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Conducted from April to July 2009, this study determined the effect of hot water extracts of medicinal plants Centella asiatica, Plantago major, Sarcandra glabra and Smallanthus sonchifolia on the cells of the Allium cepa L. root tips particularly on mitotic index. The study found out the possible cytological effects of the extracts on the occurrence of chromosome abnormalities, and determined the effects of the extracts on the gross morphological parameters of the roots.Onion root tips treated with C. asiatica, P. major, S. glabra and S. sonchifolia, plant leaf extracts showed high mitotic indices in the metaphase stage, 101.3%, 113.0%, 139.0% and 144.7%, respectively. Cells failed to divide at telophase as proven by mitotic index, 28.0% (C. asiatica), and 27.7% (P. major), 26.0% (S. glabra) and 18.3% (S. sonchifolia) respectively compared to the control with 49.3% indicating inhibition of cell division. Likewise, the plant extract-treated onion cells exhibited some abnormalities in the chromosomes such as: c metaphase or c-mitosis in S. sonchifolia, S. glabra and P. majortreated cells; and anaphase chromosome bridges in C. asiatica-treated cells. Extracts from C. asiatica and P. major showed lower degree of cytotoxicity as compared to that of S. glabra and S. sonchifolia. Comparing with the normal cell growth and development in the control (distilled water), macroscopic abnormalities were also observed such as root swellings or c-tumor in C. asiatica-treated root.