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Resistance of apples (Malus domestica Borkh) to gas diffusion changed during fruit development. The resistance increased during fruit growth and maturation showing the highest value near harvest maturity. After harvest maturity, the resistance decreased rapidly and leveled off as apples continued to ripe on the tree. From microscopic time-course observations of the skin structure, it was conceivable that lenticels were differentiated from stomata and development of lenticel structure might contribute to the seasonal changes in the resistance. During growth, hypodermal cells beneath stomatal pores and trichomc bases cohered and formed cone-shaped blocks which were progressively cutinized. As fruit matured with continuous increases in resistance to gas diffusion, the cells in the cone-shaped block were collapsed and crumpled. Near harvest maturity, crumpled cell blocks were expanded with more collapsed cells, suberized, and forced up through the fissure as cork cambium appeared under the block. These developmental events coincided with the rapid increase in the fruit resistance to gas diffusion. After that, lenticels were enlarged with the formation of closing layers resulting in the protrusion of suberized cell blocks above surface level. These final events occurred at the late stage of maturation when the resistance decreased and level off. On mature apples, however, four types of lenticels could be classified according to hypodermal differentiation below the aperture. As apples matured and ripened on the tree, the frequency of more differentiated lenticel types increased.
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