The mention of firm names or trade products does not imply endorsement or recommendation by the U.S. Department of Agriculture over other firms or similar products not men~ioned. We wish to thank Mr. C. E. eedham for drawing the diagram (Fig. 1). We thank Dr. C. G. Crawford (ARS, Peoria, IL) who coached us ably in the three-dimensional reconstructions. Ms. Lola Elam worked long and searched diligently for aflatoxin in the preliminary experiment. One of us (M. G. S.) again thanks the donor for the kidney.
Penetration pegs of Erysiphe graminis D.C. f. sp. hordei Em. Marchal are usually not impeded by normal papillae of barley coleoptiles, whereas oversize papillae are impenetrable to appressoria of the pathogen. We investigated the chemical composition of these papillae and the cell walls by classical histochemistry, in part to extend the fragmented knowledge of these structures and in part to find out if there are differences between normal and oversize papillae which would account for their different efficacies in resisting penetration. These papillae were indistinguishable from one another histochemically and contained protein, carbohydrate other than pectin, and a phenolic which was not lignin. We report also a definitive proof of callose in papillae. They do not contain cutin or suberin. The cell wall did not contain callose or cutin–suberin but did contain protein, pectin, and a phenolic (also not lignin). The results imply that different linkages between molecules in oversize papillae, or some other differences not revealed in this study, are responsible for their ability to prevent fungal penetration.
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