs and growing in full sunlight are likely to have high temperature optima for photosynthesis. A related hypothesis is that such species are likely to use the C4 pathway for carbon assimilation, as mullein apparently does. Finally, it is pointed out that dense leaf hairs increase boundary layer resistance to a value equivalent to that of larger glabrous leaves. Thus, the hypotheses suggested should also apply to plants having large leaves and growing in full sunlight.