Cell cycle kinetics were determined for roots grown either in tanks of water from 24 to 144 hr or in a moistened mixture of vermiculite and perlite (V/P) from 24 to 96 hr. For roots grown in tanks of water Tc was 23.4 hr (TG1= 6.1 hr, TG2—5.5 hr, T2= 9.6 hr, Tm= 2.3 hr) while roots grown in V/P had a shorter Tcof 17 hr (TG1= 5.6 hr, TG2= 4.0 hr, T, = 5.0 hr, Tm= 2.4 hr). Most of the difference in cycle times was due to a disproportionate increase in the duration of S for water grown roots while the duration of G1and G2showed relatively smaller changes. This difference in TSwas coupled with an increase in L.I. in water grown roots over that seen in V/P grown roots, i.e. 31.0 ± 8.6 cf. 25.9 ± 6.5. Changes in the proportions of fast, slow and non-cycling cells also occurred; i.e. in V/P grown roots there were 83.2% fast, 11.4% slow and 5.4% non-cycling cells, while in tanks of water there were 52.9% fast, 22.7% slow and 24.4% non-cycling cells. The results are evidence of differences both in mean cycle duration and phase duration within the cell cycle for roots grown in different volumes of water. They also show that roots respond to different volumes of water by altering their GF and the proportions of fast, slow and non-cycling cells in their meristems. These changes may be part of a mechanism used by roots to regulate the overall rate of cell proliferation and thus to achieve different TDTvalues under different environmental conditions.