Effects of 2,4-Dichlorophenoxyacetic Acid and 2,4,5-Trichlorophenoxyacetic Acid on Mitosis in Allium cepa

The effects of 2,4-D and of 2,4,5-T on mitosis in Allium cepa are proportional to concentration and can be grouped as physiological and structural effects. Concentrations as low as 25 p.p.m. are effective, and at 3500 p.p.m. mitosis ceases. 2. Physiological effects such as stickiness, condensation of chromosomes, and delay in spindle formation are seen immediately after 2-hour treatments. 3. Structural effects include chromatid breaks, produced by 2- or 4-hour treatments with high concentrations, and chromosome breaks produced by all concentrations. 4. These effects closely parallel the effects of X-rays. Similarities to the effects of other chemicals are discussed. Both 2,4-D and 2,4,5-T affect the nucleic acid cycle, and it is suggested that the effect on the actual chromosome structure is analogous with the action of radiations. 5. Recovery, or the reversibility of the effect of growth substances, has not been proved in this case. 6. Further investigation of these aspects may help to explain the mode of action of plant growth substances.