To test the hypothesis that xylogenesis is coupled to cell growth suppression, cell expansion in Zinnia elegans L. var. Envy mesophyll suspension cultures was manipulated by varying the extracellular osmolarity and the effect on xylogenesis was examined. Cell expansion and tracheary element differentiation were inversely related along a gradient of extracellular osmolarity ranging from 200 to 400 mOsm, supporting the hypothesis that tracheary element differentiation is coupled to cessation of cell expansion. Above 300 mOsm, reduction in the number of cells that differentiated into tracheary elements coincided with an increase in the number of plasmolyzed cells as extracellular osmolarity was increased, indicating that plasmolysis inhibits tracheary element differentiation, although not specifically. Using the plasmolysis method we showed that cellular osmolarity within populations of isolated Zinnia mesophyll cells ranges from 250 to 600 mOsm with a mean of 425 mOsm. The broad range in cellular osmolarity within Zinnia mesophyll cell populations, coupled with inhibition of differentiation in the low range due to cell expansion and in the high range due to plasmolysis, may help explain why tracheary element differentiation in Zinnia suspension cultures is never complete nor perfectly synchronous and enable further optimization of this culture system.