[en] The effects of nutrient limitation and γ-irradiation on tracheary element differentiation and cell division were investigated using single cells isolated from the mesophyll of Zinnia elegans. When the phosphate concentration of the medium was reduced to 10 μM (1/50 of Fukuda and Komamine's medium, 1980a), the frequency of cell division during 4 days of culture decreased, while the frequency of tracheary element differentiation was unaffected. γ-Irradiation with a dose of 92 Gy at 36 h of culture preferentially and thoroughly suppressed cell division without reducing the number of tracheary elements formed. The appearance of secondary cell wall thickenings was delayed by irradiation, but synchrony was maintained. Thus the Zinnia system previously reported [Fukuda and Komamine (1980a) Plant Physiol. 65 : 57] was improved to give a more useful system for the study of cytodifferentiation, in which tracheary element formation occurred from single cells without cell division. (author)