A complete kinetic analysis of temperature‐induced changes in primary root growth ofHelianthus annuuswas undertaken. Growth rate was expressed in terms of the rate of cell production per file and the average cell length at maturity. Growth rate increased from 0.014 cm · hr−1at 10 C to a maximum of 0.162 cm · hr−1at 25 C and decreased at higher temperatures. The rate of cell production per file, which is dependent on the mitotic cycle duration and the number of proliferating cells in the meristem, reached a maximum of 8.55 cells hr−1at 25 C. The mitotic cycle duration decreased from 46.4 hr at 10 C to a minimum of about 6.3 hr at 30 and 35 C. The mitotic index remained constant between 10 and 35 C. At 30 and 35 C the duration of the S period was about 65% of the total cycle time, while at 10 C it was about 50%. The number of proliferating cells in a meristem remained constant between 15 and 25 C and decreased with further increase in temperature. Therefore, the temperature at which the rate of cell production was maximum was not the temperature at which cycle time was minimum. The maximum length of mature cells occurred between 15 and 30 C. The contribution of each of the above parameters to temperature‐induced changes in growth rate is discussed.