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It has long been known that ethylene and certain other unsaturated hydrocarbon gases have the property of stimulating the ripening of various fruits. Recent data indicate that similar effects can be produced by several of the synthetic plant-growth hormones. Thus, the maturation and ripening of certain varieties of apples and pears have been observed to be directly influenced by naphthalene acetic acid when used as a spray to retard harvestdrop (1, 2). MITCHELL and MARTH (6) found that the time required for the ripening of green bananas and freshly-harvested apples and pears was reduced by treatment with 2,4-dichlorophenoxy acetic acid. These treatments were applied to fruits in the pre-climacteric stage when the quantity of ethylene metabolized by the tissues is known to be extremely small (4,5). No data are available to indicate their effect in the presence of physiologically active quantities of ethylene, such as occur in post-mature and storage fruits. In the present study, the comparative effects of 2,4-dichlorophenoxy acetic acid on the respiration and ripening of Bartlett pears, which vary in stage of maturity and capacity for ethylene production, are considered. Materials and methodsTwo collections of pears were made from a single tree located at the Experiment Station orchard near Corvallis. The first sample was picked on August 14 and represented fruit in an immature stage of development. The second sample was collected two weeks later when the fruit was at the optimum stage of maturity for commercial use. A portion of this collection was stored at 310 F. for five weeks prior to treatment.The method used in treating pears with 2,4-dichlorophenoxy acetic acid consisted of immersing the fruit in an aqueous solution containing 1000 p.p.m. of the reagent and one per cent. Carbowax. In some experiments other dilutions were used as indicated later. The treated fruits were allowed to dry before transferring to the respiration chambers.The rates of carbon dioxide and ethylene production were determined by the methods described previously (3).Results and discussion IMMATURE FRUIT Immediately after picking, the pears were divided into four uniform lots, each consisting of 15 fruits. Lot 1 was treated with 2,4-D and then con-

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