A Comparison of the Effectiveness of 2,4-Dichlorophenoxyacetic Acid and Four of Its Salts in Inducing Histological Responses in Bean Plants

1. Young kidney-bean plants were decapitated in the second internode. 2,4-Dichlorophenoxyacetic acid or one of four salts-ammonium, copper, calcium, or magnesium 2,4-dichlorophenoxyacetate-was applied to the cut surface in 0.5% concentration by weight in lanolin paste. Observations were made of material grown for a period of 30 days after treatment. 2. Gross response showed characteristic differences among the tumors. Treatments with 2,4-D and with the ammonium salt resulted in relatively deep, club-shaped tumors. Proliferation was entirely below the cut surface. The tumor induced by the copper salt was likewise formed below the cut surface but Was somewhat shallower. Those induced by calcium and magnesium salts were broadly flared at the cut surface and gradually tapered below. Growth above the cut surface occurred with both of the latter substances but to a more marked degree with the magnesium salt. 3. Histological responses in the early stages showed considerable similarity. In parenchymatous tissues of the inner cortex including the endodermis, of the pericycle, phloem, cambial zone, xylem, and rays, proliferation was initiated soon after treatment. Response induced by the magnesium salt was earlier and more marked than in other treatments, less so with the calcium salt, and least with the ammonium salt and with 2,4-D. 4. Study of the tumor as a whole disclosed a pattern of zonation for all treatments. At upper levels a zone of limited proliferation was found in the responses to 2,4-D and to the ammonium and copper salts. This zone was lacking in the other responses. A zone of major proliferation occupied the intermediate levels below the zone of limited proliferation in treatments with 2,4-D and with the ammonium and copper salts but was found at upper levels in the tumors induced by the calcium and magnesium salts. In response to the magnesium salt this zone Was only poorly represented. With all substances a root zone was formed at next lower levels, and below the roots a zone of minor proliferation terminated the tumor. Comparison on the basis of zonation disclosed a consistent sequence among the substances according to the characteristics of the responses induced: 2,4-D and the ammonium, copper, calcium, and magnesium salts. This, same series was duplicated in gross responses and in details of histological responses. 5. Maturation was earliest and most marked in the treatment with the magnesium salt and least with the ammonium salt and with 2,4-D.