Evidence obtained on the relation between the pH of the medium and the growth of intact stem sections is compatible with the acid-growth theory only if the proton conductance of the cuticle is so low that the cuticle is an effective barrier to the entry or exit of protons from the tissue. By measuring the rate at which protons cross frozen-thawed epidermal strips of sunflower (Helianthus annuus L.) and soybean hypocotyls (Glycine max Morr.) and enzymically isolated cuticles of Berberis aquifoliun Persh. and tomato (Lycopersicum eseadentum Mill.) frlit, we have now demonstrated the low proton conductance of the cuticular layer. Unless the conductance is enhanced by abrasion of the cuticle or by removal of the cuticular waxes, proton movement into and out of a tissue across the cuticle will be signlifcant only over long time periods.The aerial portions of all higher plants are covered by a cuticle, which is believed to act as a barrier to the entry and exit of materials from tissues (8). The ability of the cuticle to limit movement of cations such as K+ (7, 21), organic substances (4), and water (17) has been established, but the proton conductance2 of a cuticle has been examined only after long periods (3). Knowledge about the proton conductance of the cuticle is of importance in regard to the acid-growth theory of auxin-induced growth (15).The acid-growth theory states (3,15) that auxin causes the cells to excrete protons into the wall solution where the resulting increased acidity activates wall-loosening enzymes. The excreted protons might be expected to acidify the external solution, too, but it has been difficult to demonstrate any auxin-induced acidification (11,12,19) unless the cuticle is abraded (3, 9, 16) or removed (1, 13). This led us to suggest (1, 13, 15) that proton conductance of the cuticle is so low that protons excreted into the wall solution are trapped within the tissue unless they escape through cut surfaces. A low proton conductance would also explain why, in order to achieve a maximum rate of acid-induced growth, it is necessary to add a 50-fold greater proton concentration to intact A vena coleoptile sections as compared with sections whose cuticle had been removed (13 the cuticle may not be a barrier to protons. In this study, we have made use of a simple assay that allows us to assess the ability of protons to cross cuticular layers, and we show for the first time that the cuticle is an effective barrier to the entry and exit of protons. The cuticle of some seedlings was abraded by rubbing the hypocotyl five to ten times with a slurry of emory powder (American Optical, Seattle, WA, grade M180) before isolation of the epidermal strips. Other seedlings were wiped twice with a 3:1 (v/ v) mixture of chloroform:ethanol to remove some of the cuticular waxes. MATERIALS AND METHODSLeaves of Berberis aquifolium Dursh. were selected from a plant growing outside the laboratory. To isolate the cuticle, leaf pieces were infiltrated and incubated for at least 2 days with a solution containing...