Many cytological studies have been carried out to detect the harmful effect of different pesticides on different plants (Grover and Tyagi 1980, Njagi and Goplan 1981, Mousa 1982, Reddy and Rao 1982, Soriano 1984, Amer and Farah 1985, Amer and Ali 1986, EL-Khodary et al. 1987.The herbicide 3, 5, 6 trichloro-2-pyridinyloxy-acetic acid, is also known as Garlon-4 . The active chemical ingredient in Garlon herbicide is triclopyr.In both the laboratory and the field, triclopyr has demonstrated highly effective control of many woody plants and broad leaf weeds.The present investigation was carried out to study the cytological effect of the herbicide Garlon-4 on the process of mitosis in root tips of Allium cepa. Materials and methodsPure strains of bulbs of Allium cepa (variety Giza-6) were used as plant test material. Roots while intact, were treated with different concentrations of the herbicide, Garlon-4, ranging from 28ppm to 38400ppm for 3, 6, 12 and 24 hours. The beakers containing Gest solutions were agitated after every 30 minutes since the chemical left a settling suspension.Roots were fixed immediately in acetic alcohol (1:3) for 24 hours.Examination of the roots was done in permantent root tip squash preparation stained by the Feulgen technique.Three replicates were preformed for each treatment and the control, and scoring was made from three roots of each replicate. The mitotic index was calculated for each treatment as a number of dividing cells/100cell. The data obtained from the different treatments were statistically analysed. Results and discussionThe herbicide in the present study caused a change in the frequencies of the different stages. Their frequencies depend on the duration of treatement and concentration of the chemical ap plied. The frequency of prophase increased after treating the root tips of Allium cepa with low concentrations.With higher concentrations and long periods of treatement, the frequency of prophase dorpped and an increase in the metaphase frequency was noticed (Table 1). These changes in the stages of mitosis indicate that Garlon affects the relative duration of each stage

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