Structure and Composition of Plant Cuticle in Relation to Environmental Factors and Permeability

1. A technique permitting the isolation and the quantitative assay of plant cuticle was developed. Through this method, cuticle layers of considerable area, apparently unaltered physically or chemically and of a high degree of purity, may be obtained. By means of the assay method, a number of aspects of plant cuticle were studied. 2. The deposition of cuticle is continuous until the leaf reaches morphological maturity; beyond maturity no further deposition occurs. Leaves grown in the sun produced heavier cuticles of greater wax content than did leaves grown in the shade. Temperature conditions under which plants were grown were shown to influence the deposition of cuticle and wax. The most cuticle was produced at a median temperature, and the greatest percentage of wax at a high temperature. Plants undergoing water stress produced cuticles containing a greater proportion of waxes than did plants with more favorable moisture conditions. 3. The permeability of cuticle to water in an in vitro system was directly correlated with the amount of impregnating waxes. The cuticle was impermeable to the passage of certain salts in solution in an in vitro system. In vivo experiments indicated that the nature of the substance sprayed and the presence or absence of stomates on a particular leaf surface were largely responsible for the amount and rate of penetration of sprayed substances into a plant. 4. From the experimental observations it appears that stomates act as the major portal of entry, regardless of the nature of the sprayed substance. Small aqueous and oil droplets were shown to be actively absorbed into a leaf when a plant is sprayed while the stomates are open and the leaves are under a slight water deficit.